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Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 2002"


Journal ArticleDOI
TL;DR: In this article, the main metallic biomaterials are stainless steels, Co-based alloys, and titanium and its alloys and they are used for replacing failed hard tissue.
Abstract: Metallic biomaterials are mainly used for replacing failed hard tissue. The main metallic biomaterials are stainless steels, Co-based alloys, and titanium and its alloys. Recently, titanium alloys are getting much attention for biomaterials. The various kinds of new high strength α+β and low-modulus β-type titanium alloys composed of nontoxic elements, such as Nb, Ta, Zr, etc., are developed for biomedical applications because of the toxicity of alloying elements and lack of mechanical biocompatibility of conventional titanium alloys, such as Ti-6Al-4V. Recent research and development in other metallic alloys, such as stainless steels and Co-based alloys, also will be discussed.

1,215 citations


Journal ArticleDOI
TL;DR: In this paper, the aluminum alloys 6063-T5 and T4 were friction-stir welded at different tool rotation speeds (R), and then distributions of the microstructure and hardness were examined in these welds.
Abstract: The aluminum (Al) alloys 6063-T5 and T4 were friction-stir welded at different tool rotation speeds (R), and then distributions of the microstructure and hardness were examined in these welds. The maximum temperature of the welding thermal cycle rose with increasing R values. The recrystallized grain size of the weld increased exponentially with increasing maximum temperature. The relationship between the grain size and the maximum temperature satisfied the static grain-growth equation. In the as-welded condition, 6063-T5 Al was softened around the weld center, whereas 6063-T4 Al showed homogeneous hardness profiles. Different R values did not result in significant differences in the hardness profile in these welds, except for the width of the softened region in the weld of 6063-T5 Al. Postweld aging raised the hardness in most parts of the welds, but the increase in hardness was small in the stir zone produced at the lower R values. Transmission electron microscope (TEM) observations detected a similar distribution of the strengthening precipitates in the grain interiors and the presence of a precipitation-free zone (PFZ) adjacent to the grain boundaries in all the welds. Microstructural analyses suggested that the small increase in hardness in the stir zone produced at the lower R values was caused by an increase in the volume fraction of PFZs.

411 citations


Journal ArticleDOI
TL;DR: In this article, a comparison of the critically resolved shear stress (CRSS) values for basal and prism slip as a function of Al content was performed on Ti-6.6 pct alloys.
Abstract: Single crystals of Ti-Al alloys containing 1.4, 2.9, 5, and 6.6 pct Al (by weight) were oriented for 〈a〉 slip on either basal or prism planes or loaded parallel along the c-axis to enforce a nonbasal deformation mode. Most of the tests were conducted in compression and at temperatures between 77 and 1000 K. Trace analysis of prepolished surfaces enabled identification of the twin or slip systems primarily responsible for deformation. Increasing the deformation temperature, Al content, or both, acted to inhibit secondary twin and slip systems, thereby increasing the tendency toward strain accommodation by a single slip system having the highest resolved stress. In the crystals oriented for basal slip, transitions from twinning to multiple slip and, finally, to basal slip occurred with increasing temperature in the lower-Al-content alloys, whereas for Ti-6.6 pct Al, only basal slip was observed at all temperatures tested. A comparison of the critically resolved shear stress (CRSS) values for basal and prism slip as a function of Al content shows that prism slip is favored at room temperature in pure Ti, but the stress to activate these two systems becomes essentially equal in the Ti-6.6 pct Al crystals over a wide range of temperatures. Compression tests on crystals oriented so that the load was applied parallel to the c-axis showed extensive twinning in lower Al concentrations and 〈c+a〉 slip at higher Al concentrations, with a mixture of 〈c+a〉 slip and twinning at intermediate compositions. A few tests also were conducted in tension, with the load applied parallel to the c-axis. In these cases, twinning was observed, and the resolved shear for plastic deformation by twinning was much lower that that for 〈c+a〉 slip observed in compression loading.

304 citations


Journal ArticleDOI
TL;DR: In this article, the authors examined how the overall resistance to high-cycle fatigue in Ti-6Al-4V compares between the bimodal microstructure and a coarser lamellar (β-annealed) micro-structure.
Abstract: The high-cycle fatigue (HCF) of titanium alloy turbine engine components remains a principal cause of failures in military aircraft engines. A recent initiative sponsored by the United States Air Force has focused on the major drivers for such failures in Ti-6Al-4V, a commonly used turbine blade alloy, specifically for fan and compressor blades. However, as most of this research has been directed toward a single processing/heat-treated condition, the bimodal (solution-treated and overaged (STOA)) microstructure, there have been few studies to examine the role of microstructure. Accordingly, the present work examines how the overall resistance to high-cycle fatigue in Ti-6Al-4V compares between the bimodal microstructure and a coarser lamellar (β-annealed) microstructure. Several aspects of the HCF problem are examined. These include the question of fatigue thresholds for through-thickness large and short cracks; microstructurally small, semi-elliptical surface cracks; and cracks subjected to pure tensile (mode I) and mixed-mode (mode I+II) loading over a range of load ratios (ratio of minimum to maximum load) from 0.1 to 0.98, together with the role of prior damage due to sub-ballistic impacts (foreign-object damage (FOD)). Although differences are not large, it appears that the coarse lamellar microstructure has improved smooth-bar stress-life (S-N) properties in the HCF regime and superior resistance to fatigue-crack propagation (in pure mode I loading) in the presence of cracks that are large compared to the scale of the microstructure; however, this increased resistance to crack growth compared to the bimodal structure is eliminated at extremely high load ratios. Similarly, under mixed-mode loading, the lamellar microstructure is generally superior. In contrast, in the presence of microstructurally small cracks, there is little difference in the HCF properties of the two microstructures. Similarly, resistance to HCF failure following FOD is comparable in the two microstructures, although a higher proportion of FOD-induced microcracks are formed in the lamellar structure following high-velocity impact damage.

269 citations


Journal ArticleDOI
TL;DR: In this article, the phase-transformation kinetics during the intercritical annealing and the isothermal bainitic transformation were investigated by means of dilatometry, and the allotropic phase-boundary was determined both by thermodynamic calculations and the experimental determination of the C content in the retained austenite.
Abstract: Conventional CMnSi transformation-induced plasticity (TRIP)-aided steels are a promising solution for producing lighter, crash-resistant car bodies, due to their high-strength and large uniform elongation. The CMnSi TRIP-aided steels, with more than 1 mass pct Si, have the drawback of poor galvanizability due to the presence of complex Si-Mn oxides on the surface. The full substitution of the Si by Al in cold-rolled and intercritically annealed TRIP-aided steels, therefore, was evaluated in detail. The phase-transformation kinetics during the intercritical annealing and the isothermal bainitic transformation were investigated by means of dilatometry. The allotropic phase-boundary was determined both by thermodynamic calculations and the experimental determination of the C content in the retained austenite. The results imply that short isothermal bainitic transformation times are sufficient to obtain the TRIP microstructure and that the processing of CMnAl TRIP-aided steels in a continuous annealing line not equipped for overaging is possible. The mechanical properties were evaluated for CMnAl TRIP-aided steels obtained using an industrial thermal cycle: the properties matched those of the conventional CMnSi TRIP-aided steels, where it was found that the high-Al CMnAl TRIP-aided steel had a high strain-hardening coefficient of 0.25, which was stable up to a true strain of 0.25.

268 citations


Journal ArticleDOI
TL;DR: In this article, a ternary intermetallic phase, (Mg,Al)2Ca, was identified in the microstructure of the ACX alloys and is proposed to be responsible for the improved creep resistance of the alloys.
Abstract: This article describes the creep and microstructure of Mg-Al-Ca-based magnesium alloys (designated as ACX alloys, where A stands for aluminum; C for calcium; and X for strontium or silicon) developed for automotive powertrain applications. Important creep parameters, i.e., secondary creep rate and creep strength, for the new alloys are reported. Creep properties of the new alloys are significantly better than those of the AE42 (Mg-4 pct* Al-2 pct RE**) alloy, which is the benchmark creep-resistant magnesium die-casting alloy. Creep mechanisms for different temperature/stress regimes are proposed. A ternary intermetallic phase, (Mg,Al)2Ca, was identified in the microstructure of the ACX alloys and is proposed to be responsible for the improved creep resistance of the alloys.

258 citations


Journal ArticleDOI
TL;DR: In this article, a review of the role of dislocation cores and planar faults in activating pyramidal slip and deformation twinning in hcp metals and alloys and in D019 intermetallic compounds is presented.
Abstract: A review is presented on the role of dislocation cores and planar faults in activating the nonbasal deformation modes, pyramidal slip and deformation twinning, in hcp metals and alloys and in D019 intermetallic compounds. Material-specific mechanical behavior arises from a competition between alternate defect structures that determine the deformation modes. We emphasize the importance of accurate atomistic modeling of these defects, going beyond simple interatomic energy models. Recent results from both experiments and theory are summarized by discussing specific examples of Ti and Mg single crystals; Ti-, Zr-, and Mg-base alloys; and Ti3Al ordered alloys. Remaining key issues and directions for future research are also discussed.

252 citations


Journal ArticleDOI
TL;DR: In this paper, the formation of α″ martensite and its influence on Young's modulus and mechanical properties of forged Ti-29Nb-13Ta-4.6Zr (wt pct) alloy is reported.
Abstract: An investigation on the formation of α″ martensite and its influence on Young’s modulus and mechanical properties of forged Ti-29Nb-13Ta-4.6Zr (wt pct) alloy is reported in this article. For ice-water-quenched specimens after solution treatment at 1023, 1123, and 1223 K in the single β-phase field for 1.8, 3.6, 14.4, and 28.8 ks, X-ray diffraction and internal friction measurements showed that the volume fraction of the α″ martensite changes with both solution temperature and time. This effect has been attributed mainly to the influence of grain size of the β-parent phase on the stability of the β phase and, consequently, on the martensitic start (Ms) temperature. A critical grain size of 40 µm was identified for the β phase, below which the martensitic transformation is largely suppressed because of low MS temperature. With the β grain size increasing above this critical value, the volume fraction of the α″ martensite increases significantly at first and then decreases gradually with further grain growth. The α″ martensite was shown to possess good ductility and, compared to the β phase, lower strength and hardness but nearly identical Young’s modulus in the studied alloy.

236 citations


Journal ArticleDOI
TL;DR: In this article, a three-dimensional laser-keyhole welding model is developed, featuring the self-consistent evolution of the liquid/vapor (L/V) interface together with full simulation of fluid flow and heat transfer.
Abstract: A three-dimensional laser-keyhole welding model is developed, featuring the self-consistent evolution of the liquid/vapor (L/V) interface together with full simulation of fluid flow and heat transfer. Important interfacial phenomena, such as free surface evolution, evaporation, kinetic Knudsen layer, homogeneous boiling, and multiple reflections, are considered and applied to the model. The level set approach is adopted to incorporate the L/V interface boundary conditions in the Navier-Stokes equation and energy equation. Both thermocapillary force and recoil pressure, which are the major driving forces for the melt flow, are incorporated in the formulation. For melting and solidification processes at the solid/liquid (S/L) interface, the mixture continuum model has been employed. The article consists of two parts. This article (Part I) presents the model formulation and discusses the effects of evaporation, free surface evolution, and multiple reflections on a steady molten pool to demonstrate the relevance of these interfacial phenomena. The results of the full keyhole simulation and the experimental verification will be provided in the companion article (Part II).

234 citations


Journal ArticleDOI
TL;DR: In this article, constant extension rate tests were conducted under open-circuit conditions and under potential control in 3.5 pct NaCl on samples isothermally treated at 150 °C.
Abstract: The AA5083 (Al-4.4Mg-0.7Mn-0.15Cr) alloy is a nonheat-treatable aluminum alloy known for its excellent corrosion resistance. However, it can become susceptible to intergranular stress corrosion cracking (IGSCC) when exposed to temperatures ranging from 50 °C to 200 °C for sufficient lengths of time. This IGSCC is widely believed to be associated with dissolution of the electrochemically active β phase, Al3Mg2, which is precipitated on grain boundaries. Recently, alternative mechanisms have been invoked related to hydrogen effects and/or free Mg segregation or depletion in the grainboundary regions. To establish a baseline for the sensitization effect, constant-extension-rate tests (CERTs) were conducted under open-circuit conditions and under potential control in 3.5 pct NaCl on samples isothermally treated at 150 °C. To aid in interpreting the CERT results, grain-boundary precipitation and solute depletion were characterized by transmission electron microscopy (TEM). Additionally, the electrochemical behavior of the β phase was characterized by anodic polarization of the intermetallic compound synthesized in bulk form. In CERTs under open-circuit conditions, the measured ductility depended strongly on sensitization time, reaching a minimum at 189 hours, followed by a slight increase at longer times. This trend correlated well with the fractional coverage of β phase on grain boundaries, which increased up to 189 hours, where it existed with nearly continuous coverage. At longer times, this film coarsened and became discontinuous. Correspondingly, some resistance to IGSCC was recovered. In polarization experiments, bulk synthesized β phase was found to be spontaneously passive from its corrosion potential (−1.40 VSCE) up to about −0.92 VSCE, where passivity was observed to break down. Sensitized AA5083 samples polarized below the β-phase breakdown potential showed almost no evidence of IGSCC, indicating that a high β dissolution rate is a requirement for IGSCC. Mg-depleted zones were observed along grain boundaries in sensitized alloys, but a clear role for solute depletion in IGSCC could not be defined on the basis of the results developed in this study.

220 citations


Journal ArticleDOI
TL;DR: In this paper, an equal-channel angular pressing (ECAP) was performed on high purity aluminum (99.99 pct) at room temperature through a die with a 90 deg angle between the die channels.
Abstract: High-purity aluminum (99.99 pct) was processed by equal-channel angular pressing (ECAP) at room temperature through a die with a 90 deg angle between the die channels. Samples were examined by transmission electron microscopy (TEM) and orientation imaging microscopy (OIM) methods after one, four, and 12 passes through the die. Repetitively pressed samples were rotated by 90 deg in the same sense between successive pressing operations (route BC). After one pressing, TEM showed a subgrain structure which was elongated in the shearing direction. Corresponding OIM data illustrated an inhomogeneous microstructure in which bandlike features were also aligned with the shearing direction. The lattice orientation varied from location to location in the material. The boundary disorientation distribution determined from the OIM data exhibited a peak at 2 to 5 deg, in agreement with a predominance of subgrains in the microstructure. After four pressings, the microstructure data obtained by TEM and OIM were mutually consistent. The disorientation data revealed a decrease in the population of 2 to 5 deg boundaries accompanied by an overall upward shift in the distribution. Two orientations were generally apparent in the texture, although specific orientations varied with location. Often, a 〈111〉 orientation tended to align with the shear direction. Following 12 ECA passes, the grain size was reduced further to about 1.0 µm. The populations of high-angle boundaries (≥15 deg) increased in the disorientation distribution. A texture characteristic of shear deformation of fcc metals became apparent, although the orientations and particular components varied with location. Microstructural refinement during severe straining includes the development of large fractions of high-angle boundaries.

Journal ArticleDOI
TL;DR: In this paper, a higher density and more uniform distribution of dislocation dislocations were found in the Li-containing alloy compared to pure Mg and Mg-15 at. pct Li.
Abstract: The ductility of Mg alloys is limited due to a shortage of independent slip systems. In particular, c-axis compression cannot be accommodated by any of the easy slip or twinning modes. Basal-textured samples of pure Mg and Mg-15 at. pct Li were examined for the presence of 〈c+a〉 dislocations by post-mortem transmission electron microscopy (TEM) after a small deformation, which forced the majority of grains to compress nearly parallel to their c-axes. A higher density and more uniform distribution of 〈c+a〉 dislocations is found in the Li-containing alloy. Because the 1/3〈11\(\bar 2\)3〉 {11\(\overline {22} \)} pyramidal slip mode offers five independent slip systems, it provides a satisfying explanation for the enhanced ductility of α-solid solution Mg-Li alloys as compared to pure Mg. The issue of 〈c+a〉 dislocation dissociation and decomposition remains open from an experimental point of view. Theoretically, the most feasible configuration is a collinear dissociation into two 1/2〈c+a〉 partial dislocations, with an intervening stacking fault on the glide plane. It is speculated that Li additions may lower the fault’s energy and, thereby, increase the stability of this glissile configuration.

Journal ArticleDOI
TL;DR: In this paper, the morphologies of dislocation networks differ slightly from each other in these alloys and the mechanisms of evolution of the interfacial dislocations networks were discussed, which can be correlated with the creep behavior of these superalloys.
Abstract: The γ/γ′ interfacial dislocation networks in several creep-ruptured superalloys were analyzed. It was found that the morphologies of dislocation networks differ slightly from each other in these alloys. The fourth-generation superalloy has finer dislocation networks and keeps a relatively stable state. Comparatively, the interfacial dislocations in the third-generation superalloy show obvious curved features associated with possible climb or slip. These interfacial dislocation characteristics can be correlated with the creep behavior of these superalloys. The mechanisms of evolution of the interfacial dislocation networks were discussed.

Journal ArticleDOI
TL;DR: In this paper, a thermal model for the thermoplastic shear instability in the machining of a titanium alloy (Ti-6Al-4V) was developed based on the analysis of the shear-localized chip formation process and the temperature generated in the hear band due to various heat sources (primary, preheating and image) in machining.
Abstract: A thermal model for the thermoplastic shear instability in the machining of a titanium alloy (Ti-6Al-4V) is developed It is based on the analysis of the shear-localized chip formation process and the temperature generated in the shear band due to various heat sources (primary, preheating, and image) in machining The temperature in the shear band was determined analytically using the Jeager’s classical stationary- and moving-heat-source methods Using Recht’s classical model of catastrophic shear instability (thermal softening vs strain hardening), the onset of shear localization was determined The shear stress in the shear band is calculated at the shear-band temperature and compared with the value of the shear strength of the bulk material at the preheating temperature If the shear stress in the shear band is less than or equal to the shear strength of the bulk material, then shear localization is imminent The cutting speed at which this occurs is taken as the critical speed for the onset of shear localization, which continues at all speeds above this value In the case of titanium alloys, this speed is rather low, indicating shear localization practically at all conventional cutting speeds The effect of the depth of the cut on the onset of shear localization was also considered, as it may affect the heat transfer from the shear-localized region, ie, between the segments in the chip, to the rest of the chip and preheating of the segment For example, there can be a significant difference in the thermal aspects of shear localization in ultraprecision machining (where the depths of cuts are a few micrometers or less) compared to conventional machining (where the depths of cuts are several hundred micrometers) This is because of the differences in the distances between the segments as well as the energy inputs in each case

Journal ArticleDOI
TL;DR: In this article, a microporosity model, based on the solution of Darcy's equation and microsegregation of gas, has been developed for arbitrary two-dimensional and three-dimensional (3D) geometry and coupled for the first time with macroporeosity and pipe-shrinkage predictions.
Abstract: A microporosity model, based on the solution of Darcy’s equation and microsegregation of gas, has been developed for arbitrary two- (2-D) and three-dimensional (3-D) geometry and coupled for the first time with macroporosity and pipe-shrinkage predictions. In order to accurately calculate the pressure drop within the mushy zone, a dynamic refinement technique has been implemented: a fine and regular finite volume (FV) grid is superimposed onto the finite-element (FE) mesh used for the heat-flow computations. For each time-step, the cells, which fall in the mushy zone, are activated, and the governing equations of microporosity formation are solved only within this domain, with appropriate boundary conditions. For that purpose, it is necessary to identify automatically the various liquid regions that may appear during solidification: open regions of liquid are connected to a free surface where a pressure is imposed, partially closed liquid regions are connected to an open region via the mushy zone, and closed regions are totally surrounded by the solid and/or mold. For partially closed liquid pockets, it is shown that an integral boundary condition applies before macroporosity appears. Finally, pipe shrinkage (i.e., shrinkage appearing at a free surface) is obtained by integration of the calculated interdendritic fluid flow over the open-region boundaries, thus ensuring that the total shrinkage (microporosity plus macroporosity and pipe shrinkage) respects the overall mass balance. This very general approach is applied to Al-Cu and Al-Si alloys.

Journal ArticleDOI
TL;DR: In this article, pre-equal channel-angular-pressing (ECAP) solution treatment combined with post-ECAP aging treatment has been found to be effective in enhancing the room-temperature strength of 6061 aluminum alloy.
Abstract: Pre-equal-channel-angular-pressing (ECAP) solution treatment combined with post-ECAP aging treatment has been found to be effective in enhancing the room-temperature strength of 6061 aluminum alloy. The largest increase in ultimate tensile strength (UTS) (=460 MPa) and yield stress (YS) (=425 MPa) is obtained in post-ECAP aged 6061 Al with six pressings. The strength increases by a factor of 1.4 when compared to T6 treated commercial 6061 Al. The strength of 6061 Al obtained in the present research is higher than that of ECA-pressed 6061 Al with pre-ECAP peak-aging treatment studied by other investigators. The more effective strengthening of post-ECAP low-temperature aging may be linked with the higher dislocation accumulation rate in the solutionized matrix and the presence of higher density particles in the aged matrix. Modest low-temperature (523 K) and high-temperature (813 K) superplasticity is observed in the ECAP 6061 Al, which may be a result of increased grain bundary area from grain refinement.

Journal ArticleDOI
TL;DR: In this paper, the authors focused on a stationary tungsten-inert-gas (TIG) welding process and developed a numerical model of the process for understanding quantitative values of the balances of mass, energy, and force in the welding phenomena.
Abstract: In order to clarify the formative mechanism of weld penetration in an arc welding process, the development of a numerical model of the process is quite useful for understanding quantitative values of the balances of mass, energy, and force in the welding phenomena because there is still lack of experimentally understanding of the quantitative values of them because of the existence of complicated interactive phenomena between the arc plasma and the weld pool The present article is focused on a stationary tungsten-inert-gas (TIG) welding process for simplification, but the whole region of TIG arc welding, namely, tungsten cathode, arc plasma, workpiece, and weld pool is treated in a unified numerical model, taking into account the close interaction between the arc plasma and the weld pool Calculations in a steady state are made for stationary TIG welding in an argon atmosphere at a current of 150 A The anode is assumed to be a stainless steel, SUS304, with its negative temperature coefficient of surface tension The two-dimensional distributions of temperature and velocity in the whole region of TIG welding process are predicted The weld-penetration geometry is also predicted Furthermore, quantitative values of the energy balance for the various plasma and electrode regions are given The predicted temperatures of the arc plasma and the tungsten-cathode surface are in good agreement with the experiments There is also approximate agreement of the weld shape with experiment, although there is a difference between the calculated and experimental volumes of the weld The calculated convective flow in the weld pool is mainly dominated by the drag force of the cathode jet and the Marangoni force as compared with the other two driving forces, namely, the buoyancy force and the electromagnetic force

Journal ArticleDOI
TL;DR: In this paper, the authors measured the thermal capacities, thermalexpansion coefficients, thermal and electrical conductivities of Nb2AlC (actual Nb:Al:C mole fractions: 0.525±0.005, 0.240±
Abstract: The heat capacities, thermal-expansion coefficients, thermal and electrical conductivities of Nb2AlC (actual Nb:Al:C mole fractions: 0.525±0.005, 0.240±0.002, and 0.235±0.005, respectively), Ti2AlC and (Ti, Nb)2AlC (actual Ti:Nb:Al:C mole fractions: 0.244±0.005, 0.273±0.005, 0.240±0.003, and 0.244±0.005, respectively) were measured as a function of temperature. These ternaries are good electrical conductors, with a resistivity that increases linearly with increasing temperatures. The resistivity of (Ti, Nb)2AlC is higher than the other members, indicating a solid-solution scattering effect. The thermal-expansion coefficients, in the 25 °C to 1000 °C temperature range, are comparable and fall in the narrow range of 8.7 to 8.9 × 10−6 K−1, with that of the solid solution being the highest. They are all good conductors of heat, with thermal conductivities in the range between 15 to 45 W/m K at room temperature. The electronic component of the thermal conductivity is the dominant mechanism at all temperatures for Nb2AlC and (Ti, Nb)2AlC. The conductivity of Ti2AlC, on the other hand, is high because the phonon contribution to the conductivity is nonnegligible.

Journal ArticleDOI
TL;DR: In this article, the formation of WLs is promoted by conditions of moderate to high cutting speed in conjunction with tool flank wear and deformation of material to very large strains.
Abstract: White layers (WLs) produced in hard steels by machining have been characterized using nanoindentation, optical microscopy, transmission electron microscopy (TEM), and X-ray diffraction. The WL is found to have a hardness of 12.85±0.80 GPa, which is significantly greater than that of untempered martensite produced by various heat-treatment processes. The grain size in the WL is shown to be in the submicrometer range with values ranging, typically, between 30 and 500 nm. These two characteristics of the WL distinguish it from various structures formed in steels by heat treatment. The formation of WLs is promoted by conditions of moderate to high cutting speed in conjunction with tool flank wear. Based on a consideration of the strain, stress, and temperature states associated with the formation of WLs in machining, it is hypothesized that deformation of material to very large strains is the principal factor contributing to the formation of these layers with ultrafine grained or nanocrystalline structures. The large strain deformation and elevated temperatures prevailing in the machining zone could also trigger dynamic recrystallization or cause decomposition and partial dissolution of the cementite present in the steels.

Journal ArticleDOI
TL;DR: In this paper, the evolution of the lamellar-colony microstructure to an equiaxed morphology during heat treatment of a hot-worked, two-phase titanium alloy was established.
Abstract: The kinetics of the evolution of the lamellar-colony microstructure to an equiaxed morphology during heat treatment of a hot-worked, two-phase titanium alloy were established. For this purpose, the alpha/beta alloy Ti-6Al-4V was isothermally upset forged at 900 °C or 955 °C and subsequently annealed for times ranging from 0.5 to 100 hours. The degree of the breakup of alpha-phase lamellae into lower-aspect-ratio grains during static annealing was measured and related to the imposed strain estimated using finite-element analysis (FEA). The kinetics of the static globularization of the alpha phase were found to depend on the amount of strain and the annealing temperature but were not affected by the specific deformation temperature in the 900 °C to 955 °C range. These results demonstrated that deformation-induced dislocation substructure has a small effect on the static-globularization process. In addition, the relative globularization kinetics at 900 °C and 955 °C were rationalized in terms of classical coarsening theory.

Journal ArticleDOI
TL;DR: In this article, the simulation results of a three-dimensional mathematical model using the level set method for laser-keyhole welding are presented, which shows very interesting features in the weld pool, such as intrinsic instability of keyholes, role of recoil pressure, and effect of beam scanning.
Abstract: This article presents the simulation results of a three-dimensional mathematical model using the level set method for laser-keyhole welding. The details of the model are presented in Part I.[4] The effects of keyhole formation on the liquid melt pool and, in turn, on the weld bead are investigated in detail. The influence of process parameters, such as laser power and scanning speed is analyzed. This simulation shows very interesting features in the weld pool, such as intrinsic instability of keyholes, role of recoil pressure, and effect of beam scanning.

Journal ArticleDOI
TL;DR: In this article, a stability diagram of martensitic phases as a function of alloy concentration has been determined, and linear relationships between M petertodd s and Al and Ni concentrations have been obtained for all types of martENSitic phases.
Abstract: The martensitic transformation temperatures and the types of martensitic phases have been determined in a wide concentration range of technological interest for Cu-Al-Ni shape-memory alloys (SMAs) A stability diagram of martensitic phases as a function of alloy concentration has been determined. It is found that when the aluminum content increases, the transformation changes from β 3 ⇒ β′3 to β 3 ⇒ γ′3, with an intermediate concentration range where both martensites coexist due to a β 3 ⇒ γ′3+β′3 transformation. On the other hand, an increase of nickel content stabilizes the martensite β′3, changing from a mixed β 3 ⇒ γ′3 + β′3 to a single β 3 ⇒ β′3 transformation. Furthermore, linear relationships between M s and Al and Ni concentrations have been obtained for all types of martensitic phases.

Journal ArticleDOI
TL;DR: In this paper, a mathematical model and computer programs were developed for numerical simulation of the processes of nucleation and growth of the α-phase Widmanstatten plates during the course of the β ⇒ α phase transformation in a Ti-6Al-4V alloy.
Abstract: In the present work, a mathematical model and computer programs were developed for numerical simulation of the processes of nucleation and growth of the α-phase Widmanstatten plates during the course of the β ⇒ α phase transformation in a Ti-6Al-4V alloy. The α-phase appearance at the grain boundary of β phase is described by a numerical procedure for random nucleation as a function of the vanadium concentration and the temperature. The rate at which an interface moves depends both on the intrinsic mobility and on the rate at which diffusion can remove the excess of vanadium atoms ahead of the interface. The finite-element method (FEM) was used for solving the diffusion equation on the domain occupied by β phase. The elements chosen have dimensions in both space and time. A computer code based on the finite-element modeling and the volume of fluids method was developed to trace the movement of the α/β interface. The influences of the cooling rate and the temperature of isothermal exposure on the Widmanstatten morphology were simulated and analyzed. The developed models and program packages are capable of one-dimensional (1-D) and two-dimensional (2-D) simulations of the morphology of the β ⇒ α phase transformation in Ti-6Al-4V alloy for continuous cooling with any cooling path and for an arbitrary combination between continuous cooling and isothermal exposure.

Journal ArticleDOI
TL;DR: In this article, a ternary carbide (Ti3SiC2) was synthesized by using the mixture method for 24 hours in an Ar atmosphere, and the synthesis process was conducted at 1200 °C to 1400 °C under a pressure of 50 MPa.
Abstract: Ti/Si/TiC powders with molar ratios of 1:1:2 (M1) and 2:2:3 (M2) were prepared for the synthesis of a ternary carbide Ti3SiC2 by using the mixture method for 24 hours in an Ar atmosphere. The synthesis process was conducted at 1200 °C to 1400 °C under a pressure of 50 MPa, using the pulse-discharge sintering (PDS) technique. After sintering, the phase constituents and microstructures of the samples were analyzed by X-ray diffraction (XRD) technique and observed by optical microscopy and scanning electron microscopy. The results showed that the phases in all the samples consisted of Ti3SiC2 and small amounts of TiC, and the optimum sintering temperature was found to be in the relatively low range of 1250 °C to 1300 °C. By the standard additive method, the relative content of Ti3SiC2 was calculated. For the M1 samples, the lowest TiC content can be only decreased to about 3 to 4 wt pct, whereas the content of Ti3SiC2 in the M2 samples is always lower than that in the M1 samples. When the M2 powder was sintered at 1300 °C for 8 to 240 minutes, the TiC peaks were found to show a very low intensity, and the corresponding content of Ti3SiC2 was calculated to be higher than 99 wt pct. The grain size of Ti3SiC2 increased from 5 to 10 µm to 80 to 100 µm in the entire applied sintering temperature range. The relative density of the M2 samples was measured to be higher than 99 pct at sintering temperatures above 1275 °C. It indicates that the PDS technique can rapidly synthesize high-content Ti3SiC2 from the Ti/Si/TiC powders in a relatively low temperature range.

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TL;DR: In this article, a self-consistent model was applied to predict the plastic flow behavior during hot working of alpha/beta titanium alloys with wrought (equiaxed alpha) microstructures as a function of flow behavior and volume fractions of the individual phases.
Abstract: A self-consistent model was applied to predict the plastic flow behavior during hot working of alpha/beta titanium alloys with wrought (equiaxed alpha) microstructures as a function of the flow behavior and volume fractions of the individual phases. For this purpose, constitutive relations that incorporated composition-dependent strength coefficients were determined for the alpha and beta phases. With these constitutive relations and measurements of the specific compositions and volume fractions of the two phases at hot-working temperatures, the flow stress dependence on temperature under nominally isothermal conditions and the (average) strain rates in the individual phases were predicted for Ti-6Al-4V. The effect of temperature transients during hot deformation on the flow stress under nonisothermal (conventional) forging conditions and under nominally isothermal, high strain-rate conditions was also established using the self-consistent modeling approach. In these instances, the effect of a rapid temperature drop or rise, respectively, on the retention of a metastable microstructure was quantified. The predicted flow behaviors showed good agreement with experimental measurements.

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TL;DR: In this article, the columnar-to-equiaxed transition (CET) during upward unsteady-state directional solidification of Al-Cu and Sn-Pb alloys, under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface was analyzed.
Abstract: Experiments were conducted to analyze the columnar-to-equiaxed transition (CET) during the upward unsteady-state directional solidification of Al-Cu and Sn-Pb alloys, under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal parameters: transient heat-transfer coefficients, tip growth rates, thermal gradients, and cooling rates. The observed results do not give support to CET criteria based individually either on tip growth rate or temperature gradients ahead of the liquidus isotherm. Rather, the analysis has indicated that a more convenient criterion should encompass both thermal parameters through the tip cooling rate. The columnar growth is expected to prevail throughout the casting for a tip cooling rate higher than a critical value, which depends only on the alloy system and was observed to be about 0.2 K/s for Al-Cu alloys and 0.01 K/s for Sn-Pb alloys in the present investigation.

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TL;DR: In this article, high-resolution electron microscopy (HREM) has been applied to elucidate the nature of the phase transformation and the evolution of the unique microstructure and defect structure characterizing the structural state of the ferromagnetic τ phase.
Abstract: Manganese-aluminum alloys in the vicinity of the equiatomic composition exhibit an attractive combination of magnetic properties for technological applications, including bulk permanent magnets and thin-film devices. The technical magnetic properties derive from the formation of a metastable L10 intermetallic phase (τ-MnAl) characterized by a high, uniaxial magnetocrystalline anisotropy with an “easy” c-axis. Carbon is generally added to stabilize the tetragonal τ phase with respect to the stable phases in the system. The magnetic hysteresis behavior of the Mn-Al-C genre of permanent magnet alloys is extremely sensitive to the microstructure and defect structure produced during the formation of the τ phase (L10) within the high-temperature e phase (hcp). In this study, modern metallographic techniques, including high-resolution electron microscopy (HREM), have been applied to elucidate the nature of the phase transformation and the evolution of the unique microstructure and defect structure characterizing the structural state of the ferromagnetic τ phase. It is concluded that the metastable τ phase is the product of a compositionally invariant, diffusional nucleation and growth process or massive transformation. The massive product nucleates preferentially at the grain boundaries of the parent e phase and is propagated by the migration of incoherent interphase interfaces. The interphase interfaces are revealed to be faceted on various length scales. It is concluded that this faceting is not a feature of the bicrystallography of the parent and product phases. The high density of lattice defects within the τ phase, generated by the phase transformation, is attributed to growth faults produced during atomic attachment at the migrating interfaces. Classical nucleation theory has been applied quantitatively to the grain-boundary nucleation process and was found to be consistent with the observed time-temperature-transformation (TTT) behavior. Analysis of the growth kinetics gives an ΔHD value of 154 kJ mol−1 for the activation energy of the transboundary diffusional process controlling boundary migration.

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TL;DR: In this article, the effects of a Mo addition on both the precipitation kinetics and high-temperature strength of a Nb carbide have been investigated in the hot-rolled high-strength, low-alloy (HSLA) steels containing both Nb and Mo.
Abstract: The effects of a Mo addition on both the precipitation kinetics and high-temperature strength of a Nb carbide have been investigated in the hot-rolled high-strength, low-alloy (HSLA) steels containing both Nb and Mo. These steels were fabricated by four-pass hot rolling and coiling at 650°C, 600°C, and 550°C. Microstructural analysis of the carbides has been performed using field-emission gun transmission electron microscopy (TEM) employing energy-dispersive X-ray spectroscopy (EDS). The steels containing both Nb and Mo exhibited a higher strength at high temperatures (∼600 °C) in comparison to the steel containing only Nb. The addition of Mo increased the hardenability and led to the refinement of the bainitic microstructure. The proportion of the bainitic phase increased with the increase of Mo content. The TEM observations revealed that the steels containing both Nb and Mo exhibited fine (<10 nm) and uniformly distributed metal carbide (MC)-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb only. The EDS analysis also indicated that the fine MC carbides contain both Nb and Mo, and the ratio of Mo/Nb was higher in the finer carbides. In addition, electron diffraction analysis revealed that most of the MC carbides had one variant of the B-N relationship ((100)MC//(100)ferrite, [011]MC//[010]ferrite) with the matrix, suggesting that they were formed in the ferrite region. That is, the addition of Mo increased the nucleation sites of MC carbides in addition to the bainitic transformation, which resulted in finer and denser MC carbides. It is, thus, believed that the enhanced high-temperature strength of the steels containing both Nb and Mo was attributed to both bainitic transformation hardening and the precipitation hardening caused by uniform distribution of fine MC particles.

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TL;DR: In this article, the hydrogen-uptake capacity and mobility in ultra-high-strength AERMET 100 are characterized for various electrochemical charging and baking conditions, with a relatively high apparent activation energy for diffusion of 17.7 to 18.2 kJ/mol at the 95 pct confidence level.
Abstract: The hydrogen-uptake capacity and mobility in ultrahigh-strength AERMET 100 are characterized for various electrochemical charging and baking conditions. From thermal desorption spectroscopy, the apparent hydrogen diffusivity (D H < 3 × 10−8 cm2/s at 23 °C) is over tenfold less than the values typical of tempered martensitic steels such as AISI 4130. The value of D H decreases with decreasing temperature below 200 °C, with a relatively high apparent activation energy for diffusion of 17.7 to 18.8 ± 0.2 kJ/mol at the 95 pct confidence level. The value of D H also decreases with decreasing diffusible H concentration from less-severe charging or increased baking. Potentiostatic charging in saturated Ca(OH)2 produced total and diffusible H concentrations in AERMET 100 which increase with (H+/H) overpotential and are significantly higher than results for AISI 4130 steel under the same conditions. A significant H concentration was produced by zero overpotential deposition. These characteristics are explained by extensive reversible and irreversible H trapping involving at least three unique trap states in the ultrafine AERMET 100 microstructure. The former likely include coherent M2C carbides, soluble Ni, or precipitated austenite, and the latter include larger incoherent M x C y or martensite lathed-packet interfaces. Baking at 23 °C and 200 °C removes H from the lowest binding-energy sites, but results in reduced D H levels to prolong outgassing time. Additionally, substantial H was retained in stronger trap states. These trapping effects are pertinent to hydrogen embrittlement of AERMET 100 steel.

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TL;DR: In this paper, the intergranular corrosion (IGC) behavior of 6061 aluminum alloy extrusions was investigated and it was considered that IGC was caused by the existence of precipitate-free zones (PFZs).
Abstract: The intergranular corrosion (IGC) behavior of 6061 aluminum alloy extrusions was investigated. After the IGC test in accordance with ISO/DIS 11846 (method B), heavy IGC was observed at the surface of the extrusion. However, little IGC occurred at the center plane of the extrusion thickness. It was considered that IGC was caused by the existence of precipitate-free zones (PFZs) because PFZs were clearly observed in the surface layer of the extrusion but were not clearly observed in the center position of the extrusion thickness. Furthermore, it was considered that the formation of PFZs was associated with the grain boundary characteristics. That is, most of the grain boundaries had random high angles in the surface layer of the extrusion, but 60 pct of the boundaries were lower than 0.26 rad (15 deg) in the center plane of the thickness. To verify this hypothesis, the center plane was cold rolled, recrystallized, and examined using the IGC test. As a result, heavy IGC was observed, while the center plane, in this case, had almost all random high-angle boundaries.