Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 2004"

Formation of simple crystal structures in Cu-Co-Ni-Cr-Al-Fe-Ti-V alloys with multiprincipal metallic elements

Abstract: Crystalline solid solutions are typically formed in conventional alloys based on one or two host elements. Here, in this research, four alloys containing multiprincipal metallic elements (≥5 elements) were prepared by casting, splat quenching, and sputtering. Their microstructures and crystal structures were investigated. It was interestingly found that solid solutions with simple fcc or bcc crystal structure were also practically formed in these alloys with multiprincipal elements. All different atoms are regarded as solutes and expected to randomly distribute in the crystal lattices without any matrix element defined.

Wear resistance and high-temperature compression strength of Fcc CuCoNiCrAl0.5Fe alloy with boron addition

Abstract: This study discusses the wear resistance and high-temperature compression strength of CuCoNiCrAl0.5Fe alloy with various amounts of boron addition. Experiments show that within the atomic ratio of boron addition from x=0 to x=1.0 in CuCoNiCrAl0.5FeBx (referred to as B-0 to B-1.0 alloys), the alloys are of fcc structure with boride precipitation. The volume fraction of borides increases with increasing boron addition. The corresponding hardness increases from HV 232 to HV 736. Wear resistance and high-temperature compression strength are significantly enhanced by the formation of boride. The alloys with boride are less tough. The superior wear resistance of B-1.0 alloy, which is even better than SUJ2 wear-resistant steel, indicates that the CuCoNiCrAl0.5FeBx alloys have potential applications as ambient- and high-temperature mold, tool, and structural materials.

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Abstract: Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.

Microstructural evolution in laser-deposited multilayer Ti-6Al-4V builds: Part I. Microstructural characterization

Abstract: The macro and microstructure of laser-deposited Ti-6Al-4V has been investigated to determine the evolution of unique microstructural features in mutilayer builds. The macro and microstructures exhibited in the build include large, columnar prior-beta grains, a gradient in the individual alpha-lath thickness between the deposited layers, and the presence of layer bands within each layer, except for the last three layers deposited. The layer band consists of a colony Widmanstatten alpha morphology, while the nominal microstructure between layer bands exhibits a basketweave morphology. Optical microscopy, hardness, and composition measurements were used to determine that the layer-band and gradient morphologies are resultant from the complex thermal history the build experiences and not a result of segregation or oxidation. The gradient alpha and layer-band morphologies form in layer n after the deposition of layer n+3.

Microstructural evolution in laser-deposited multilayer Ti-6Al-4V builds: Part II. Thermal modeling

Abstract: The thermal history developed in laser metal deposition (LMD) processes has been shown to be quite complex and results in the evolution of an equally complex microstructure. A companion article (Part I. Microstructural Characterization) discussed the LMD of Ti-6Al-4V, where the resultant microstructure consists of a periodic, scale-graded layer of basketweave Widmanstatten alpha and a banding that consists of colony Widmanstatten alpha. In order to understand the microstructural evolution in Ti-6Al-4V, a numerical thermal model based on the implicit finite-difference technique was developed to model LMD processes. The effect of different laser-scan velocities on the characteristics of the thermal history was investigated using an eight-layer single-line build. As the laser-scan speed decreases and the position within a layer increases, the peak temperature increases. The heating rate and the peak thermal gradient within a deposited layer were shown to follow the same trend as the peak temperature after two layers were deposited on top of the substrate. In general, the laser-scan speed or z-position within a layer did not have a significant effect on the cooling rate. The cooling rate in a newly deposited layer decreases as the number of layer additions increases. Given the predicted temperature vs time profile from the thermal model, the evolution of phase transformations occurring in the deposit is mapped as each layer is deposited. As a result of the thermal cycling imposed by the periodic deposition of material, a characteristic layer, consisting of two regions heated above and below the beta transus, forms in layer n due to the deposition of layer n+1.

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

Abstract: The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.

Austenite formation during intercritical annealing

Abstract: A systematic experimental study has been conducted on ferrite recrystallization and intercritical austenite formation for two low-carbon steels with chemical compositions typically used for dual-phase and transformation-induced plasticity (TRIP) steels. Different initial heating rates, holding temperatures, and times were applied to the materials to examine the ferrite recrystallization and austenite formation kinetics. An Avrami model was developed to describe the isothermal ferrite recrystallization behavior and was applied successfully to the nonisothermal conditions. It was found that the initial heating rate affects the isothermal austenite formation kinetics for both the hot-rolled and cold-rolled materials albeit the effect is more pronounced for the cold-rolled material. This can be attributed to the interaction between the ferrite recrystallization and austenite formation processes. Furthermore, it was found that the distribution of austenite phase is also affected by the ferrite recrystallization process. When ferrite recrystallization is completed before the austenite formation (i.e., under sufficiently slow heating rate conditions), austenite is to a large extent randomly distributed in the ferrite matrix. On the other hand, incomplete recrystallization of ferrite due to higher heating rates leads to the formation of banded austenite grains. It is proposed that this observation is characteristic of simultaneous recrystallization and austenite formation where moving ferrite grain boundaries do not provide suitable sites for austenite nucleation.

Effect of carbon concentration on precipitation behavior of M23C6 carbides and MX carbonitrides in martensitic 9Cr steel during heat treatment

Abstract: The distributions and precipitated amounts of M23C6 carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M23C6 carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 pct C steel, there are no M23C6 carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount of M23C6 carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.

Measuring the five-parameter grain-boundary distribution from observations of planar sections

Abstract: A stereological method is described for estimating the distribution of grain-boundary types in polycrystalline materials on the basis of observations from a single planar section. The grain-boundary distribution is expressed in terms of five macroscopically observable parameters that include: three parameters that describe the lattice misorientation across the boundary and two parameters that describe the orientation of the grain-boundary plane normal. The grain-boundary distribution is derived from measurements of grain orientations and the orientations of the lines formed where grain boundaries intersect the plane of observation. Tests of the method on simulated observations illustrate that the distribution of boundaries in a material with cubic symmetry can be reliably determined with about 10° of resolution from the analysis of 5×104 or more line segments. Furthermore, grain-boundary distributions directly observed from serial sections of a SrTiO3 polycrystal are compared to those resulting from the stereological analysis of a single plane. The comparison shows that the stereological method provides a reasonable estimate of the measured distribution. The differences between the directly observed grain-boundary distribution and that derived from the stereological analysis are consistent with the results from the simulation.

Grain Refinement Induced by Electromagnetic Stirring : A Dendrite Fragmentation Criterion

Abstract: The influence of electromagnetic stirring (EMS) on grain refinement has been studied for two copper-base alloys (Cu-1 wt pct Ni-1 wt pct Pb-0.2 wt pct P and Cu-4 wt pct Zn-4 wt pct Sn-4 wt pct Pb) solidified in a Bridgman furnace. Metallographic inspection of the specimens, temperature measurements during solidification, and numerical simulations performed with CALCOSOFT revealed that the efficiency of EMS is strongly dependent upon the penetration of the liquid in the mushy zone and therefore upon the position of the convection vortices with respect to the liquidus front. In particular, the low-concentration alloy could be grain refined only at high power and when the coil was moved close to the liquidus front. These results were analyzed on the basis of a dendrite fragmentation criterion similar to Flemings’ criterion for local remelting of the mushy zone. Considering that the component of the fluid flow velocity along the thermal gradient, \(u_{l,G} = \frac{{u_e \cdot abla T}}{{\left\| { abla T} \right\|}}\), must be larger than the casting speed, Vc, dendrite fragmentation occurs if $$C_R \approx \frac{1}{{V_c }}\frac{K}{{g_l \cdot \mu }}\frac{{B_0^2 }}{{\mu _0 d_{ind} }} > 1$$ at some depth within the mushy zone where dendrite arms are sufficiently developed, typically 8 λ2, where λ2 is the final secondary dendrite arm spacing, K is the permeability of the mushy zone, gl is the volume fraction of liquid, μ is the dynamic viscosity, B0 is the magnetic field, μ0 is the permeability of vacuum, and dind is the distance between the inductor and the liquidus front.

A Quantitative Dendrite Growth Model and Analysis of Stability Concepts

Abstract: While a number of cellular automaton (CA) based models for dendrite growth have been proposed, none so far have been validated, casting doubt on their quantitative capabilities. All these models are mesh dependent and cannot correctly describe the influence of crystallographic orientation on growth morphology. In this work, we present an improved version of our previously developed CA based model for dendrite growth controlled by solutal effects in the low Peclet number regime. The model solves the solute and heat conservation equations subject to the boundary conditions at the interface, which is tracked with a new virtual front tracking method. It contains an expression equivalent to the stability constant required in analytical models, termed stability parameter, which is not a constant. The process determines its value, changing with time and angular position during dendrite formation. The article proposes solutions for the evaluation of local curvature, solid fraction, trapping rules, and anisotropy of the mesh, which eliminates the mesh dependency of calculations. Several tests were performed to demonstrate the mesh independence of the calculations using Fe-0.6 wt pct C and Al-4 wt pct Cu alloys. Computation results were validated in three ways. First, the simulated secondary dendrite arm spacing (SDAS) was compared with literature values for an Al-4.5 wt pct Cu alloy. Second, the predictions of the classic Lipton-Glicksman-Kurz (LGK) analytical model for steady-state tip variables, such as velocity, radius, and composition, were compared with simulated values as a function of melt undercooling for Al-4 wt pct Cu alloy. In this validation, it was found that the stability parameter approaches the experimentally and theoretically determined value of 0.02 of the stability constant. Finally, simulated results for succinonitrile-0.29 wt pct acetone (SCN-0.29 wt pct Ac) alloy are compared with experimental data. Model calculations were found to be in very good agreement with both the analytical model and the experimental data. The model is used to simulate equiaxed and columnar growth of Fe-0.6 wt pct C and Al-4 wt pct Cu alloys offering insight into microstructure formation under these conditions.

Mechanism of primary MC carbide decomposition in Ni-base superalloys

Abstract: Three industrial gas turbine blades made of conventionally cast (CC) IN-738 and GTD-111 and directionally solidified GTD-111 Ni-base superalloys were examined after long-term exposures in service environments. All three blades exhibit similar, service-induced microstructural changes (MCs) including γ′ coarsening and coalescence, excessive secondary M23C6 precipitation, and primary MC degeneration, regardless of the chemical composition and the grain size. Special attention was paid to the primary MC decomposition. It is shown that the primary MC decomposition occurs by carbon diffusion out of the carbide into the γ + γ′ matrix, resulting in the formation of Cr-rich M23C6 carbides near the initial carbide/matrix interface. A transition zone is shown to develop between the original MC core and its perimeter, demonstrating the gradual outward diffusion of carbon and a slight inward increase in nickel concentration. The hexagonal Ni3(TiTa) η-phase was also found in the MC transition zone and on the MC-γ/γ′ interface. The primary MC decomposition can be expressed by the reaction MC + γ/γ′ → M23C6 + η. Finally, it is shown that the grain-boundary (GB) MC decomposes more rapidly than that in the grain interiors. This is consistent with the more rapid GB diffusion that leads to the acceleration of the MC diffusional decomposition processes.

Directional solidification of large superalloy castings with radiation and liquid-metal cooling: A comparative assessment

Abstract: A columnar-grain variant of single-crystal RENE N4 has been directionally solidified (DS) over a range of conditions in order to assess the possible benefits of the use of liquid metal-enhanced cooling for large cross-sectional castings. Castings were solidified at a rate of 2.5 mm/min using conventional radiation cooling and at rates between 2.5 and 8.5 mm/min using liquid-metal cooling (LMC) with tin as a cooling medium. Thermocouples inserted in the casting directly measured thermal gradients during solidification. The LMC process exhibited higher gradients at all withdrawal rates. The higher thermal gradients resulted in a refined structure measurable by the finer dendrite-arm spacing. Additionally, the conventionally cast material exhibited several freckle-type defects, while none were observed in the liquid-metal-cooled castings.

Effect of microstructural length scales on spall behavior of copper

Abstract: A systematic study to quantify the effects of specific microstructural features on the spall behavior of 99.999 pct copper has revealed a strong dependence of the failure processes on length scale. Shock loading experiments with Cu flyer plates at velocities ranging from 300 to 2000 m/s (or impact pressures from 5 to 45 GPa) using a 35-mm single/two-stage light gas gun revealed that single crystals exhibit a higher spallation resistance than fine-grained polycrystals and internally oxidized single crystals. However, in contrast to previously reported results, the fine-grained (∼8-µm) polycrystalline samples exhibit lower damage resistance than the coarse-grained (50- and 133-µm) samples. These observations have been analyzed in the context of the length scale inherent in each of these microstructures, and modeled using an analytical model developed recently.

Grain-size dependence of the flow stress of Cu from millimeters to nanometers

Abstract: Data in the literature on the effect of grain size (d) from millimeters to nanometers on the flow stress of Cu are evaluated. Three grain-size regimes are identified: regime I, d>∼10−6 m; regime II, d ≈10−8 to 10−6 m; and regime III, d<∼10−8 m. Grain-size hardening occurs in regimes I and II; grain-size softening occurs in regime III. The deformation structure in regime I consists of dislocation cells; in regime II, the dislocations are mostly restricted to their slip planes; in regime III, computer simulations indicate that dislocations are absent and that deformation occurs by the shearing of grain-boundary atoms. The transition from regime I to II occurs when the dislocation cell size becomes larger than the grain size, and the transition from regime II to III occurs when the dislocation spacing due to elastic interactions becomes larger than the grain size. The rate-controlling mechanism in regime I is concluded to be the intersection of dislocations; in regime II, it is proposed to be grain-boundary shear promoted by the pileup of dislocations; in regime III, it appears to be grain-boundary shear by the applied stress alone.

Hydrogen trap states in ultrahigh-strength AERMET 100 steel

Abstract: Hydrogen (H) trap states and binding energies were determined for AERMET 100 (Fe-13.4Co-11Ni-3Cr-1.2Mo-0.2C), an ultrahigh-strength steel using thermal desorption methods. Three major H desorption peaks were identified in the precipitation-hardened microstructure, associated with three distinct metallurgical trap states, and apparent activation energies for desorption were determined for each. The lattice diffusivity (D L ) associated with interstitial H was measured experimentally and verified through trapping theory to yield H-trap binding energies (E b ). Solid-solution elements in AERMET 100 reduce D L by decreasing the pre-exponential diffusion coefficient, while the activation energy for migration is similar to that of pure iron. M2C precipitates are the major reversible trap states, with E b of 11.4 to 11.6 kJ/mol and confirmed by heat treatment that eliminated these precipitates and the associated H-desorption peak. A strong trap state with E b of 61.3 to 62.2 kJ/mol is likely associated with martensite interfaces, austenite grain boundaries, and mixed dislocation cores. Undissolved metal carbides and highly misoriented grain boundaries trap H with a binding energy of 89.1 to 89.9 kJ/mol. Severe transgranular hydrogen embrittlement in peak-aged AERMET 100 at a low threshold-stress intensity is due to H repartitioning from a high density of homogeneously distributed and reversible M2C traps to the crack tip under the influence of high hydrostatic tensile stress.

Statistically representative three-dimensional microstructures based on orthogonal observation sections

Abstract: Techniques are described that have been used to create a statistically representative three-dimensional model microstructure for input into computer simulations using the geometric and crystallographic observations from two orthogonal sections through an aluminum polycrystal. Orientation maps collected on the observation planes are used to characterize the sizes, shapes, and orientations of grains. Using a voxel-based tessellation technique, a microstructure is generated with grains whose size and shape are constructed to conform to those measured experimentally. Orientations are then overlaid on the grain structure such that distribution of grain orientations and the nearest-neighbor relationships, specified by the distribution of relative misorientations across grain boundaries, match the experimentally measured distributions. The techniques are applicable to polycrystalline materials with sufficiently compact grain shapes and can also be used to controllably generate a wide variety of hypothetical microstructures for initial states in computer simulations.

The solidification characteristics of Fe-rich intermetallics in Al-11.5Si-0.4Mg cast alloys

Abstract: After the nucleation and sedimentation of primary Fe-rich phases, the microstructures of Al-11.5Si-0.4Mg cast alloys with 0.35–1.03Fe and 0.18–0.59Mn have been studied to investigate the solidification characteristics of Fe-rich phases. Depending on the iron and manganese contents as well as cooling rates, Fe-rich phases may solidify as predendritic (primary), pre-eutectic, coeutectic, and posteutectic intermetallics at the different stages of solidification through three types of reactions: (1) predendritic (primary), (2) eutectic, and (3) peritectic reactions. It seems that Fe-rich phases may nucleate on the wetted sides of double oxide films, while the gap of the dry sides of oxide films constitutes the cracks commonly observed in the Fe-rich phases and aluminum matrix. Conventional metallurgical observations also suggest that the Fe-rich phases nucleated early during the solidification might act as nuclei for those formed subsequently, although it has not been ruled out that these phases may share the same oxide substrates. It is probable that these nucleation events may all work as suggested in the possible nucleation hierarchy for Al-11.5Si-0.4Mg cast alloys.

A study of coarsening, recrystallization, and morphology of microstructure in Al-Sc-(Zr)-(Mg) alloys

Abstract: Minor additions of Sc are effective in controlling the recrystallization resistance of 5xxx, 2xxx, and 7xxx aluminum. The addition of Sc to aluminum results in the rapid precipitation of homogeneously distributed Al3Sc dispersoids, which are coherent with the matrix and have the L12 structure. The presence of Al3Sc dispersoids increases the recrystallization resistance of wrought alloys. The higher coarsening rate of Al3Sc compared to that of Al3Zr may limit its applications as a single ancillary addition. When both scandium and zirconium are used in the same alloy, Al3(Sc1-x , Zr x ) dispersoids form. These dispersoids are more effective recrystallization inhibitors than either Al3Sc or Al3Zr. The Al3(Sc1-x , Zr x ) dispersoids precipitate more rapidly than Al3Zr but have a slower coarsening rate than Al3Sc. Furthermore, the distribution of Al3(Sc1-x , Zr x ) is significantly more homogeneous than Al3Zr. It was also established that alloys containing up to 3.5Mg showed improvement in recrystallization resistance when both Sc and Zr were present. Several morphologies of Al3Sc and Al3(Sc1-x , Zr x ) were also observed.

Microstructural modification of as-cast NiAl bronze by friction stir processing

Abstract: The application of friction stir processing (FSP) to a cast NiAl bronze (NAB) material is presented as a means for selective modification of the near-surface layers by converting as-cast microstructures to a wrought condition in the absence of macroscopic shape change. This may enable selective surface hardening of cast components. The complex physical metallurgy of the NAB is reviewed, and microstructure changes associated with FSP for a selected set of processing parameters are examined by optical microscopy (OM) and transmission electron microscopy (TEM) methods. Direct temperature measurement in the stir zone is infeasible and, so, these microstructure changes are used to estimate peak temperatures in the stir zone. The persistence of a Fe3Al phase (κii) indicates that peak temperatures are below the solvus for this phase, while the presence of transformation products of the β phase, including fine Widmanstatten α, bainite, and martensite, indicates that peak temperatures exceed the eutectoid temperature for the reaction β → α+κiii throughout the stir zone.

Fatigue of cold-work tool steels: Effect of heat treatment and carbide morphology on fatigue crack formation, life, and fracture surface observations

Abstract: The fatigue properties of two types of cold-work tool steels tempered at various temperatures were evaluated. The microstructure and fracture surface morphology were correlated to the fatigue behavior. Cold-work tool steels using this study were a conventional tool steel (JIS SKD11; 1.4C-11Cr-0.8Mo-0.2V) and its modified steel (M-SKD11; 0.8C-8Cr-2Mo-0.5V). The fatigue strength of the M-SKD11 steel increased 20 pct over that of the SKD11 steel for any number of cycles. This is attributed to the refinement of primary M7C3 carbides. These M7C3 carbides fractured during fatigue and were found at the sites of fatigue crack initiation. Change in crack initiation behavior was confirmed by acoustic emission testing. The S-N curves of the steels are similar to those of most structural steels. However, the subsurface fatigue crack initiation was dominant at lower alternating stresses. This study points to a general approach of carbide refinement that can be used for the enhancement of fatigue properties.

Coarsening behavior of an alpha-beta titanium alloy

Abstract: The static-coarsening behavior of the alpha-beta titanium alloy, Ti-6Al-4V, was established via a series of heat treatments at typical forging-preheat and final-heat-treatment temperatures followed by quantitative metallography. For this purpose, samples of an ultra-fine-grain (UFG) size billet with a microstructure of equiaxed alpha in a beta matrix were heated at temperatures of 843 °C, 900 °C, 955 °C, and 982 °C for times between 0.25 and 144 hours followed by water quenching. The coarsening of the primary alpha particles was found to follow r 3-vs-time kinetics, typical of volume-diffusion-controlled behavior, at the three lower temperatures. At the highest temperature, the kinetics appeared to be fit equally well by an r 3 or r 4 dependence on time. The observations were interpreted in terms of the modified LSW theory considering the effect of volume fraction on kinetics and the fact that the phases are not terminal solid solutions. Prior models, which take into account the overall source/sink effects of all particles on each other, provided the best description of the observed dependence of coarsening on the volume fraction of primary alpha. In addition, the volume-diffusion kinetics derived for the UFG material were found to be capable of describing the coarsening behavior observed for industrial-scale billet of Ti-6Al-4V with a coarser starting equiaxed-alpha microstructure.

Effects of temperature and strain rate on tensile properties and activation energy for dynamic strain aging in alloy 625

Abstract: Alloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873 K. These were then subjected to a solution-annealing treatment at 1473 K for 0.5 hours. The effects of temperature and strain rate on the tensile properties of the solution-annealed alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3×10−5 s−1 to 3×10−3 s−1. At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow, peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T<823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T≥823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3×10−5 s−1 and 3×10−3 s−1 and beyond 923 K for 3×10−4 s−1 strain rates seen in this alloy has been ascribed to reduction in ductility due to precipitation of carbides and δ phase on the grain boundaries.

Tensile behavior of friction-stri-welded Al 6061-T651

Abstract: In the present study, tensile behavior of friction-stir-welded Al 6061-T651 with varying welding parameters, including rotating and welding speeds, was examined. The 4-mm-thick Al 6061-T651 alloy plates were FSW with varying tool rotating speeds, 1000, 1400, 1600, 2000, and 2500 rpm, and welding speeds, 0.1, 0.2, 0.3, to 0.4 mpm (m/min). Tensile specimens were prepared with the tensile direction perpendicular to the welding direction, so that the weld zone is located in the middle of the specimen. It was found that the tensile elongation of friction-stir-welded Al 6061-T651 decreased with decreasing welding speed or increasing rotating speed. The yield and ultimate tensile strength were also affected, but to a significantly lesser degree, with varying welding parameters. The micrographic and fractographic observations strongly suggested that the change in tensile behavior of friction-stri-welded Al 6061-T651 was largely related to the clustering of coarse Mg2Si precipitates, due to the whirling and hurling action by severe plastic flow in the weld zone. Low welding speed or high rotating speed tended to encourage the plastic flow per unit time and consequently the clustering of coarse precipitates.

Dislocation mechanics-based constitutive equations

Abstract: A review of constitutive models based on the mechanics of dislocation motion is presented, with focus on the models of Zerilli and Armstrong and the critical influence of Armstrong on their development. The models were intended to be as simple as possible while still reproducing the behavior of real metals. The key feature of these models is their basis in the thermal activation theory propounded by Eyring in the 1930’s. The motion of dislocations is governed by thermal activation over potential barriers produced by obstacles, which may be the crystal lattice itself or other dislocations or defects. Typically, in bcc metals, the dislocation-lattice interaction is predominant, while in fcc metals, the dislocation-dislocation interaction is the most significant. When the dislocation-lattice interaction is predominant, the yield stress is temperature and strain rate sensitive, with essentially athermal strain hardening. When the dislocation-dislocation interaction is predominant, the yield stress is athermal, with a large temperature and rate sensitive strain hardening. In both cases, a significant part of the athermal stress is accounted for by grain size effects, and, in some materials, by the effects of deformation twinning. In addition, some simple strain hardening models are described, starting from a differential equation describing creation and annihilation of mobile dislocations. Finally, an application of thermal activation theory to polymeric materials is described.

The role of microstructure in localized corrosion of magnesium alloys

Abstract: The article presents new findings on the influence of microstructural changes on corrosion behavior of magnesium alloy AZ91 in chloride solution, with a particular attention to the role of the β phase (Mg17Al12) and the surrounding Al-rich-α area. The as-cast alloy was subjected to solutionizing and aging heat treatments, in order to incorporate variation in morphology and distribution of the intermetallic β phase and the surrounding Al-rich-α (also known as eutectic α). Although previous workers have ascribed the higher corrosion resistance of a fine-grained alloy to the formation of the finely distributed β phase, the present work suggests that it is the ratio of the β phase to the Al-rich-α that governs the localized corrosion of the aged alloy. Corrosion characteristics were investigated by immersion and electrochemical tests. Surface microtopography, optical microscopy, and scanning electron microscopy (SEM) were employed to characterize the localized corrosion.

Microstructural evolution in the heat-affected zone of a friction stir weld

Abstract: The microstructure and crystallographic texture spanning the soft region at the thermomechanically affected zone/heat-affected zone (TMAZ/HAZ) boundary of a friction stir weld in 2519 Al were systematically investigated to determine their contributions to the properties of that region. The microstructure was shown to be the primary cause of softening at the TMAZ/HAZ boundary. During welding, fine ϑ′ precipitates responsible for much of the strength in this alloy coarsen and transform to the equilibrium ϑ phase in the HAZ and into the TMAZ, accounting for the observed softening through the HAZ region. The higher temperatures achieved in the TMAZ partially resolutionize the precipitates and allow the subsequent formation of Guinier-Preston (GP) zones during cooling. These two processes are responsible for the variation in microhardness observed in the TMAZ/HAZ region. Texture analyses revealed significant differences in the crystallographic texture across this region that were primarily due to macroscopic rigid-body rotations of the grains, but do not account for the observed softening. The effect of the observed microstructural evolutions on the friction stir welding (FSW) deformation field and on the fracture behavior of the weld are also discussed.

Multiscale structure and properties of cast and deformation processed polycrystalline NiTi shape-memory alloys

Abstract: The objective of this study is to examine fundamental processing-structure-property relationships in polycrystalline NiTi bars. Three different polycrystalline Ti-50.9 at. pct Ni (Ti-55.7 wt pct Ni) materials were examined: (1) cast, (2) cast then hot rolled, and (3) cast, hot rolled, then cold drawn. The structure of the materials was investigated at various scales ranging from nanometers to micrometers. The cast materials contained random crystallographic textures along the loading axis of the extracted samples. The hot-rolled and cold-drawn materials contained a strong 〈111〉 texture parallel to the deformation-processing direction. The high-temperature hot-rolling process facilitated recrystallization and recovery, and curtailed precipitate formation, leaving the hot-rolled and cold-drawn materials in near solutionized states. The cold-drawn material contained a high density of dislocations and martensite. After a mild aging treatment, all three materials contained distributed coherent Ti3Ni4 precipitates on the order of 10 nm in size. The cast material was capable of full shape-memory transformation strain recovery up to approximately 5 pct strain at room temperature under both tension and compression. The hot-rolled and cold-drawn materials demonstrated significant tension-compression stress-strain asymmetry owing to their strong crystallographic texture. Under compression, the deformation-processed materials were only capable of 3 pct transformation strain recovery while under tension they were capable of nearly 7 pct transformation strain recovery. Based on the present results, the presence of small coherent Ti3Ni4 precipitates is determined to be the driving force for the favorable strain transformation strain recovery properties in all three materials, despite drastically different grain sizes and crystallographic textures. The unique dependence of elastic modulus on stress-state, temperature, and structure is also presented and discussed for the deformation-processed materials. In addition, we demonstrate that the appearance of a Luders band transformation under tensile loading can be controlled by material structure. Specifically, the presence of significant martensite and dislocations in the cold-drawn materials was shown to mitigate the Luders band propagation and result in a more gradual transformation.

Microstructure evolution and mechanical behavior of bulk copper obtained by consolidation of micro- and nanopowders using equal-channel angular extrusion

Abstract: The consolidation of copper micro- and nanoparticles (325 mesh, 130 nm, and 100 nm) was performed using room-temperature equal-channel angular extrusion (ECAE). The effects of extrusion route, number of passes, and extrusion rate on consolidation performance were evaluated. The evolution of the microstructure and the mechanical behavior of the consolidates were investigated and related to the processing route. Possible deformation mechanisms are proposed and compared to those in ECAE-processed bulk Cu. A combined high ultimate tensile stress (470 MPa) and ductility (∼20 pct tensile fracture strain) with near-elasto-plastic behavior was observed in consolidated 325-mesh Cu powder. On the other hand, early plastic instability took place, leading to a continuous softening in flow stress of bulk ECAE-processed copper. Increases in both strength and ductility were evident with an increasing number of passes in the bulk samples, which appears to be inconsistent with grain-boundary-moderated deformation mechanisms for a microstructure with an average grain size of 300 to 500 nm. Instead, this increase is attributed to microstructural refinement and to dynamic recovery and bimodal grain-size distribution. Near-perfect elastoplasticity in consolidated 325-mesh Cu powder is explained by a combined effect of strain hardening accommodated by large grains in the bimodal structure and softening caused by recovery mechanisms. Compressive strengths as high as 760 MPa were achieved in consolidated 130-nm copper powder. Although premature failure occurred during tensile loading in 130-nm consolidated powder, the fracture strength was still about 730 MPa. The present study shows that ECAE consolidation of nanoparticles opens a new possibility for the study of mechanical behavior of bulk nanocrystalline (NC) materials, as well as offering a new class of bulk materials for practical engineering applications.

Formability and strength of friction-stir-welded aluminum sheets

Abstract: Friction stir welding was investigated as a viable process for joining thin aluminum sheets in order to manufacture tailored blanks. In the present study three alloys were tested: 5182-O, 5754-O, and 6022-T4. All three of these alloys are being used to fabricate stamped automotive parts. The gas tungsten arc welding process has been used to make aluminum-tailored welded blanks industrially, so results using this process were compared to FSW results. Blanks of the same gage of all three alloys were welded and then evaluated using tensile and formability testing. The 5xxx series alloys had similar tensile ductility and formability regardless of the welding process. However, the 6022-T4 sheets joined using friction stir welding had better formability than those joined using gas tungsten arc welding because friction stir welding caused less softening in the heat-affected zone.