Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 2006"

Numerical simulation of three-dimensional heat transfer and plastic flow during friction stir welding

Abstract: Three-dimensional visco-plastic flow of metals and the temperature fields in friction stir welding have been modeled based on the previous work on thermomechanical processing of metals. The equations of conservation of mass, momentum, and energy were solved in three dimensions using spatially variable thermophysical properties and non-Newtonian viscosity. The framework for the numerical solution of fluid flow and heat transfer was adapted from decades of previous work in fusion welding. Non-Newtonian viscosity for the metal flow was calculated considering strain rate, temperature, and temperature-dependent material properties. The computed profiles of strain rate and viscosity were examined in light of the existing literature on thermomechanical processing. The heat and mass flow during welding was found to be strongly three-dimensional. Significant asymmetry of heat and mass flow, which increased with welding speed and rotational speed, was observed. Convective transport of heat was an important mechanism of heat transfer near the tool surface. The numerically simulated temperature fields, cooling rates, and the geometry of the thermomechanically affected zone agreed well with independently determined experimental values.

Strengthening mechanisms in solid solution aluminum alloys

Abstract: A number of commercial and high-purity non-heat-treatable aluminum alloys are investigated in this work. It is found that both magnesium and manganese in solid solution give a nearly linear concentration dependence of the strength at a given strain for commercial alloys. This deviates from high-purity AlMg binary alloys, where a parabolic concentration dependence is found. Mn in solid solution is found to give a considerably higher strengthening effect per atom than Mg, both in terms of yield stress and initial work hardening rate. This strengthening effect is stronger comparing commercial grades to high-purity alloys. This enhanced strengthening is believed to be a synergy or clustering effect caused by interaction between Mn atoms and trace elements, probably silicon, in solid solution.

An overall aspect of electroless Ni-P depositions—A review article

Abstract: Literature on electroless Ni-P deposition, in recent decades, has dwelled primarily on surface engineering and corrosion-resistant applications. By contrast, we have many research articles devoted to the engineering aspects of the electroless Ni-P depositions and their technology. The present article deals with the development of electroless Ni-P bath, advantages and mechanisms of deposition, and applications of the Ni-P deposits. We also present a comparison of the properties of electroless Ni-P and Ni-B as well as the recent developments in nickel-phosphorous research. We attempt to review these in a detailed manner. We also briefly discuss the future developments of electroless Ni-P.

Effect of vanadium addition on the microstructure, hardness, and wear resistance of Al0.5CoCrCuFeNi high-entropy alloy

Abstract: The authors studied the effect of vanadium addition on the microstructure and properties of Al0.5CoCrCuFeNi high-entropy alloy. The microstructure of Al0.5CoCrCuFeNiV x (x=0 to 2.0 in molar ratio) alloys was investigated by scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction. With little vanadium addition, the alloys are composed of a simple fcc solid-solution structure. As the vanadium content reaches 0.4, a BCC structure appears with spinodal decomposition and envelops the FCC dendrites. From x=0.4 to 1.0, the volume fraction of bcc structure phase increases with the vanadium content increase. When x=1.0, fcc dendrites become completely replaced by bcc dendrites. Needle-like σ-phase forms in bcc spinodal structure and increases from x=0.6 to 1.0 but disappears from x=1.2 to 2.0. The hardness and wear resistance of the alloys were measured and explained with the evolution of the microstructure. The hardness values of the alloys increase when the vanadium content increases from 0.4 to 1.0 and peak (640 HV) at a vanadium content of 1.0. The wear resistance increases by around 20 pct as the content of vanadium increases from x=0.6 to 1.2 and levels off beyond x=1.2. The optimal vanadium addition is between x=1.0 and 1.2. Compared with the previous investigation of Al0.5CoCrCuFeNi alloy, the vanadium addition to the alloy promotes the alloy properties.

Quantitative analysis on hydrogen trapping of TiC particles in steel

Abstract: The change in the hydrogen-trapping behavior of a TiC particle accompanying its coherent to incoherent interfacial-character transition in a 0.05C-0.20Ti-2.0Ni steel that was quenched and tempered in a partially protective argon atmosphere and in ultrahigh vacuum (UHV) has been studied by thermal desorption spectrometry (TDS). The results indicated that (semi)coherent TiC precipitates demonstrate distinctly different hydrogen-trapping features from that of incoherent TiC particles with respect to hydrogen capacity, interaction energy with hydrogen, locations available for hydrogen occupation, and the capability of hydrogen absorption from the environment. The broad (semi)coherent interface of the disc-shaped (semi)coherent TiC precipitate does not trap hydrogen during tempering in a partially protected argon atmosphere, but traps hydrogen during cathodic charging at room temperature. The semicoherent interface traps 1.3 atoms/nm2 of hydrogen at the core of the misfit dislocation with short-time charging (1 hour), which is characterized by a desorption activation energy of 55.8 kJ/mol. The side interface of the (semi)coherent TiC precipitate acts like the broad interface when the precipitate is small. As the precipitate grows, the side interface gradually loses its coherency and results in a simultaneous increase in the trapping activation energy and the binding energy. An increase in the trapping activation energy, i.e., the energy barrier for trapping, makes hydrogen trapping more difficult in cathodic charging at room temperature, while an increase in the binding energy enhances the capability of hydrogen absorption from the atmosphere during heat treatment. An incoherent TiC particle is not able to trap hydrogen during cathodic charging at room temperature due to its high energy barrier for trapping, but absorbs hydrogen during heat treatment at high temperatures. The amount of hydrogen that is trapped by incoherent TiC particles depends on their volume, which strongly indicates that incoherent TiC particles trap hydrogen within them rather than at the particle/matrix interface. Octahedral carbon vacancies are supposedly the hydrogen trap sites in incoherent TiC particles.

Strain hardening due to deformation twinning in α-titanium: Mechanisms

Abstract: Novel experiments were conducted to elucidate the effect of deformation twinning on the mechanical response of high-purity α-titanium deformed at room temperature. Orientation-imaging microscopy (OIM), microhardness, and nanohardness evaluations were employed in conjunction with optical microscopy and quasi-static compression testing to obtain insight into the deformation mechanisms. Hardness measurements revealed that the newly formed deformation twins were harder than the matrix. This observation is perhaps the first experimental evidence for the Basinski mechanism for hardening associated with twinning, arising from the transition of glissile dislocations to a sessile configuration upon the lattice reorientation by twinning shear. This work also provided direct evidence for two competing effects of deformation twinning on the overall stress-strain response: (1) hardening via both a reduction of the effective slip length (Hall-Petch effect) and an increase in the hardness of twinned regions (Basinski mechanism) and (2) softening due to the lattice reorientation of the twinned regions.

Effect of strain rate on stress-strain behavior of alloy 309 and 304L austenitic stainless steel

Abstract: The effect of strain rate on stress-strain behavior of austenitic stainless steel 309 and 304L was investigated. Tensile tests were conducted at room temperature at strain rates ranging from 1.25×10−4s−1 to 400 s−1. The evolution of volume fraction martensite that formed during plastic deformation was measured with X-ray diffraction and characterized with light microscopy. Alloy 304L was found to transform readily with strain, with martensite nucleating on slip bands and at slip band intersections. Alloy 309 did not exhibit strain-induced transformation. Variations in ductility and strength with strain rate are explained in terms of the competition between hardening, from the martensitic transformation and a positive strain rate sensitivity, and softening due to deformational heating. Existing models used to predict the increase in volume fraction martensite with strain were examined and modified to fit the experimental data of this study as well as recent data for alloys 304 and 301LN obtained from the literature.

Deformation-induced phase transformation and strain hardening in type 304 austenitic stainless steel

Abstract: Deformation-induced phase transformation in a type 304 austenitic stainless steel has been studied in tension at room temperature and −50 °C. The evolution of transformation products was monitored using X-ray diffraction (XRD) line profile analysis of diffraction peaks from a single XRD scan employing the direct comparison method. Crystallographic texture transitions due to deformation strain have been evaluated using (111) γ pole figures. The tensile stress-strain data have been analyzed to explain the influence of underlying deformation-induced microstructural changes and associated texture changes in the steel. It is found that the initial stage of rapidly decreasing strain hardening rate in type 304 steel is primarily influenced by hcp ɛ-martensite formation, and the second stage of increasing strain hardening rate is associated with an increase in the α′-martensite formation. The formation of ɛ-martensite is associated with a gradual strengthening of the copper-type texture components up to 15 pct strain and decreasing with further strain at −50 °C. Texture changes during low-temperature deformation not only change the mechanism of ɛ-martensite formation but also influence the strain rate sensitivity of the present steel.

Microstructural modification of as-cast Al-Si-Mg alloy by friction stir processing

Abstract: Friction stir processing (FSP) has been applied to cast aluminum alloy A356 plates to enhance the mechanical properties through microstructural refinement and homogenization. The effect of tool geometry and FSP parameters on resultant microstructure and mechanical properties was investigated. The FSP broke up and dispersed the coarse acicular Si particles creating a uniform distribution of Si particles in the aluminum matrix with significant microstructural refinement. Further, FSP healed the casting porosity. These microstructural changes led to a significant improvement in both strength and ductility. Higher tool rotation rate was the most effective parameter to refine coarse Si particles, heal the casting porosity, and consequently increase strength. The effect of tool geometry was complicated and no systematic trend was observed. For a standard pin design, maximum strength was achieved at a tool rotation rate of 900 rpm and traverse speed of 203 mm/min. Post-FSP aging increased strength for materials processed at higher tool rotation rates of 700 to 1100 rpm, but exerted only a marginal effect on samples prepared at the lower rotation rate of 300 rpm. Two-pass FSP with 100 pct overlapping passes resulted in higher strength for both as-FSP and post-FSP aged conditions.

Controlled dissolution of colossal quantities of nitrogen in stainless steel

Abstract: The solubility of nitrogen in austenitic stainless steel was investigated thermogravimetrically by equilibrating thin foils of AISI 304 and AISI 316 in ammonia/hydrogen gas mixtures. Controlled dissolution of colossal amounts of nitrogen under metastable equilibrium conditions was realized, with nitrogen contents as high as corresponding to an occupancy of y N=0.61 of the interstitial sublattice, i.e., about 38 at. pct N. Associated with the dissolution of these unprecedented nitrogen contents in an austenitic matrix a reversible volume expansion of the austenite lattice occurred for y N > 0.17. A simplistic model based on a statistical distribution fo the nitride forming elements over the octahedrons constituting the solid state agrees favorably with the experimental data.

Brief Review of Oxidation Kinetics of Copper at 350 °C to 1050 °C

Abstract: Copper’s oxication mechanism and purity effects were elucidated by oxidizing 99.99 pct (4N), 99.9999 pct (6N), and floating zone refined (>99.9999 pct) specimens in 0.1 MPa oxygen at 350 °C to 1050 °C. Throughout the temperature range, the oxidation kinetics for all specimens obeys the parabolic oxidation rate law. The Cu2O scale grows predominantly, and the rate-determining step is concluded to be outward diffusion of copper atoms in Cu2O. The activation energy at high temperatures, where the lattice diffusion predominates, is 173 kJ/mol, but it becomes lower at intermediate temperatures and even lower at low temperatures because of the contribution of the grain boundary diffusion. At high temperatures, oxidation kinetics is almost uninfluenced by purity, but the lattice-diffusion temperature range is wider for higher-purity copper. At intermediate temperatures, copper oxidation is enhanced because trace impurities can impede growth of Cu2O grains to facilitate grain boundary diffusion. At low temperatures, grain boundary diffusion is possibly hindered by impurities segregated at grain boundaries.

Influence of alloying elements on the kinetics of strain-induced martensitic nucleation in low-alloy, multiphase high-strength steels

Abstract: Low-alloy multiphase transformation-induced-plasticity (TRIP) steels offer excellent mechanical properties in terms of elongation and strength. This results from the complex synergy between the different phases, i.e., ferrite, bainite, and retained austenite. The precise knowledge of the austenite-to-martensite transformation kinetics is required to understand the behavior of TRIP steels in a wide array of applications. The parameters determining the stability of the metastable austenite were reviewed and investigated experimentally, with special attention paid to the effect of the chemical composition, the temperature, and the size of the austenite particles. The results show that the stability and rate of transformation of the austenite particles in TRIP steels have a pronounced composition dependence: austenite particles transform at a faster rate in CMnSi TRIP steel than in TRIP steels in which Si is fully or partially replaced by Al and P. The results clearly support the view that (1) both a high C content and a submicron size are required for the room-temperature stability of the austenite particles and (2) the effect of the chemical composition on the transformation is due to its influence on the intrinsic stacking-fault energy. In addition, the composition dependence of the Md 30 temperature was derived by regression analysis of experimental data. The influence of the size of the retained austenite particles on their Ms σ temperature was studied by means of a thermodynamic model. Both the analysis of the transformation-kinetics data and the microstructural analysis by transmission electron microscopy revealed the very limited role of autocatalysis in the transformation.

Crystal fragmentation and columnar-to-equiaxed transitions in Al-Cu studied by synchrotron X-ray video microscopy

Abstract: Recent improvements in detectors combined with the eminent brightness and collimation offered with modern synchrotron sources open the way forin situ X-radiographic investigations of solidification fundamentals and phenomena in real alloys at resolutions approaching regular video microscopy. Here, the authors present observations on dendrite fragmentation from columnar fronts in Al-Cu and subsequent transport phenomena. From directional solidification experiments it has been found that the tendency for crystal fragments to detach by remelting of branch roots in the mush dendrite network strongly depends upon the relative buoyant and settling motions of crystal fragments and mush liquid, respectively. At the copper concentrations studied (20 to 30 wt pct), primary aluminum dendrites are lighter than the melt and solidification experiments parallel and anti-parallel to gravity show significant differences in detachment tendency. The experimental results compare well with three different models proposed for fragmentation at different mush locations; however, the results also demonstrate that these models differ substantially both with respect to detachment frequency and the ability for detached fragments to cause eventual columnar to equiaxed transitions. Under particular conditions it has been found that crystal fragmentation could lead to an alternating mesoscale segregation.

A three-phase model for mixed columnar-equiaxed solidification

Abstract: A three-phase model for mixed columnar-equiaxed solidification is presented in this article. The three phases are the parent melt as the primary phase, as well as the solidifying columnar dendrites and globular equiaxed grains as two different secondary phases. With an Eulerian approach, the three phases are considered as spatially coupled and interpenetrating continua. The conservation equations of mass, momentum, species, and enthalpy are solved for all three phases. An additional conservation equation for the number density of the equiaxed grains is defined and solved. Nucleation of the equiaxed grains, diffusion-controlled growth of both columnar and equiaxed phases, interphase exchanges, and interactions such as mass transfer during solidification, drag force, solute partitioning at the liquid/solid interface, and release of latent heat are taken into account. Binary steel ingots (Fe-0.34 wt pct C) with two-dimensional (2-D) axis symmetrical and three-dimensional (3-D) geometries as a benchmark were simulated. It is demonstrated that the model can be used to simulate the mixed columnar-equiaxed solidification, including melt convection and grain sedimentation, macrosegregation, columnar-to-equiaxed-transition (CET), and macrostructure distribution. The model was evaluated by comparing it to classical analytical models based on limited one-dimensional (1-D) cases. Satisfactory results were obtained. It is also shown that in order to apply this model for industrial castings, further improvements are still necessary concerning some details.

A semianalytical Sachs model for the flow stress of a magnesium alloy

Abstract: A semianalytical Sachs-type equation for the flow stress of magnesium-base alloys is developed using the Schmid law, power law hardening, and a sigmoidal increase in the twinning volume fraction with strain. Average Schmid factors were estimated from electron backscattered diffraction (EBSD) data. With these, the equation provides a reasonable description of the flow curves obtained in compression and tension for samples of Mg-3Al-1Zn cut in different orientations from rolled plate. The model illustrates the general importance of basal slip and twinning in magnesium alloys. The significance of prismatic slip in room temperature tension testing is also highlighted. This is supported with EBSD slip line trace analysis and rationalized in terms of a possible sensitivity of the critical resolved shear stress for prismatic (cross) slip to the stress on the basal plane.

Observations on the faceted initiation site in the dwell-fatigue tested Ti-6242 alloy : Crystallographic orientation and size effects

Abstract: A novel use of the electron backscattered diffraction (EBSD) characterization technique for study of fracture has been demonstrated. This new approach has been employed for characterization and analysis that contribute to the understanding of crack initiation in Ti-6242 under dwell-fatigue loading conditions. A faceted crack initiation site is typically observed on the dwell-fatigue fracture surface of Ti-6242. The level of microtexture has a major influence on the dwell-fatigue failures in near-α titanium alloys, such as Ti-6242. In this study, serial sectioning and EBSD techniques were used to obtain the orientation images of almost the entire specimen cross section at different depths below the fracture surface. The orientation images are color coded on three different bases: the angle between the loading axis and basal plane normal, the Schmid factor for prism slip, and the Schmid factor for basal slip. The aim was to determine the important aspects of the crystallographic orientation and the size of the microtextured region that is associated with the faceted initiation site. The results of this study are used to explain the possible locations of crack initiation in a test specimen under dwell-fatigue loading condition. These results are also used to better understand the role of size of microtextured regions in determining which crack will outgrow the other cracks (for the case of multiple cracking typically observed in the alloy of current study under the dwell-fatigue loading conditions) to become the dominant crack that leads to eventual specimen failure. This understanding has important practical implications because the dominant crack effectively determines the specimen life.

Anisotropy of Segregation at Grain Boundaries and Surfaces

Abstract: The purpose of this article is to review analytical models of the anisotropy of segregation to grain boundaries (GBs) and surfaces, and to evaluate their predictions. A summary of Gibbsian interfacial thermodynamics is provided as an introduction to the topic. This is followed by a historical overview of previous analytical models. A recently developed model of the dependence of GB segregation on the five macroscopic parameters of GB orientation is outlined, and illustration of how this formulation reduces to the particular cases of segregation to simpler types of interfaces is provided. In addition, some specific aspects of interfacial segregation, which have either been problematic or have lacked satisfactory explanation, are addressed. These include (a) the relationship between the compositions on the two sides of a given GB; (b) the difficulty of meaningful definitions of segregation-free energy (and related thermodynamic quantities such as enthalpy and entropy); (c) the so-called compensation temperature, at which the anisotropy of interfacial segregation seems to vanish; (d) the relationship between surface and GB segregation; and finally (e) an attempt to determine whether segregation increases or decreases interfacial energy anisotropy, and the consequences thereof on the equilibrium crystal shape of alloys. Where possible, comparisons are made with the results of experiments or computer simulations.

Dissimilar friction stir welds in AA5083-AA6082. Part I: Process parameter effects on thermal history and weld properties

Abstract: The aim of this study was to explore the so-called processing window, within which good-quality welds can be produced, for the friction stir welding of AA5083 to AA6082. To that end a systematic set of nine instrumented welds were made using rotation speeds of 280, 560, and 840 rpm and traverse speeds of 100, 200, and 300 mm/min with AA5083 on the advancing side and another nine with the materials reversed. For comparison a smaller series of AA5083-AA5083 and AA6082-AA6082 welds were also made. Thermocouple measurements, tool torque, extent of material mixing, and macrostructural observations all indicate that the temperature under the tool is more strongly dependent on the rotation than the traverse speed. It was found that in the current case, the power (energy/s) and heat input (energy/mm) do not correlate simply with the weld temperature. As a result, such metrics may not be suitable for characterizing the conditions under which welds are produced.

Crystal plasticity modeling of deformation and creep in polycrystalline Ti-6242

Abstract: This paper develops an experimentally validated computational model based on crystal plasticity for the analysis of two-phase α/β Ti-6242 polycrystalline alloys. A rate-dependent elastic-crystal plasticity model is incorporated in this model to accommodate anisotropy in material behavior and tension-compression asymmetry inherent to this alloy. A combination of microtesting, orientation imaging microscopy, computational simulations, and minimization process, involving genetic algorithms, is implemented in this study for careful characterization and calibration of the material parameters. Size effects are considered in this analysis through a simple scaling process. A homogenized equivalent model of the primary α with transformed β colonies is developed for incorporation in the Ti-6242 FE model. The polycrystalline Ti-6242 computational model incorporates accurate phase volume fractions, as well as statistically equivalent orientation distributions to those observed in the orientation imaging microscopy scans. The effects of orientation, misorientations, and microtexture distributions are investigated through simulations by this computational model. The model is used to simulate constant strain rate and creep tests in compression and tension, and the results are compared with experiments. The effects of microstructure and creep-induced load-shedding on the localization of microstructural stresses and strains are studied for potential crack initiation criteria.

Stir zone microstructure and strain rate during Al 7075-T6 friction stir spot welding

Abstract: The factors determining the temperature, heating rate, microstructure, and strain rate in Al 7075-T6 friction stir spot welds are investigated. Stir zone microstructure was examined using a combination of transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD) microscopy, while the strain rate during spot welding was calculated by incorporating measured temperatures and the average subgrain dimensions in the Zener-Hollomon relation. The highest temperature during friction stir spot welding (527 °C) was observed in spot welds made using a tool rotational speed of 3000 rpm. The stir zone regions comprised fine-grained, equiaxed, fully recrystallized microstructures. The calculated strain rate in Al 7075-T6 spot welds decreased from 650 to about 20 s−1 when the tool rotational speed increased from 1000 to 3000 rpm. It is suggested that the decrease in strain rate results when tool slippage occurs when the welding parameter settings facilitate transient local melting during the spot welding operation. Transient local melting and tool slippage are produced when the welding parameters produce sufficiently high heating rates and temperatures during spot welding. However, transient local melting and tool slippage is not produced in Al 7075-T6 spot welds made using a rotational speed of 1000 rpm since the peak temperature is always less than 475 °C.

Effect of processing and heat treatment on behavior of Cu-Cr-Zr alloys to railway contact wire

Abstract: A new series of Cu-Cr-Zr alloys to be used as railway contact wire, Cu-0.26 wt pct Cr-0.15 wt pct Zr, Cu-0.13 wt pct Cr-0.41 wt pct Zr, and Cu-0.34 wt pct Cr-0.41 wt pct Zr, were studied. The results indicated that processing and aging treatment had an effect on the microstructure, tensile strength, and electrical conductivity behavior of the Cu-Cr-Zr alloys. Process I (solution treatment + cold work + aging) was superior to process II (cold work + solution treatment + aging), because precipitation can occur heterogeneously at the dislocations and subcells. An appropriate processing and aging treatment may improve the properties of the alloys due to the formation of fine, dispersive, and coherent precipitates within the matrix. It is demonstrated that the best combination of tensile strength and electrical conducitivity, on the order of 599 MPa and 82 pct IACS (International Annealed Copper Standard), respectively, can be obtained in alloy Cu-0.34 wt pct Cr-0.41 wt pct Zr in the solution-heat-treated, cold-worked, and aged condition. The mechanism of tensile and conductive properties of Cu-Cr-Zr alloy is also discussed.

Effect of dendritic arm spacing on mechanical properties and corrosion resistance of Al 9 Wt Pct Si and Zn 27 Wt Pct Al alloys

Abstract: It has been reported that the mechanical properties and the corrosion resistance (CR) of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of heat-transfer solidification variables on the microstructural array of both Al 9 wt pct Si and Zn 27 wt pct Al alloy castings and to develop correlations between the as-cast dendritic microstructure, CR, and tensile mechanical properties. Experimental results include transient metal/mold heat-transfer coefficient (h i), secondary dendrite arm spacing (λ2), corrosion potential (E Corr), corrosion rate (i Corr), polarization resistance (R 1), capacitances values (Z CPE), ultimate tensile strength (UTS, σ u ), yield strength (YS, σ y ), and elongation. It is shown that σ U decreases with increasing λ2 while the CR increases with increasing λ2, for both alloys experimentally examined. A combined plot of CR and σ U as a function of λ2 is proposed as a way to determine an optimum range of secondary dendrite arm spacing that provides good balance between both properties.

Crystallography of fracture facets in a near-alpha titanium alloy

Abstract: A faceted initiation site is observed in Ti-6242 alloy for both the cyclic and static-loading test conditions. In this experimental study, the crystallographic orientation of the facets has been determined using the electron backscattered diffraction (EBSD) technique in conjunction with the quantitative tilt fractography in a scanning electron microscope (SEM). Quantitative tilt fractography analysis has been used to determine the spatial orientation of fracture facets. The results indicate that the normal-fatigue (no-dwell) fracture facets are oriented at ∼5 deg with respect to the basal plane; the dwell-fatigue fracture facets are oriented at ∼10 to 15 deg with respect to the basal plane and the static-loading fracture facets are oriented at ∼20 deg with respect to the basal plane. These crystallographic orientation determinations of the fracture facets at the crack-initiation site can be used to obtain an idea about the type of loading that produced them.

Effects of Ru on the high-temperature phase stability of Ni-base single-crystal superalloys

Abstract: Microstructural instabilities associated with the precipitation of refractory-rich topologically-close-packed (TCP) phases within the microstructure of advanced Ni-base single-crystal superalloys were quantified in two nominally identical alloys with and without additions of Ru. Differences in the microstructural kinetics associated with the formation of TCP precipitates in these experimental single-crystal superalloys enabled the influence of Ru to be assessed. Detailed microstructural investigations were carried out on specimens subjected to prolonged isothermal exposures at elevated temperature. Even after 1000 hours at temperatures in excess of 1100 °C, the microstructure of the Ru-bearing alloy was highly resistant to the formation of TCP phases. Transmission electron micro-analysis (TEM) coupled with X-ray diffraction (XRD) was used to identify the characteristic crystal structures of the TCP precipitates in both alloys as being primarily the orthorhombic P and tetragonal σ phase. The sluggish precipitation kinetics of TCP phases in the Ru-bearing single-crystal Ni-base superalloy prevents the breakdown of the parent γ-γ′ microstructure and greatly enhances the high-temperature creep characteristics.

Study of the fusion zone and heat-affected zone microstructures in tungsten inert gas-welded INCONEL 738LC superalloy

Abstract: The fusion zone and heat-affected zone (HAZ) microstructures obtained during tungsten inert gas (TIG) welding of a commercial superalloy IN 738LC were examined. The microsegregation observed during solidification in the fusion zone indicated that while Co, Cr, and W segregated to the γ dendrites, Nb, Ti, Ta, Mo, Al, and Zr were rejected into the interdendritic liquid. Electron diffraction and energy-dispersive X-ray microanalyses using a transmission electron microscope (TEM) of secondary phases, extracted from the fusion zone on carbon replicas, and of those in thin foils prepared from the fusion zone showed that the major secondary solidification constituents, formed from the interdendritic liquid, were cubic MC-type carbides and γ-γ’ eutectic. The terminal solidification reaction product was found to consist of M3B2 and Ni7Zr2 formed in front of the interdendritic γ-γ’ eutectic. Based on the knowledge of the Ni-Ti-C ternary system, a pseudoternary solidification diagram was adapted for IN 738 superalloy, which adequately explained the as-solidified microstructure. The HAZ microfissuring was observed in regions surrounding the fusion zone. Closer and careful microstructural examination by analytical scanning electron microscopy revealed formation of re-solidified constituents along the microfissured HAZ grain boundaries, which suggest that HAZ cracking in this alloy involves liquation cracking. Liquation of various phases present in preweld alloy as well as characteristics of the intergranular liquid film contributing to the alloy’s low resistance to HAZ cracking were identified and are discussed.

Effect of TiN inclusions on the impact toughness of low-carbon microalloyed steels

Abstract: Microalloying with various elements, including titanium, coupled with thermomechanically controlled processing, has become a major technology for the manufacture of high-quality steel plate. In this research, the influence of TiN inclusions on the impact toughness of low-carbon plate steels microalloyed with titanium, vanadium, and boron was investigated. The three experimental steels had Ti/N ratios of 2.44, 3.5, and 4.2, and all three had a granular bainite microstructure. However, Charpy V-notch testing showed that steel A had very high toughness at both room temperature and −20 °C, whereas steels B and C showed very low toughness at −20 °C and moderate toughness at room temperature. Scanning electron microscope fractography revealed that coarse TiN inclusions had acted as cleavage fracture initiation sites in steels B and C. The effect of Ti and N levels on TiN formation and growth is analyzed using alloy thermodynamics. It is shown that not only is the Ti/N ratio important, but also the product of total Ti and N plays a most important role in TiN formation and growth. It is concluded that the product of the total Ti and N contents should not be greater than the solubility product of TiN at the solidus temperature to prevent the precipitation of TiN particles before solidification. Furthermore, the ratio of Ti to N should also be maintained lower than the stoichiometric ratio of 3.42 to ensure a low coarsening rate for the TiN inclusions during soaking before rolling.

Effect of strontium on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy

Abstract: The effect of strontium (Sr) on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy and its sensitivity to cooling rate are investigated. Three phases—blocky-shaped Mg17Al12, acicular Mg20Al20Mn5Sr, and insular Mg16(Al,Zn)2Sr—are identified in the Sr-containing AZ31 alloys. With increasing cooling rate, the blocky-shaped Mg17Al12 phase increases, the acicular Mg20Al20Mn5Sr phase diminishes, and the insular Mg16(Al,Zn)2Sr phase is refined and granulated. The study suggests that the grain size decreases with increasing cooling rate for a given composition. However, the grain size decreases first, then increases, and finally decreases again with increasing Sr for a given cooling rate. The yield strength (σ y ) of AZ31 magnesium alloy can be improved by grain refinement and expressed as σ y =35.88+279.13d −1/2 according to the Hall-Petch relationship. The elongation increases when Sr is added up to 0.01 pct and then decreases with increasing Sr addition. Grain refinement changes the fracture behavior from quasicleavage failure for the original AZ31 alloy to mixed features of quasicleavage and microvoid coalescence fracture.

Influence of annealing treatment on the formation of nano/submicron grain size AISI 301 Austenitic stainless steels

Abstract: Nano/submicron austenitic stainless steels have attracted increasing attention over the past few years due to fine structural control for tailoring engineering properties. At the nano/submicron grain scales, grain boundary strengthening can be significant, while ductility remains attractive. To achieve a nano/submicron grain size, metastable austenitic stainless steels are heavily cold-worked, and annealed to convert the deformation-induced martensite formed during cold rolling into austenite. The amount of reverted austenite is a function of annealing temperature. In this work, an AISI 301 metastable austenitic stainless steel is 90 pct cold-rolled and subsequently annealed at temperatures varying from 600 °C to 900 °C for a dwelling time of 30 minutes. The effects of annealing on the microstructure, average austenite grain size, martensite-to-austenite ratio, and carbide formation are determined. Analysis of the as-cold-rolled microstructure reveals that a 90 pct cold reduction produces a combination of lath type and dislocation cell-type martensitic structure. For the annealed samples, the average austenite grain size increases from 0.28 µm at 600 °C to 5.85 µm at 900 °C. On the other hand, the amount of reverted austenite exhibits a maximum at 750 °C, where austenite grains with an average grain size of 1.7 µm compose approximately 95 pct of the microstructure. Annealing temperatures above 750 °C show an increase in the amount of martensite. Upon annealing, (Fe, Cr, Mo)23C6 carbides form within the grains and at the grain boundaries.

Precipitation and grain refinement in a 2195 Al friction stir weld

Abstract: The microstructure across a friction stir weld in aluminum alloy 2195 was analyzed to reveal the precipitation processes, grain evolution mechanisms, and crystallographic texture within that weld. The complex microhardness variations across the weld are explained by the observed precipitation sequence, in which the original precipitates coarsen and dissolve during welding, and are then replaced by different precipitates, which form during cooling. The grain development from the thermomechanically affected zone (TMAZ) into the weld nugget reveals that subgrains form within the TMAZ grains and develop increasing boundary misorientations through continuous dynamic recrystallization by subgrain rotation to eventually form the refined grains observed within the weld nugget. Within the weld nugget, a {112}〈110〉 texture is observed, corresponding to a high strain/high temperature shear strain component.

Influence of β grain size on tensile behavior and ductile fracture toughness of titanium alloy Ti-10V-2Fe-3Al

Abstract: The β grain size of the alloy Ti-10V-2Fe-3Al was varied by heat treatment, and the tensile behavior and fracture toughness were evaluated as a function of β grain size at room temperature. The alloy showed stress-induced martensitic transformation, and the triggering stress for this transformation varied with grain size. The 0.2 pct yield stress exhibited a Hall-Petch relationship with grain size. The ductile fracture toughness was found to increase with decrease in grain size, and it was also shown to follow a Hall-Petch kind of relationship. The grain boundary and the stress-induced martensitic contribution to fracture toughness were separated out.