Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 2017"

Deformation Microstructure and Deformation-Induced Martensite in Austenitic Fe-Cr-Ni Alloys Depending on Stacking Fault Energy

Abstract: The deformation microstructure of austenitic Fe-18Cr-(10-12)Ni (wt pct) alloys with low stacking fault energies, estimated by first-principles calculations, was investigated after cold rolling The

Formation of the Ni3Nb δ-Phase in Stress-Relieved Inconel 625 Produced via Laser Powder-Bed Fusion Additive Manufacturing

Abstract: The microstructural evolution of laser powder-bed additively manufactured Inconel 625 during a post-build stress-relief anneal of 1 hour at 1143 K (870 °C) is investigated. It is found that this industry-recommended heat treatment promotes the formation of a significant fraction of the orthorhombic D0a Ni3Nb δ-phase. This phase is known to have a deleterious influence on fracture toughness, ductility, and other mechanical properties in conventional, wrought Inconel 625; and is generally considered detrimental to materials’ performance in service. The δ-phase platelets are found to precipitate within the inter-dendritic regions of the as-built solidification microstructure. These regions are enriched in solute elements, particularly Nb and Mo, due to the micro-segregation that occurs during solidification. The precipitation of δ-phase at 1073 K (800 °C) is found to require up to 4 hours. This indicates a potential alternative stress-relief processing window that mitigates δ-phase formation in this alloy. Ultimately, a homogenization heat treatment is recommended for additively manufactured Inconel 625 because the increased susceptibility to δ-phase precipitation increases the possibility for significant degradation of materials' properties in service.

Corrosion Behavior of Additive Manufactured Ti-6Al-4V Alloy in NaCl Solution

Abstract: The microstructures, potentiodynamic curves, and electrochemical impedance spectroscopy are characterized for Ti-6Al-4V samples produced by selective laser melting (SLM), SLM followed by heat treatment (HT), wire and arc additive manufacturing (WAAM), and traditional rolling to investigate their corrosion behaviors. Results show that the processing technology acts a significant role in controlling the microstructures, which in turn directly determine their corrosion resistance. The order of corrosion resistance of these samples is SLM < WAAM < rolling < SLM+HT. Among these microstructural factors for influencing corrosion resistance, type of constituent phase is the main one, followed by grain size, and the last is morphology. Finally, the application potentials of additive manufactured Ti-6Al-4V alloy are verified in the aspect of corrosion resistance.

Structural Characteristics and In Vitro Biodegradation of a Novel Zn-Li Alloy Prepared by Induction Melting and Hot Rolling

Abstract: Zinc shows great promise as a bioabsorbable metal; however, the low tensile strength of pure zinc limits its application for endovascular stent purposes. In this study, a new Zn-xLi alloy (with x = 2, 4, 6 at. pct) was prepared by induction melting in an argon atmosphere and processed through hot rolling. Structures of the formulated binary alloys were characterized by X-ray diffraction and optical microscopy. Mechanical testing showed that the incorporation of Li into Zn increased ultimate tensile strength from 560 MPa (x = 6 at. pct). In vitro corrosion behavior was evaluated by immersion tests in simulated body fluid. The Zn-2Li and Zn-4Li corrosion study demonstrated that corrosion rates and products resemble those observed for pure Zn in vivo, and in addition, the Zn-4Li alloy exhibits higher resistance to corrosion as compared to Zn-2Li. The findings herein encourage further exploration of Zn-Li systems for structural use in biomedical vascular support applications with the ultimate goal of simplifying stent procedures, thereby reducing stent-related complications.

Thermophysical Properties of Liquid Aluminum

Abstract: Ohmic pulse-heating with sub-microsecond time resolution is used to obtain thermophysical properties for aluminum in the liquid phase. Measurement of current through the sample, voltage drop across the sample, surface radiation, and volume expansion allow the calculation of specific heat capacity and the temperature dependencies of electrical resistivity, enthalpy, and density of the sample at melting and in the liquid phase. Thermal conductivity and thermal diffusivity as a function of temperature are estimated from resistivity data using the Wiedemann–Franz law. Data for liquid aluminum obtained by pulse-heating are quite rare because of the low melting temperature of aluminum with 933.47 K (660.32 °C), as the fast operating pyrometers used for the pulse-heating technique with rise times of about 100 ns generally might not be able to resolve the melting plateau of aluminum because they are not sensitive enough for such low temperature ranges. To overcome this obstacle, we constructed a new, fast pyrometer sensitive in this temperature region. Electromagnetic levitation, as the second experimental approach used, delivers data for surface tension (this quantity is not available by means of the pulse-heating technique) and for density of aluminum as a function of temperature. Data obtained will be extensively compared to existing literature data.

Evolution of LPSO Phases and Their Effect on Dynamic Recrystallization in a Mg-Gd-Y-Zn-Zr Alloy

Abstract: The effect of long-period stacking ordered (LPSO) phases on the dynamic recrystallization (DRX) of the compressed Mg-6.9Gd-3.2Y-1.5Zn-0.5Zr (wt pct) alloy was investigated. LPSO phases of various morphologies were obtained by homogenization at 783 K (510 °C) for various durations. During homogenization, intragranular thin-platelet LPSO phases form near the α-Mg grain boundaries first, then penetrate through the grain interior, and finally coagulate. During compression at 753 K (480 °C), intragranular thin-platelet LPSO phases reduce the DRX ratio by hindering the migration of α-Mg grain boundaries. Intragranular-coagulated LPSO phases stimulate DRX near the α-Mg grain boundaries, while faintly affecting the DRX behavior in the interior of α-Mg grains. However, interdendritic phases enhance the nucleation of DRX-ed grains via the particle-stimulated nucleation mechanism. Among all the compressed alloys, the alloy rich in thin-platelet LPSO phases exhibits the optimal mechanical properties.

Microstructural Analyses of ATI 718Plus® Produced by Wire-ARC Additive Manufacturing Process

Abstract: A detailed microstructural study of ATI 718Plus superalloy produced by the wire-arc additive manufacturing (WAAM) process was performed through the use of scanning electron microscopy (SEM), transm ...

Comparative Evaluation of Cast Aluminum Alloys for Automotive Cylinder Heads: Part II—Mechanical and Thermal Properties

Abstract: The first part of this study documented the as-aged microstructure of five cast aluminum alloys namely, 206, 319, 356, A356, and A356+0.5Cu, that are used for manufacturing automotive cylinder heads (Roy et al. in Metall Mater Trans A, 2016). In the present part, we report the mechanical response of these alloys after they have been subjected to various levels of thermal exposure. In addition, the thermophysical properties of these alloys are also reported over a wide temperature range. The hardness variation due to extended thermal exposure is related to the evolution of the nano-scale strengthening precipitates for different alloy systems (Al-Cu, Al-Si-Cu, and Al-Si). The effect of strengthening precipitates (size and number density) on the mechanical response is most obvious in the as-aged condition, which is quantitatively demonstrated by implementing a strength model. Significant coarsening of precipitates from long-term heat treatment removes the strengthening efficiency of the nano-scale precipitates for all these alloys systems. Thermal conductivity of the alloys evolve in an inverse manner with precipitate coarsening compared to the strength, and the implications of the same for the durability of cylinder heads are noted.

Fracture Toughness to Understand Stretch-Flangeability and Edge Cracking Resistance in AHSS

Abstract: The edge fracture is considered as a high risk for automotive parts, especially for parts made of advanced high strength steels (AHSS). The limited ductility of AHSS makes them more sensitive to the edge damage. The traditional approaches, such as those based on ductility measurements or forming limit diagrams, are unable to predict this type of fractures. Thus, stretch-flangeability has become an important formability parameter in addition to tensile and formability properties. The damage induced in sheared edges in AHSS parts affects stretch-flangeability, because the generated microcracks propagate from the edge. Accordingly, a fracture mechanics approach may be followed to characterize the crack propagation resistance. With this aim, this work addresses the applicability of fracture toughness as a tool to understand crack-related problems, as stretch-flangeability and edge cracking, in different AHSS grades. Fracture toughness was determined by following the essential work of fracture methodology and stretch-flangeability was characterized by means of hole expansions tests. Results show a good correlation between stretch-flangeability and fracture toughness. It allows postulating fracture toughness, measured by the essential work of fracture methodology, as a key material property to rationalize crack propagation phenomena in AHSS.

Unraveling the Initial Microstructure Effects on Mechanical Properties and Work-Hardening Capacity of Dual-Phase Steel

Abstract: Ferritic-martensitic, dual-phase (DP) microstructures with different size, morphology, and distribution of martensite were produced by altering the initial microstructures using heat treatment and thermomechanical processing routes. It was revealed that the strength, ductility, and work-hardening rate of DP steels strongly depend on the volume fraction and the morphology of the martensite phase. In this regard, the fine-grained DP microstructure showed a high work-hardening ability toward an excellent combination of strength and ductility. Such a microstructure can be readily obtained by intercritical annealing of an ultrafine grained (UFG) microstructure, where the latter can be produced by cold-rolling followed by tempering of a martensite starting microstructure. Conclusively, the enhancement of mechanical properties of DP steels through microstructural refinement was found to be more beneficial compared with increasing the volume fraction of martensite. Finally, it was also demonstrated that the work-hardening rate analysis based on the instantaneous (incremental) work-hardening exponents might be an advantageous approach for characterizing DP steels along with the conventional approaches.

Microstructures and Grain Refinement of Additive-Manufactured Ti-xW Alloys

Abstract: It is necessary to better understand the composition–processing–microstructure relationships that exist for materials produced by additive manufacturing To this end, Laser Engineered Net Shaping (LENS™), a type of additive manufacturing, was used to produce a compositionally graded titanium binary model alloy system (Ti-xW specimen (0 ≤ x ≤ 30 wt pct), so that relationships could be made between composition, processing, and the prior beta grain size Importantly, the thermophysical properties of the Ti-xW, specifically its supercooling parameter (P) and growth restriction factor (Q), are such that grain refinement is expected and was observed The systematic, combinatorial study of this binary system provides an opportunity to assess the mechanisms by which grain refinement occurs in Ti-based alloys in general, and for additive manufacturing in particular The operating mechanisms that govern the relationship between composition and grain size are interpreted using a model originally developed for aluminum and magnesium alloys and subsequently applied for titanium alloys The prior beta grain factor observed and the interpretations of their correlations indicate that tungsten is a good grain refiner and such models are valid to explain the grain-refinement process By extension, other binary elements or higher order alloy systems with similar thermophysical properties should exhibit similar grain refinement

Strength–Ductility Property Maps of Powder Metallurgy (PM) Ti-6Al-4V Alloy: A Critical Review of Processing-Structure-Property Relationships

Abstract: A comprehensive assessment of tensile properties of powder metallurgical (PM) processed Ti-6Al-4V alloy, through the mapping of strength–ductility property domains, is performed in this review. Tensile property data of PM Ti-6Al-4V alloys made from blended element (BE) and pre-alloyed powders including that additive manufactured (AM) from powders, as well as that made using titanium hydride powders, have been mapped in the form of strength–ductility domains. Based on this, porosity and microstructure have been identified as the dominant variables controlling both the strength and the tensile ductility of the final consolidated materials. The major finding is that tensile ductility of the PM titanium is most sensitive to the presence of pores. The significance of extreme-sized pores or defects in inducing large variations in ductility is emphasized. The tensile strength, however, has been found to depend only weakly on the porosity. The effect of microstructure on properties is masked by the variations in porosity and to some extent by the oxygen level. It is shown that any meaningful comparison of the microstructure can only be made under a constant porosity or density level. The beneficial effect of a refined microstructure is also brought out by logically organizing the data in terms of microstructure groups. The advantages of new processes, using titanium hydride powder to produce PM titanium alloys, in simultaneously increasing strength and ductility, are also highlighted. The tensile properties of AM Ti-6Al-4V alloys are also brought to light, in comparison with the other PM and wrought alloys, through the strength–ductility maps.

Tensile Properties of Medium Mn Steel with a Bimodal UFG α + γ and Coarse δ-Ferrite Microstructure

Abstract: While the tensile strength and elongation obtained for medium Mn steel would appear to make it a candidate material in applications which require formable ultra-high strength materials, many secondary aspects of the microstructure–properties relationships have not yet been given enough attention. In this contribution, the microstructural and tensile properties of medium Mn steel with a bimodal microstructure consisting of an ultra-fine grained ferrite + austenite constituent and coarse-grained delta-ferrite are therefore reviewed in detail. The tensile properties of ultra-fine-grained intercritically annealed medium Mn steel reveal a complex dependence on the intercritical annealing temperature. This dependence is related to the influence of the intercritical annealing temperature on the activation of the plasticity-enhancing mechanisms in the microstructure. The kinetics of deformation twinning and strain-induced transformation in the ultra-fine grained austenite play a prominent role in determining the strain hardening of medium Mn steel. While excellent strength–ductility combinations are obtained when deformation twinning and strain-induced transformation occur gradually and in sequence, large elongations are also observed when strain-induced transformation plasticity is not activated. In addition, the localization of plastic flow is observed to occur in samples after intercritical annealing at intermediate temperatures, suggesting that both strain hardening and strain rate sensitivity are influenced by the properties of the ultra-fine-grained austenite.

Crystal Plasticity Model Validation Using Combined High-Energy Diffraction Microscopy Data for a Ti-7Al Specimen

Abstract: High-Energy Diffraction Microscopy (HEDM) is a 3-d X-ray characterization method that is uniquely suited to measuring the evolving micro-mechanical state and microstructure of polycrystalline materials during in situ processing. The near-field and far-field configurations provide complementary information; orientation maps computed from the near-field measurements provide grain morphologies, while the high angular resolution of the far-field measurements provides intergranular strain tensors. The ability to measure these data during deformation in situ makes HEDM an ideal tool for validating micro-mechanical deformation models that make their predictions at the scale of individual grains. Crystal Plasticity Finite Element Models (CPFEM) are one such class of micro-mechanical models. While there have been extensive studies validating homogenized CPFEM response at a macroscopic level, a lack of detailed data measured at the level of the microstructure has hindered more stringent model validation efforts. We utilize an HEDM dataset from an alpha-titanium alloy (Ti-7Al), collected at the Advanced Photon Source, Argonne National Laboratory, under in situ tensile deformation. The initial microstructure of the central slab of the gage section, measured via near-field HEDM, is used to inform a CPFEM model. The predicted intergranular stresses for 39 internal grains are then directly compared to data from 4 far-field measurements taken between ~4 and ~80 pct of the macroscopic yield strength. The evolution of the elastic strain state from the CPFEM model and far-field HEDM measurements up to incipient yield are shown to be in good agreement, while residual stress at the individual grain level is found to influence the intergranular stress state even upon loading. Implications for application of such an integrated computational/experimental approach to phenomena such as fatigue are discussed.

Enhanced Relative Slip Distance in Gas-Tungsten-Arc-Welded Al 0.5 CoCrFeNi High-Entropy Alloy

Abstract: Gas-tungsten-arc-welded (GTAW) Al0.5CoCrFeNi high-entropy alloy (HEA) was analyzed using scanning electron microscopy (SEM), microhardness, and tensile testing. The weld metal having refined equiaxed and elongated columnar dendritic microstructure experienced 6.38 pct reduction in strength and marginally reduced hardness compared to the base metal (BM). Lower work hardening with enhanced relative slip distance, which was observed through the Kocks–Mecking plot and slip distance–true strain plots, was attributed to the reduced bcc fraction in the weld.

Review of Grain Refinement of Cast Metals Through Inoculation: Theories and Developments

Abstract: The inoculation method of grain refinement is widely used in research and industry. Because of its commercial and engineering importance, extensive research on the mechanisms/theories of grain refinement and development of effective grain refiners for diverse cast metals/alloys has been conducted. In 1999, Easton and St. John reviewed the mechanisms of grain refinement of cast Al alloys. Since then, grain refinement in alloys of Al, Mg, Fe, Ti, Cu, and Zn has evolved a lot. However, there is still no full consensus on the mechanisms/theories of grain refinement. Moreover, some new grain refiners developed based on the theories do not ensure efficient grain refinement. Thus, the factors that contribute to grain refinement are still not fully understood. Clarification of the prerequisite issues that occur in grain refinement is required using recent theories. This review covers multiple metals/alloys and developments in grain refinement from the last twenty years. The characteristics of effective grain refiners are considered from four perspectives: effective particle/matrix wetting configuration, sufficiently powerful segregating elements, preferential crystallographic matching, and geometrical features of effective nucleants. Then, recent mechanisms/theories on the grain refinement of cast metals/alloys are reviewed, including the peritectic-related, hypernucleation, inert nucleant, and constitutional supercooling-driven theories. Further, developments of deterministic and probabilistic modeling and nucleation crystallography in the grain refinement of cast metals are reviewed. Finally, the latest progress in the grain refinement of cast Zn and its alloys is described, and future work on grain refinement is summarized.

Comparative Evaluation of Cast Aluminum Alloys for Automotive Cylinder Heads: Part I—Microstructure Evolution

Abstract: The present study stages a comparative evaluation of microstructure and associated mechanical and thermal response for common cast aluminum alloys that are used for manufacturing automotive cylinder heads. The systems considered are Al-Cu (206-T6), Al-Si-Cu (319-T7), and Al-Si (356-T6, A356-T6, and A356 + 0.5Cu-T6). The focus of the present manuscript is on the evaluation of microstructure at various length scales after aging, while the second manuscript will deal with the mechanical and thermal response of these alloys due to short-term (aging) and long-term (pre-conditioning) heat treatments. At the grain-scale, the Al-Cu alloy possessed an equiaxed microstructure as opposed to the dendritic structure for the Al-Si-Cu or Al-Si alloys which is related to the individual solidification conditions for these alloy systems. The composition and morphology of intermetallic precipitates within the grain and at the grain/dendritic boundary are dictated by the alloy chemistry, solidification, and heat treatment conditions. At the nanoscale, these alloys contain various metastable strengthening precipitates (GPI and $$ \theta^{\prime\prime} $$ in Al-Cu alloy, $$ \theta^{\prime} $$ in Al-Si-Cu alloy, and $$ \beta^{\prime} $$ in Al-Si alloys) with varying size, morphology, coherency, and thermal stability.

The Influence of Composition on the Clustering and Precipitation Behavior of Al-Mg-Si-Cu Alloys

Abstract: The natural aging (NA) and artificial aging (AA) behavior of Al-Mg-Si-Cu alloys with different Mg/Si ratios and Cu additions were systematically investigated by means of hardness test, atom probe tomography, transmission electron microscopy, and Monte Carlo simulation. The Si-rich low-Cu alloys displayed higher hardness compared to the Mg-rich equivalents because Si atoms play a dominant role in clustering of solute atoms during both natural and artificial aging. In the high-Cu alloys, Cu did not obviously change the cluster distribution during NA, but significantly refines the clusters and precipitates due to the strong interaction of Cu atoms with Mg atoms during AA. In contrast to the low-Cu alloys, the Mg-rich high-Cu alloys exhibit higher hardness in the early and over-aged stages of artificial aging, with similar or slightly higher hardness in the peak aging condition compared to their Si-rich counterparts. Three types of precipitates (β″, Q′, and L) are favored in the high-Cu alloys. The Mg-rich high-Cu alloy has more L phase, while the Si-rich variant is abundant in Q′ phase. The negative effect of NA on subsequent AA behavior is less dependent on Mg/Si ratio in the high-Cu alloys due to a synergistic action of the residual Si and Cu atoms, but is closely related to Mg/Si ratio in low-Cu alloys.

Composite Behavior of Lath Martensite Steels Induced by Plastic Strain, a New Paradigm for the Elastic-Plastic Response of Martensitic Steels

Abstract: Based on high-resolution neutron diffraction experiments, we will show that in lath martensite steels, the initially homogeneous dislocation structure, i.e., homogeneous on the length scale of grain size, is disrupted by plastic deformation, which, in turn, produces a composite on the length scale of martensite lath packets. The diffraction patterns of plastically strained martensitic steel reveal characteristically asymmetric peak profiles in the same way as has been observed in materials with heterogeneous dislocation structures. The quasi homogeneous lath structure, formed by quenching, is disrupted by plastic deformation producing a composite structure. Lath packets oriented favorably or unfavorably for dislocation glide become soft or hard. Two lath packet types develop by work softening or work hardening in which the dislocation densities become smaller or larger compared to the initial average dislocation density. The decomposition into soft and hard lath packets is accompanied by load redistribution and the formation of long-range internal stresses between the two lath packet types. The composite behavior of plastically deformed lath martensite opens a new way to understand the elastic-plastic response in this class of materials.

Influence of Temperature and Grain Size on Austenite Stability in Medium Manganese Steels

Abstract: With an aim to elucidate the influence of temperature and grain size on austenite stability, a commercial cold-rolled 7Mn steel was annealed at 893 K (620 °C) for times varying between 3 minutes and 96 hours to develop different grain sizes. The austenite fraction after 3 minutes was 34.7 vol pct, and at longer times was around 40 pct. An elongated microstructure was retained after shorter annealing times while other conditions exhibited equiaxed ferrite and austenite grains. All conditions exhibit similar temperature dependence of mechanical properties. With increasing test temperature, the yield and tensile strength decrease gradually, while the uniform and total elongation increase, followed by an abrupt drop in strength and ductility at 393 K (120 °C). The Olson–Cohen model was applied to fit the transformed austenite fractions for strained tensile samples, measured by means of XRD. The fit results indicate that the parameters α and β decrease with increasing test temperature, consistent with increased austenite stability. The 7Mn steels exhibit a distinct temperature dependence of the work hardening rate. Optimized austenite stability provides continuous work hardening in the temperature range of 298 K to 353 K (25 °C to 80 °C). The yield and tensile strengths have a strong dependence on grain size, although grain size variations have less effect on uniform and total elongation.

Combined Intercritical Annealing and Q&P Processing of Medium Mn Steel

Abstract: The microstructure and mechanical properties of intercritically annealed medium Mn steel are dependent on the selection of the intercritical annealing (IA) temperature. While the yield strength (YS) decreases with increasing IA temperature, the ultimate tensile strength increases with increasing IA temperature. Strain aging phenomena, both static and dynamic, are also often observed. The present contribution shows that, by combining IA with the quench and partitioning processing of the intercritical austenite, it is possible to obtain non-aging mechanical properties which combine a high YS with an ultra-high tensile strength. These properties are particularly suitable for automotive parts related to passenger safety.

Microstructural and Phase Composition Differences Across the Interfaces in Al/Ti/Al Explosively Welded Clads

Abstract: The microstructure and phase composition of Al/Ti/Al interfaces with respect to their localization were investigated. An aluminum-flyer plate exhibited finer grains located close to the upper interface than those present within the aluminum-base plate. The same tendency, but with a higher number of twins, was observed for titanium. Good quality bonding with a wavy shape and four intermetallic phases, namely, TiAl3, TiAl, TiAl2, and Ti3Al, was only obtained at the interface closer to the explosive material. The other interface was planar with three intermetallic compounds, excluding the metastable TiAl2 phase. As a result of a 100-hour annealing at 903 K (630 °C), an Al/TiAl3/Ti/TiAl3/Al sandwich was manufactured, formed with single crystalline Al layers. A substantial difference between the intermetallic layer thicknesses was measured, with 235.3 and 167.4 µm obtained for the layers corresponding to the upper and lower interfaces, respectively. An examination by transmission electron microscopy of a thin foil taken from the interface area after a 1-hour annealing at 825 K (552 °C) showed a mixture of randomly located TiAl3 grains within the aluminum. Finally, the hardness results were correlated with the microstructural changes across the samples.

The Investigation of Strain-Induced Martensite Reverse Transformation in AISI 304 Austenitic Stainless Steel

Abstract: This paper presents a comprehensive study on the strain-induced martensitic transformation and reversion transformation of the strain-induced martensite in AISI 304 stainless steel using a number of complementary techniques such as dilatometry, calorimetry, magnetometry, and in-situ X-ray diffraction, coupled with high-resolution microstructural transmission Kikuchi diffraction analysis. Tensile deformation was applied at temperatures between room temperature and 213 K (−60 °C) in order to obtain a different volume fraction of strain-induced martensite (up to ~70 pct). The volume fraction of the strain-induced martensite, measured by the magnetometric method, was correlated with the total elongation, hardness, and linear thermal expansion coefficient. The thermal expansion coefficient, as well as the hardness of the strain-induced martensitic phase was evaluated. The in-situ thermal treatment experiments showed unusual changes in the kinetics of the reverse transformation (α′ → γ). The X-ray diffraction analysis revealed that the reverse transformation may be stress assisted—strains inherited from the martensitic transformation may increase its kinetics at the lower annealing temperature range. More importantly, the transmission Kikuchi diffraction measurements showed that the reverse transformation of the strain-induced martensite proceeds through a displacive, diffusionless mechanism, maintaining the Kurdjumov–Sachs crystallographic relationship between the martensite and the reverted austenite. This finding is in contradiction to the results reported by other researchers for a similar alloy composition.

Mechanical Properties and Microstructural Characterization of Cu-4.3 Pct Sn Fabricated by Selective Laser Melting

Abstract: Components were fabricated via selective laser melting (SLM) of prealloyed Cu-43 pct Sn powder and heat treated at 873 K and 1173 K (600 °C and 900 °C) for 1 hour Tensile testing, conductivity measurement, and detailed microstructural characterization were carried out on samples in the as-printed and heat-treated conditions Optimization of build parameters resulted in samples with around 97 pct density with a yield strength of 274 MPa, an electrical conductivity of 241 pct IACS, and an elongation of 56 pct Heat treatment resulted in lower yield strength with significant increases in ductility due to recrystallization and a decrease in dislocation density Tensile sample geometry and surface finish also showed a significant effect on measured yield strength but a negligible change in measured ductility Microstructural characterization indicated that grains primarily grow epitaxially with a submicron cellular solidification substructure Nanometer scale tin dioxide particles identified via X-ray diffraction were found throughout the structure in the tin-rich intercellular regions

Analysis of Particle-Stimulated Nucleation (PSN)-Dominated Recrystallization for Hot-Rolled 7050 Aluminum Alloy

Abstract: In 7xxx series aluminum alloys, the constituent large and small second-phase particles present during deformation process. The fraction and spatial distribution of these second-phase particles significantly influence the recrystallized structure, kinetics, and texture in the subsequent treatment. In the present work, the Monte Carlo Potts model was used to model particle-stimulated nucleation (PSN)-dominated recrystallization and grain growth in high-strength aluminum alloy 7050. The driving force for recrystallization is deformation-induced stored energy, which is also strongly affected by the coarse particle distribution. The actual microstructure and particle distribution of hot-rolled plate were used as an initial point for modeling of recrystallization during the subsequent solution heat treatment. Measurements from bright-field TEM images were performed to enhance qualitative interpretations of the developed microstructure. The influence of texture inhomogeneity has been demonstrated from a theoretical point of view using pole figures. Additionally, in situ annealing measurements in SEM were performed to track the orientational and microstructural changes and to provide experimental support for the recrystallization mechanism of PSN in AA7050.

Work Hardening, Dislocation Structure, and Load Partitioning in Lath Martensite Determined by In Situ Neutron Diffraction Line Profile Analysis

Abstract: A lath martensite steel containing 0.22 mass pct carbon was analyzed in situ during tensile deformation by high-resolution time-of-flight neutron diffraction to clarify the large work-hardening behavior at the beginning of plastic deformation. The diffraction peaks in plastically deformed states exhibit asymmetries as the reflection of redistributions of the stress and dislocation densities/arrangements in two lath packets: soft packet, where the dislocation glides are favorable, and hard packet, where they are unfavorable. The dislocation density was as high as 1015 m−2 in the as-heat-treated state. During tensile straining, the load and dislocation density became different between the two lath packets. The dislocation character and arrangement varied in the hard packet but hardly changed in the soft packet. In the hard packet, dislocations that were mainly screw-type in the as-heat-treated state became primarily edge-type and rearranged towards a dipole character related to constructing cell walls. The hard packet played an important role in the work hardening in martensite, which could be understood by considering the increase in dislocation density along with the change in dislocation arrangement.

Characterization of austenitic stainless steels deformed at elevated temperature

Abstract: Highly alloyed austenitic stainless steels are promising candidates to replace more expansive nickel-based alloys within the energy-producing industry. The present study investigates the deformatio ...

Bulk Nanostructured Materials

Abstract: This paper will address three topics of importance to bulk nanostructured materials. Bulk nanostructured materials are defined as bulk solids with nanoscale or partly nanoscale microstructures. This category of nanostructured materials has historical roots going back many decades but has relatively recent focus due to new discoveries of unique properties of some nanoscale materials. Bulk nanostructured materials are prepared by a variety of severe plastic deformation methods, and these will be reviewed. Powder processing to prepare bulk nanostructured materials requires that the powders be consolidated by typical combinations of pressure and temperature, the latter leading to coarsening of the microstructure. The thermal stability of nanostructured materials will also be discussed. An example of bringing nanostructured materials to applications as structural materials will be described in terms of the cryomilling of powders and their consolidation.

A Model for Static Recrystallization with Simultaneous Precipitation and Solute Drag

Abstract: In the present work, we introduce a state parameter-based microstructure evolution model, which incorporates the effect of solute atoms and precipitates on recrystallization kinetics. The model accounts for local precipitate coarsening at grain boundaries, which promotes an average grain boundary movement even if the Zener pinning force exceeds the driving force for recrystallization. The impact of solute drag on the grain boundary mobility as well as simultaneous precipitation is discussed in detail. The model is validated on experimental data on recrystallization in V-micro-alloyed steel, where excellent agreement is achieved.

Microstructural Characteristics and Mechanical Properties of Friction Stir Welded Thick 5083 Aluminum Alloy

Abstract: Joining thick sections of aluminum alloys by friction stir welding (FSW) in a single pass needs to overcome many challenges before it comes to full-scale industrial use. Important parameters controlling the structure-properties relationships both across weld cross-section and through thickness direction were investigated through mechanical testing, electron backscatter diffraction technique, transmission electron microscopy, and occurrence of serrated plastic flow. The evolution of the properties in the weld cross-section shows that the presence of undissolved and fragmented Al $$_6$$ MnFe particles cause discrepancies in establishing the Hall-Petch relationship, and derive the strengthening from the Orowan strengthening mechanism. A ‘stop action’ friction stir weld has been prepared to understand the role of geometrical features of the tool probe in the development of the final microstructure after complete weld. Sectioning through the ‘stop action’ weld with the probe in situ displays the individual effect of thread and flat on the grain structure formation. The material at the thread surface experiences more severe deformation than the material at flat surface. Both the high-angle boundaries and mean grain size are found to be higher at the thread surface. The strain hardening capacity, stress serration amplitude, and frequency are observed to be higher in the stir zone than other weld regions.