Abstract: Additive Manufacturing of titanium components holds promise to deliver benefits such as reduced cost, weight and carbon emissions during both manufacture and use. To capitalize on these benefits, it must be shown that the mechanical performance of parts produced by Additive Manufacturing can meet design requirements that are typically based on wrought material performance properties. Of particular concern for safety critical structures are the fatigue properties of parts produced by Additive Manufacturing. This research evaluates the fatigue properties of Ti–6Al–4V specimens produced by the Selective Laser Melting additive manufacturing process. It was found that the fatigue life is significantly lower compared to wrought material. This reduction in fatigue performance was attributed to a variety of issues, such as microstructure, porosity, surface finish and residual stress. There was also found to be a high degree of anisotropy in the fatigue performance associated with the specimen build orientation.

Abstract: Al-12Si specimens are produced by selective laser melting (SLM) from gas atomized powders. An extremely fine cellular structure is observed with residual free Si along the cellular boundaries. Room temperature tensile tests reveal a remarkable mechanical behavior: the samples show yield and tensile strengths of about 260. MPa and 380. MPa, respectively, along with fracture strain of ~3%. The effect of annealing on microstructure and related tensile properties is examined and the results demonstrate that the mechanical behavior of the Al-12Si SLM samples can be tuned within a wide range of strength and ductility through proper annealing treatment.

Abstract: Commercially pure titanium (CP-Ti) has received a great deal of attention in medical applications. Improvement of its mechanical properties plays a key role in enhancing the biomechanical compatibility of Ti implants, leading to avoid revision surgeries. Emerging advanced manufacturing technologies such as selective laser melting (SLM) is providing an ideal platform for producing components with almost no geometric constraints and is economically feasible down to a batch size of one. This study presents the results of using SLM to produce CP-Ti parts starting from powder with a wide grain size range up to 100 μm. Accurate manipulation of SLM manufacturing parameters were applied to produce nearly full dense (>99.5%) CP-Ti parts without any post-treatments. Compared with the properties of those manufactured by traditional processing technologies, the microhardness, compressive, and tensile strengths of SLM-processed CP-Ti parts have been improved to 261 Hv, 1136 MPa, and 757 MPa, respectively, due to the formation of refined martensitic α′ grains during SLM. The optimal manufacturing parameters could enhance the strength and hardness of CP-Ti and yet maintaining the ductility of titanium. Fractography study of the tensile-failed SLM-processed specimens showed that incompletely melted particles and porosities caused early fracture in porous sample. Mixture of dimples and minor quasi-cleavage facets covered most fracture surface of full dense sample.

Abstract: Selective laser melting (SLM) technology has been used to manufacture the AZ91D magnesium alloy. The relative density, microstructure, microhardness and tensile properties of the deposited AZ91D samples at different laser energy inputs were characterized. The results indicate that laser energy input plays a significant role in determining formation qualities of the SLMed samples. High density samples without obvious macro-defects can be obtained between 83 J/mm 3 and 167 J/mm 3 . The SLMed AZ91D presents a unique layerwise feature in which the fully divorced eutectic β-Mg 17 Al 12 distributing along the boundary of the equiaxed α-Mg matrix. The average size of α-Mg in overlapping regions is a little larger than that in the center of the scanning tracks due to the remelting process though the element distributions of Mg and Al are quite uniform. The microhardness of all samples shows directional independence. The microhardness and tensile strengths of the SLMed AZ91D at room temperature are superior to those of the die-cast AZ91D due to the combined effect of grain refinement and solid solution strengthening.

Abstract: As one of the most important engineering materials, aluminum alloys have been widely applied in many fields. However, the requirement of enhancing their mechanical properties without sacrificing the ductility is always a challenge in the development of aluminum alloys. Thanks to the excellent physical and mechanical properties, graphene nanoflakes (GNFs) have been applied as promising reinforcing elements in various engineering materials, including polymers and ceramics. However, the investigation of GNFs as reinforcement phase in metals or alloys, especially in aluminum alloys, is still very limited. In this study, the aluminum alloy reinforced by GNFs was successfully prepared via powder metallurgy approach. The GNFs were mixed with aluminum alloy powders through ball milling and followed by hot isostatic pressing. The green body was then hot extruded to obtain the final GNFs reinforced aluminum alloy nanocomposite. The scanning electron microscopy and transmission electron microscope analysis show that GNFs were well dispersed in the aluminum alloy matrix and no chemical reactions were observed at the interfaces between the GNFs and aluminum alloy matrix. The mechanical properties׳ testing results show that with increasing filling content of GNFs, both tensile and yield strengths were remarkably increased without losing the ductility performance. These results not only provided a pathway to achieve the goal of preparing high strength aluminum alloys with excellent ductilitybut they also shed light on the development of other metal alloys reinforced by GNFs.

Abstract: In order to clarify the effects of the martensite distribution on the mechanical properties of low-carbon dual-phase steel, four types of dual-phase steel with different ferrite grain sizes and martensite distributions were prepared using a thermomechanical treatment. The tensile properties of these steels were investigated; in particular, the strain hardening and the ductile fracture behaviors were discussed in terms of the strain partitioning between the ferrite and martensite and the formation and growth of micro-voids, respectively. When the martensite grains surround the ferrite grains and form a chain-like networked structure, the strain hardenability is greatly improved without a significant loss of elongation, while the necking deformability is considerably reduced. A digital-image correlation analysis revealed that the tensile strain in the martensite region in the chain-like networked dual-phase structure is markedly increased during tensile deformation, which leads to an improvement in the strain hardenability. On the other hand, the joint part of the martensite grains in the structure acts as a preferential formation site for micro-voids. The number density of the micro-voids rapidly increases with increasing tensile strain, which would cause the lower necking deformability.

Abstract: In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual stresses, microstructure changes and mechanical properties of austenitic stainless steel 304 were investigated. The dynamic impacts induced by UNSM leads to surface nanocrystallization, martensite formation, and the generation of high magnitude of surface compressive residual stresses (−1400 MPa) and hardening. Highly dense deformation twins were generated in material subsurface to a depth of 100 µm. These deformation twins significantly improve material work-hardening capacity by acting both as dislocation blockers and dislocation emission sources. Furthermore, the gradually changing martensite volume fraction ensures strong interfacial strength between the ductile interior and the two nanocrystalline surface layers and thus prevents early necking. The microstructure with two strong surface layers and a compliant interior embedded with dense nanoscale deformation twins and dislocations leads to both high strength and high ductility. The work-hardened surface layers (3.5 times the original hardness) and high magnitude of compressive residual stresses lead to significant improvement in fatigue performance; the fatigue endurance limit was increased by 100 MPa. The results have demonstrated that UNSM is a powerful surface engineering technique that can improve component mechanical properties and performance.

Abstract: The present work reports the mechanical improvement in Cu matrix composites reinforced with graphene nanosheets decorated with Ni nanoparticles (GNS–Ni) hybrids. The GNS–Ni hybrids were firstly synthesized by an in situ chemical reduction method and then incorporated into the Cu matrix to fabricate bulk GNS–Ni/Cu composites by spark plasma sintering. Benefiting from the unique characteristic of GNS–Ni hybrids, the GNS–Ni/Cu composites exhibited homogeneously dispersed GNSs and a strong GNS–Cu interface interaction, therefore leading to a 61% increase in Young׳s modulus (132 GPa) and a 94% improvement in yield strength (268 MPa) by addition of only 1.0 vol% GNSs. The GNS–Ni/Cu composites exhibited a load transfer mechanism as verified by a modified shear-lag model. Our study thus shows the potential for GNS–Ni hybrids to be successfully used as a reinforcing phase in metal matrix composites.

Abstract: The hot compressive deformation behaviors of a typical Ni-based superalloy are investigated over wide ranges of forming temperature and strain rate. Based on the experimental data, the efficiencies of power dissipation and instability parameters are evaluated and processing maps are developed to optimize the hot working processing. The microstructures of the studied Ni-based superalloy are analyzed to correlate with the processing maps. It can be found that the flow stress is sensitive to the forming temperature and strain rate. With the increase of forming temperature or the decrease of strain rate, the flow stress significantly decreases. The changes of instability domains may be related to the adiabatic shear bands and the evolution of δ phase(Ni 3 Nb) during the hot formation. Three optimum hot deformation domains for different forming processes (ingot cogging, conventional die forging and isothermal die forging) are identified, which are validated by the microstructural features and adiabatic shear bands. The optimum window for the ingot cogging processing is identified as the temperature range of 1010–1040 °C and strain rate range of 0.1–1 s −1 . The temperature range of 980–1040 °C and strain rate range of 0.01–0.1 s −1 can be selected for the conventional die forging. Additionally, the optimum hot working domain for the isothermal die forging is 1010–1040 °C and near/below 0.001 s −1 .

Abstract: Restrictions on fuel consumption and safety in the automotive industry have stimulated the development of quenching and partitioning (Q&P) steel. This steel is expected to have very high strength in combination with acceptable ductility owing to its microstructure consisting of martensite with a considerable amount of retained austenite. The effect of retained austenite on the mechanical properties and its transformation stability were determined by stepwise uniaxial micro-tensile testing and subsequent electron backscatter diffraction (EBSD) study of a pre-selected region. The austenite fraction evolution with increasing plastic deformation and the influence of fresh martensite on the local strain distribution were quantified based on the orientation data. The decrease of the retained austenite as a function of the applied strain was described by an exponential function with the pre-exponential and exponential factors related to the starting austenite fraction and its transformation stability respectively. It was proven that the presence of fresh martensite has a negative influence on this austenite transformation stability due to its constraining effect on the strain distribution. This effects the mechanical properties manifested by changes in the strain hardening behavior and total elongation. The results suggest that the ductility of the Q&P steels can be improved by an appropriate design of the heat treatment schedule in order to ensure high retained austenite fractions without the presence of fresh martensite in the final microstructure.

Abstract: The present work deals with achieving improvement in the mechanical properties of SUS304L stainless steel through the application of a unique microstructure design termed as ‘Harmonic structure’, and establishing a co-relationship between various microstructural characteristics and mechanical properties. Harmonic structure essentially means a bimodal grain size distribution with a specific periodic arrangement of coarse- and ultrafine-grain fractions. SUS304L stainless steel samples having such microstructure were fabricated by a powder metallurgy route involving the mechanical milling of pre-alloyed steel powder followed by spark plasma sintering. Due to these peculiar microstructural characteristics, the harmonic-structured SUS304L stainless steels demonstrated a winning combination of high strength, large uniform elongation, and large total elongation to failure, simultaneously. It was also found that the fraction of a shell area (a three-dimensional continuously connected network of ultrafine-grained structure) is an important parameter controlling the balance of the mechanical properties of the harmonic-structured SUS304L steel compacts.

Abstract: A 99.995% pure Ni sample, compressed to 25%, was annealed in a SEM chamber and changes in the density of annealing twins were monitored in situ during recrystallization and grain growth. In addition to average microstructural measurements, the evolution of individual grains was also observed. Both the average annealing twin density in the recrystallized domain and the annealing twin density per grain increased during recrystallization. The rate of increase in twin density correlates with the velocity of the recrystallization front. During grain growth, however, the average annealing twin density decreased. The in situ EBSD observations showed both the formation of new twins and the extension of existing twins during annealing. The observations reported here suggest that the existing models for annealing twin formation are incomplete.

Abstract: The strength of polycrystalline metals increases with a decrease in grain size according to the Hall–Petch relationship. However, heterogeneous microstructures deviate from this relationship depending on the distribution of grain sizes. This paper introduces a rule of mixtures based approach for determining the characteristic length of the microstructure for heterogeneous weld metal. The proposed grain size parameter, the volume-weighted average grain size, is measured experimentally for nine structural steel weld metals and two base materials. The weld metals are found to have a large variety of grain size distributions that are noticeably broader than those of the base material due to differences in phase contents. The results show that the volume-weighted average grain size is able to capture the influence of grain size distribution on the strength of welded structural steel. Based on the experimental results, a modified Hall–Petch relationship is formulated for the strength prediction of heterogeneous microstructures. The modified relationship is also found to be applicable to data from the literature.

Abstract: The effects of rare earth (RE) containing Ce and La elements addition on the microstructures characteristics, tensile properties and fracture behavior of A357 alloy under as-cast and T6 conditions were systematically investigated in this study. Obtained results showed that the addition of RE obviously reduced the sizes of the α-Al primary phase and eutectic silicon particles as well as SDAS value and improved the morphology of eutectic silicon particles. The optimum level of added RE content were 0.2 wt%, and the aspect ratio of eutectic silicon particles of the A357 modified alloy under as-cast and T6 conditions decreased 142% and 174%, respectively, compared with the unmodified alloy. In addition, the addition of RE greatly improved the tensile properties of A357 alloy as result of the significant improvement in microstructure, especially in elongation under T6 condition. The fracture surfaces of the A357 unmodified alloy tensile samples showed a clear brittle fracture nature, and its fracture path passed through the eutectic silicon particles and displayed a transgranular fracture mode, leading to poorer ductility. The fracture path of the A357 modified alloys passed through the eutectic phase along the grain boundaries of the α-Al primary phase, and the fracture generated by dimple rupture with cracked eutectic silicon particles, and it showed an intergranular fracture mode, resulting in superior ductility.

Abstract: This paper presents a systematic study of the influence of the state of precipitation on the plasticity of an Al–Cu–Li alloy. By varying the heat treatment, a variety of T1 precipitate morphologies are obtained. Atomic resolution electron microscopy observations show that T1 precipitates remain shearable even during early stages of over-ageing when their thickness is of several nanometres. They appear to be sheared only by single dislocations at a given location, which prevents catastrophic strain localisation at a microscopic scale. In later stages of over-ageing, the study of macroscopic strain hardening rate and of slip line localisation strongly suggests a transition to precipitate by-passing. The influence of the strengthening mechanism on strain reversal experiments (Bauschinger effect) is discussed.

Abstract: Structure–mechanical property relationship studies were carried out on Gleeble simulated intercritically reheated coarse-grained heat affected zone (ICCGHAZ) of 700 MPa linepipe steel microalloyed with Nb. The design of experiments was aimed at varying reheat temperature in the first pass to obtain different coarse grain size in the HAZ. This enabled the study of the effect of prior austenite grain size on martensite–austenite (M–A) constituent during the second pass reheating and its consequent influence on impact toughness. We elucidate here the role of phase transformation and the fraction, size, shape, distribution, and carbon content of M–A constituent on impact toughness. The data suggests that the fraction of M–A constituent is not influenced by grain size, but the size of M–A constituent is influenced by the prior austenite grain size, which consequently governs toughness. Coarse austenite grain size increases the size of M–A constituent and lowers the HAZ toughness. Coarse austenite grain associated with coarse M–A constituent along grain boundary is the dominant factor in promoting brittle fracture. The combination of fine prior austenite grain size and smaller M–A constituent is favorable in obtaining high toughness. Good toughness is obtained on refining the prior austenite grain size in the CGHAZ during first pass and hence ICCGHAZ in the second pass.

Abstract: Laser shock peening (LSP) is an innovative surface treatment technique, and can significantly improve the fatigue performance of metallic components. In this paper, the objective of this work was to improve the fatigue resistance of TC6 titanium alloy by laser shock peening. Firstly, the effects on the microstructure and mechanical properties with different LSP impacts were investigated, which were observed and measured by X-ray diffraction (XRD), transmission electron microscope (TEM), residual stress tester and microhardness tester. Specially, nanostructure was detected in the laser-peened surface layer with multiple LSP impacts. Whereafter, a better parameter was chosen to be applied on the standard vibration fatigue specimens. Via the high-cycle vibration fatigue tests, the high cycle fatigue limits of the specimens without and with LSP were obtained and compared. The fatigue results demonstrate that LSP can effectively improve the fatigue limit of TC6 titanium alloy. The strengthening mechanism was indicated by analyzing the effects on the microstructure and mechanical properties comprehensively.

Abstract: The present work aims to investigate the relationship between the mechanical behavior and composite structure of silicon carbide (SiC) particle reinforced aluminum matrix composites. On account of newly developed particle size analysis technique, a large number of SiC particles are experimentally measured to provide statistical particular structural information. According to the statistical analysis and physical observations of SiC particles, the composite structures of SiC/Al composites are numerically reproduced in line with their actual microscopic structures, in which a developed structural modeling program can build the randomly dispersions of the particle sizes, the particle shapes, the particle positions and the volume fractions of SiC particles. Elastoplastic material properties, strengthened matrix properties and particle–matrix interfacial behaviors are introduced to simulate the mechanical behavior of SiC/Al composites. Enough fine meshes and reasonable loads and boundaries conditions can efficiently guarantee the computing accuracy and reduce the computing cost. A lot of simulating results of SiC/Al composites are provided and verified with the related experimental results. This work makes an effective attempt to establish the relationship between the actual composite structures and the mechanical behaviors within the particle reinforced metal matrix composites.

Abstract: AZ31 magnesium alloys were deformed to 10% and to failure strain by tensile loading at room temperature. Scribed grids were drawn by a focused ion beam system (FIB) to visualize the local deformation in each grain. This showed that the magnitude of the strain was distributed non-uniformly in each grain. It was found that the low-strain grains accompanied {10–12} twins, while the severely strained grains accompanied {10–11}–{10–12} double twins. Cracks nucleated at the double twins and tended to propagate along {10–12} twin interfaces as well as within grains. Furthermore, fractography revealed three types of microstructural features: dimples, elliptic facets and sheared dimples. Most abundant were the dimples formed by ductile failure. The elliptic facets appeared to be due to crack propagation along the {10–12} twin interfaces. The sheared dimples were frequently observed in connection with localized shear deformation within the double twins. These results led us to conclude that premature and catastrophic failure of Mg alloys is mainly associated with double twins. Prevention of double twinning is essential to improve the ductility of Mg alloys.

Abstract: To comprehensively analyze the relationship between the microstructure and mechanical properties in a one-step quenched and partitioned (Q&P) steel, Q&P processes with different partitioning time were applied to a low-carbon steel. Microstructures were characterized by means of EPMA, XRD, EBSD and TEM. The dislocation density of martensite was calculated using the Williamson–Hall method. Mechanical properties were measured by uniaxial tensile tests. Results show that the microstructures consist of lath martensite accompanying with both film-like inter-lath retained austenite and blocky retained austenite. Martensite laths broaden with prolonged partitioning time. The amount of retained austenite increases first and decreases with the critical partitioning time of 100 s. The relation between the microstructure and properties was clarified by analyzing the stress–strain curves stage by stage combining with the substructure of martensite and the condition of retained austenite. The presence of retained austenite decreases the elastic limit and influences on the yield strength for its early plastic deformation. Two kinds of nano-scaled carbide appearing in the specimens partitioned longer than 100 s promote the austenite decomposition and play the main role in increasing the yield strength. The interaction of the dislocations in martensite and the transformation induced plasticity (TRIP) effect of retained austenite increase the work hardening rate and improve both the ultimate tensile strength (UTS) and the uniform elongation.

Abstract: Twinning is an important deformation mechanism for Mg alloys, which is intimately related to their mechanical behaviors, such as yield strength, strain hardening, ductility and so on. Therefore, a fundamental understanding of twinning behavior and mechanism under various conditions is essential for improving the mechanical properties of Mg alloys. This study aims to extend understanding the mechanism of {10–12} twin variant selection and twin patterns in compression of basal textured AZ31 Mg alloy sheet along transverse direction. Various twin patterns such as independent or intersecting multi-twins in a grain, paired twins in two neighboring grains, and twin chains across many grains were observed in the 3% compressed AZ31 sample. The variant selection of {10–12} twins in the various twin patterns were studied by a combined analysis of Schmid factor and strain compatibility factor, m′. Multiple twin variants can be formed in a grain and the non-Schmid twin variant could be induced by the impingement of a neighboring twin at grain boundary. Paired {10–12} twins are likely formed when the Schmid factors of both twins and their strain compatibility factor are very high. The paired twins can propagate throughout the grains and transfer to more neighboring grains (termed as twin–twin transfer behavior). Such continuous twin–twin transfer behavior across more than three neighboring grains results in the formation of twin chains. In addition, strain accommodation among the different twin variants in the same grain, and the orientation relationship between the paired twins and their common grain boundary direction are discussed. A comprehensive understanding of the twin patterns and variant selection was achieved by such a combined analysis of Schmid factor and strain accommodation factor.

Abstract: In this study, single line scans at different laser powers were carried out using selective laser meting (SLM) equipment on a pre-fabricated porous Al86Ni6Y4.5Co2La1.5 metallic glass (MG) preform. The densification, microstructural evolution, phase transformation and mechanical properties of the scan tracks were systematically investigated. It was found that the morphology of the scan track was influenced by the energy distribution of the laser beam and the heat transfer competition between convection and conduction in the melt pool. Due to the Gaussian distribution of laser energy and heat transfer process, different regions of the scan track experienced different thermal histories, resulting in a gradient microstructure and mechanical properties. Higher laser powers caused higher thermal stresses, which led to the formation of cracks; while low power reduced the strength of the laser track, also inducing cracking. The thermal fluctuation at high laser power produced an inhomogeneous chemical distribution which gave rise to severe crystallization of the MG, despite the high cooling rate. The crystallization occurred both within the heat affected zone (HAZ) and at the edge of melt pool. However, by choosing an appropriate laser power crack-free scan tracks could be produced with no crystallization. This work provides the necessary fundamental understanding that will lead to the fabrication of large-size, crack-free MG with high density, controllable microstructure and mechanical properties using SLM.

Abstract: The hybrid fabricating technique by laser additive manufacturing provides an attractive potential for manufacturing titanium alloy components. Microstructure, micro-hardness and room tensile mechanical properties of hybrid fabricated TC11 titanium alloy sample were examined. Results show that the hybrid manufactured sample consists of three typical zones: the laser additive manufactured zone (LAMZ), the wrought substrate zone (WSZ), and the bonding zone without any metallurgical defects. Superfine basket-wave microstructure forms in LAMZ and heat affected zone (HAZ) due to the rapid cooling rate. No obvious grain growth or recrystallization occurs in the HAZ. A special bimodal microstructure consisting of coarse fork-like primary α and fine β transformed microstructure is found in the transition zone due to the heat effect in α+β region. The hybrid fabricated TC11 sample has good mechanical properties with tensile strength of 1033±13 MPa and elongation of 6.8±0.2%. The fracture of hybrid sample occurs in the substrate in tensile testing, meaning that the bonding zone has better mechanical properties than the substrate.

Abstract: Uniaxial tensile tests of a typical Ni-based superalloy are conducted under the deformation temperature range of 920–1010 °C and strain rate range of 0.01–0.001 s −1 . The effects of initial δ phase (Ni 3 Nb) on the hot tensile deformation behaviors and fracture characteristics are discussed in detail. The results show that: (1) For the studied Ni-based superalloy with a large amount of δ phase, the flow stress curves are composed of three distinct stages, i.e., work hardening stage, flow softening stage and the final fracture stage. (2) The initial δ phase has significant effects on the deformation behaviors of the studied superalloy. δ phase can cause the obvious work hardening at the beginning of hot deformation, and then accelerates the flow softening by promoting the dynamic recrystallization with further straining. With the increase of initial δ phase, the strain rate sensitivity coefficient decreases firstly and then increases. (3) The combined effects of localized necking and microvoid coalescence cause the final fracture of specimens. The increase of initial δ phase increases the density of nucleus for the formation of microvoids, and promotes the nucleation and coalescence of microvoids.

Abstract: The present study concerns laser surface hardening (LSH) and melting (LSM) of AISI H13 tool steel using a high power continuous wave diode laser. Depth of surface hardened or melted layer increases with increase in incident laser energy density. Surface melting occurs at a higher laser energy density (>75 J/mm 2 ) and leads to the formation of inhomogeneous microstructure comprising non-uniform distribution of retained austenite, carbides (along inter-dendritic boundary) and martensite with their respective volume fractions varying with depth. Application of intermediate laser energy density (50–75 J/mm 2 ) yields a hardened layer with dispersion of ultrafine mixed carbides (M 23 C 6 , M 7 C 3 , MC or M 2 C). Laser treatment with a very low laser energy density ( 2 ) leads to formation of an over-tempered microstructure consisting of low carbon martensite and coarse carbide precipitates. Micro-tensile studies with specially machined samples from the surface melted zone following LSM with a laser energy density of 100 J/mm 2 records a high yield strength of 1310 MPa along with poor ductility, marked by brittle failure. On the other hand, a similar sample from laser surface hardened zone treated with a laser energy density of 62.5 J/mm 2 yielded even higher yield strength of 1460 MPa with a maximum elongation of 3.6%. Though both LSH and LSM produced higher yield strength compared to hardened and tempered AISI H13 tool steel, LSH yielded a combination of higher elongation (3.6%), than that after LSM (0.97%), with high yield strength and hence was considered a better option.

Abstract: The effects of extrusion parameters on the microstructure and tensile properties of the Mg–2Zn–0.5Ce, Mg–1Zn–1Mn–0.5Ce, and Mg–2Zn–1.5Gd alloys were investigated by conducting indirect extrusion at different temperatures, ram speeds, and extrusion ratios. All of the extruded alloys exhibited a bimodal grain structure composed of equiaxed fine recrystallized (DRXed) grains and elongated coarse unDRXed grains, except for some alloys extruded at a high temperature and a fast speed. With an increase in the temperature, speed, and ratio of extrusion, the average size and area fraction of the DRXed grains increased as a result of the temperature rise at the deformation zone caused by deformation heating. The yield strength increased with a decrease in the size of the DRXed grains according to the Hall–Petch relation, while the elongation increased with an increase in the fraction of the DRXed grains due to the suppression of {10−11}–{10−12} double twinning, irrespective of alloy composition. Due to these relationships between the microstructural characteristics and tensile properties, the yield strength decreased but the elongation increased when the temperature, speed, and ratio were increased.

Abstract: Microstructure, mechanical and corrosion properties of four alloys, Mg–2Gd–2Zn, Mg–2Gd–6Zn, Mg–10Gd–2Zn and Mg–10Gd–6Zn (all are in weight percentages), prepared by gravity permanent mold casting were investigated. The results indicated that the intermetallic phases in the Mg–2Gd–2Zn alloy consisted mainly of (Mg, Zn) 3 Gd phase whereas the Mg–2Gd–6Zn alloy consisted of both I (Mg 3 Zn 6 Gd) and (Mg, Zn) 3 Gd phases. In addition, few Mg–Gd and Mg–Zn binary phases were also present in both the alloys. Lamellar long period stacking ordered (LPSO) phase was observed in alloys containing high concentrations of Gd (Mg–10Gd–2Zn and Mg–10Gd–6Zn alloys) in addition to the continuously distributed (Mg,Zn) 3 Gd phase along the interdendritic regions and grain boundaries. A small fraction of X phase (Mg 12 ZnGd) was also present in Mg–10Gd–2Zn alloy. Mg–10Gd– x Zn alloys ( x =2,6) exhibited higher yield strength due to the higher solute contents and the presence of LPSO phase in the matrix, but showed poor elongation due to the coarse continuous second phase at the boundary. Low Gd-containing alloys showed better elongation to failure and moderate strength due to the lower volume fraction of fine scale second phases. Corrosion resistances of the alloys decreased with increase in the total amount of alloying elements. Increase in Zn content from 2% to 6% in Mg–2Gd– x Zn alloys did not alter the corrosion properties much; however, this increase in the high Gd-containing alloys had significant detrimental effects on the corrosion properties due to the significant increase in the volume of the second phases. In all the alloys, galvanic corrosion due to the second phase and filiform corrosion dominated the earlier stages of corrosion, and after long immersion times, the second phase, (Mg,Zn) 3 Gd, was found to become unstable and dissolved, leading to intergranular corrosion.

Abstract: The effects of Zr, Er and Cr additions on the microstructures, mechanical properties and corrosion resistance of Al–Zn–Mg–Cu alloy were investigated. The combined additions of Zr, Er and Cr to Al–Zn–Mg–Cu alloy led to the formation of coherent Zn, Mg, Cu, Cr-containing Al 3 (Zr, Er). These 15–25 nm secondary precipitates remarkably inhibited the recrystallization of Al matrix, and numerous fine subgrain boundaries were retained. Compared to the partial recrystallized Al–Zn–Mg–Cu–Zr alloy, the unrecrystallized Al–Zn–Mg–Cu–Zr–Er–Cr alloy exhibited higher resistance to intergranular corrosion and stress corrosion with the improved mechanical properties and fracture toughness. The synergetic effects of Zr, Er and Cr on the precipitates, recrystallization and corrosion resistance of ultra-high strength Al–Zn–Mg–Cu alloys have been discussed.

Abstract: A series of candidate alumina-forming austenitic (AFA) stainless steels designed to evaluate the effects of variation in Al, C, Cr, Mn, Nb, and Ni content on high-temperature tensile properties, creep, and oxidation/corrosion resistance were studied. The compositions assessed were based on medium Ni (20–25 wt%) and low Ni (12 wt%) AFA variations strengthened primarily by MC and/or M23C6 carbide precipitates, and a high Ni (32 wt%) AFA superalloy variation strengthened primarily by γ′-Ni3Al intermetallic precipitates. Tensile and creep properties were measured at 650 and 750/760 °C, oxidation resistance from 650 to 900 °C in air with water vapor and steam environments, and sulfidation–oxidation resistance in Ar–20%H2–20%H2O–5% H2S at 550 and 650 °C. Optimized composition ranges for different use temperatures ranges based on these evaluations are presented.