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Abstract Selective laser melting, due to the high energy density input and the small interaction time (106 K/s), can result in an ultrafine microstructure and excellent mechanical properties. However, due to the nonuniform nature of the temperature distribution and the transition from liquid to solid, high residual stresses exist. High residual stresses in the parts can increase the risk of material distortion and cause many problems, such as dimensional inaccuracy or cracks. This work systematically investigated the influence of annealing on the microstructure and mechanical properties of SLM-processed AlSi7Mg alloy parts. The residual stresses relaxed significantly after annealing. The Vickers hardness and the tensile stress greatly reduced, while the elongation increased. The fracture mode of the as-fabricated sample was ductile and brittle mixed fracture, whereas the fracture morphology of the annealed sample presented many more dimples, and the elongation also increased, which indicated the ductile mode. [ABSTRACT FROM AUTHOR]

Abstract Stir tools with different pin eccentricities were applied to the friction stir processing of Al-5052 alloy. Results show that pin eccentricity enhances material flow and refines grains of stir zone. The stir zone produced by a stir tool with 0.4 mm pin eccentricity performs the highest hardness and yield strength, attributing to the enhanced grain-boundary strengthening. [ABSTRACT FROM AUTHOR]

Abstract Revealing the microstructural parameters-mechanical properties relationship is very critical for the microstructure tailoring and properties optimization of titanium alloy. In this work, the dependence of mechanical properties on microstructural parameters of TA15 titanium alloy with tri-modal microstructure was investigated. The results indicate that at a certain lamellar α (α l) content (28%), both of yield strength (σ 0.2) and ultimate tensile strength (σ b) decrease continuously with the increase of equiaxed α (α p) content, while the decrease speed slows down at higher α p content. At a certain content of α p of 20%, σ 0.2 and σ b both increase first and then decrease as the content of α l increases. The above effect laws result from the competition between the softening effect caused by increasing the softer α p and α l contents and the strengthening effect relating to α l /β t interfaces and β t hardening. As for the elongation (δ) and reduction of area (Φ), they show an increase trend with α p content increasing at a certain α l content of 28%. However, they decrease first and then increase with α l content increasing at a certain α p content of 20%. The early plasticity decrease at lower α l content is due to the popular rapid void nucleation and cleavage fracture along α l /β t interfaces. When α p or α l content increases to higher level, the deformation compatibility and homogeneity improves, which suppress the rapid void nucleation and cleavage fracture along α l /β t interfaces. Meanwhile, the fracture mechanism changes from the coexistence of void coalescence and cleavage fracture to the void coalescence dominated fracture, thus the plasticity increases. Moreover, a back-propagation neural network model was developed to correlate the mechanical properties with microstructural parameters of tri-modal microstructure. The prediction results suggest that better combination of strength and plasticity can be achieved by controlling the contents of α p and α l in the range of 10–15% and 22–27%, respectively. [ABSTRACT FROM AUTHOR]

Commercially pure Ti grade 2 was multi-directionally forged (MDFed) at room temperature. MDFing could be successfully carried out without any cracking to a cumulative strain of ΣΔε = 2.0. Ultrafine-grained (UFGed) structure gradually developed with increasing cumulative strain mainly by mechanical twinning and kinking. An average grain size of approximately 400 nm was attained. The mechanical properties were drastically modified by the grain refinement and a superior balance of tensile strength and ductility, 710 MPa and 21%, was achieved. Additional cold rolling of the UFGed Ti possessed further better mechanical properties of 930 MPa tensile strength and 19% ductility. [ABSTRACT FROM AUTHOR]

In order to improve the plasticity of AlCoCrFeNi high entropy alloys (HEA) without obviously decreasing the strength, Co has been added to this alloy as Co is a FCC phase stabilizer. In this study, the alloying effects of Co addition on the phase types, microstructure and mechanical properties of (AlCoCrFeNi) 100- x Co x are investigated systematically. The result shows that Co element has the capacity to induce phase transformation from BCC phase to FCC phase in the (AlCoCrFeNi) 100- x Co x alloy system, and the volume fraction of FCC phase increases from 0% to 77% as the Co content increases. Compressive testing shows that Co addition has a positive effect in improving the plasticity of AlCoCrFeNi HEA, and the fracture strain increases gradually from 27% to 40%, while the ultimate strength only reduces from 2743 MPa to 2680 MPa as the Co content changes from 0 to 16 at%. The maximum ultimate compressive strength reaches 2974 MPa, while the fracture strain reaches 30%. The increased volume fraction of FCC phase is the main factor in the plasticity increases, and the valence electron concentration (VEC) increase induced by Co addition is the main factor in FCC phase volume fraction increases in this alloy. [ABSTRACT FROM AUTHOR]

During the Selective Laser Melting (SLM) process large temperature gradients can form, generating a mismatch in elastic deformation that can lead to high levels of residual stress within the additively manufactured metallic structure. Rapid melt pool solidification causes SLM processed Ti6Al4V to form a martensitic microstructure with a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. The use of high temperature pre-heating during an SLM build can assist in reducing thermal gradients, enable a more controlled cooling with the possibility to control/tailor as-built mechanical properties. In this work a high temperature SLM powder bed capable of pre-heating to 800 °C is used during processing of Ti6Al4V feedstock. The effect of powder bed temperature on residual stress formation, microstructure and mechanical properties was investigated. It was found that increasing the bed temperature to 570 °C significantly reduced residual stress formation within components and enhanced yield strength and ductility. This pre-heating temperature enabled the decomposition of α ′ martensitic microstructure into an equilibrium α+β microstructure. At 570 °C the yield strength and elongation of components was improved by 3.2% and 66.2% respectively. [ABSTRACT FROM AUTHOR]

The effects of beryllium (Be) addition on the microstructure and mechanical properties of hot-rolled Zr–3Al– χ Be alloys ( χ =0, 0.4, 0.6, and 0.8 wt%) were characterized experimentally. X-ray diffraction and optical microscopic results indicated that the complete α phase was replaced by α phase+Be 2 Zr compound in the alloys after Be addition. Meanwhile, with an increase of Be content from 0.4 wt% to 0.8 wt%, the content of Be 2 Zr particles increased in the examined Zr–3Al– χ Be alloys. Moreover, partial α recrystallization was observed in the hot-rolled Zr–3Al alloy. The degree of α recrystallization tended to increase in the alloys with Be addition. Adding Be is beneficial to the improvement of the tensile properties. The Zr–3Al–0.8Be alloy exhibited maximum ultimate tensile strength of 1103 MPa, which increased by 27% compared with that of the hot-rolled Zr–3Al alloy and the elongation remained at 6.07%. Fractography results indicated that the fracture modes of the Zr–3Al– χ Be alloys transformed from ductile to a combination of ductile and brittle with the gradual addition of Be. [ABSTRACT FROM AUTHOR]

The effects of boron additions in an amount of 0.1–2 wt%, thermomechanical processing and heat treatment on microstructure and mechanical properties of a two-phase titanium alloy Ti–6.5Al–3.3Mo–0.3Si alloy have been investigated. Depending on the boron amount, the materials under study were divided into two groups: (1) boron modified alloys containing ~0.1 wt% of boron and (2) discontinuously reinforced metal matrix Ti–TiB based composites containing 1.5–2 wt% of boron. Boron additions led to formation of TiB whiskers, which were predominantly located along boundaries of prior β-grains and α-colonies resulting in refined as-cast microstructure. Multiple 3D forging at T =650–700 °C applied for the boron modified alloys resulted in formation of ultrafine-grained microstructure and intensive breaking of TiB whiskers. Tensile properties of the Ti–6.5Al–3.3Mo–0.3Si–0.2 wt% B alloy after multiple 3D forging followed by β-heat treatment were found to be appreciably higher than those of the alloy free of boron after the same processing route that was ascribed to better controlling the β-grain size during β heat treatment. The composite materials were subjected to multiple isothermal 2D forging at T =950 °C that provided effective alignment of TiB whiskers while retaining their high aspect ratio. The hot forged composites demonstrated appreciably higher strength, creep resistance in comparison with those of the base alloy without drastic reduction in ductility. The effect of TiB whiskers orientation and morphology on the tensile properties of the composite materials is discussed. [ABSTRACT FROM AUTHOR]

Repetitive upsetting (RU), a newly developed SPD technique, was employed to process AZ91D magnesium alloy plates at 400 °C for 0, 2, 4 and 8 passes. The microstructure and texture evolution of the plates after RU were investigated and correlated with the mechanical performances at room temperature. Results show that the initial coarse grains are effectively refined, however an abnormal grain coarsening appears after 8 passes of RU. A compression texture and a shear texture, induced by specific local strains, are observed within the RU-processed plates. Both the ultimate tensile strength (UTS) and elongation to fracture of the RU-processed AZ91D alloy are improved with the increase of RU passes, exhibiting a maximum increase of 35.8% and over 75%, respectively. The yield strength (YS) increases steadily in the first 4 RU passes and then decreases slightly after another 4 passes. The mechanical anisotropy is discussed from the perspective of texture and Schmid factor. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure–property relationship of an NbTi-microalloyed ferritic steel was studied as a function of thermo-mechanical schedule using a Gleeble 3500 simulator, optical and scanning electron microscope, and atom probe tomography. Contributions to the yield stress from grain size, solid solution, work hardening, particle and cluster strengthening were calculated using the established equations and the measured microstructural parameters. With a decrease in the austenite deformation temperature the yield stress decreased, following a decrease in the number density of >20nm Nb-rich particles and ≈5nm Nb-C clusters, although the grain refinement contribution increased. To achieve the maximum cluster/precipitation strengthening in ferrite, the austenite deformation should be carried out in the recrystallisation temperature region where there is a limited tendency for strain-induced precipitation. Based on the analysis of cluster strengthening increment, it could be suggested that the mechanism of dislocation–cluster interaction is closer to shearing than looping. [Copyright &y& Elsevier]

Abstract: We report on the formation of a new bulk multimodal-grained (MGed) iron with a large true strain of 58% and high strength of 955MPa by spark plasma sintering. The new MGed structure contains fine-grained α-Fe matrix and ultrafine-grained equiaxed and acicular α-Fe enhancing phases. The large plastic strain can be attributed to the increased dislocation motion and improved shear deformation ability resulted from such MGed structure containing three different α-Fe morphologies. [Copyright &y& Elsevier]

Abstract: The microstructure and mechanical properties of Mg–8.2Gd–3.8Y–1.0Zn–0.4Zr (wt%) alloy sheets produced by combined processes of extrusion, large-strain hot rolling and ageing treatment were investigated. The sheet rolled at 300°C was mainly composed of deformed grains while that rolled at 400°C was fully recrystallized, and the LPSO phase distributed at the grain boundaries in both sheets. The sheet rolled at 300°C and then peak-aged at 200°C exhibits 0.2% proof stress (PS) of 454MPa, ultimate tensile strength (UTS) of 469MPa and elongation to failure of 1.3% at ambient temperature, and 0.2% PS of 413MPa, UTS of 457MPa and elongation to failure of 6.8% at 200°C, respectively. While the sheet rolled at 400°C and peak aged at 200°C had lower 0.2% PS and superior ductility. The improvement of strength is ascribed to the fine β′ phase precipitated within the grains, the LPSO phase and dispersed cuboid Mg–Gd–Y containing phase located at the grain boundaries. [Copyright &y& Elsevier]

Abstract: Bulk dense composites of Al–Fe alloys containing 2.5–10at% Fe were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). Supersaturated Al(Fe) solid solutions were obtained by MA for 80h. The sintered samples are composed of nano and ultrafine needle-like Al13Fe4 phase and angular-shaped Al13Fe4 phase with a size range of 1–2μm uniformly distributed in the Al matrix. Needle-like Al13Fe4 phase originates from the precipitation of supersaturated Al(Fe) solid solutions, and angular-shaped Al13Fe4 phase originates from the reaction of undissolved Fe particles and the Al melt. The hardness of the alloys varied from 1.44 to 1.97GPa and Al–10%Fe exhibited highest specific yield strength of 391.1kNm/kg. The yield strength varied from 657 to 1130MPa, depending on the Fe content. Excellent plastic strain (>14.5%) were achieved in the alloys containing 2.5–5%Fe, but the Al–10%Fe exhibited elastic behavior only, failing without any macroscopic yielding or plastic strain. The lack of plastic performance of Al–10%Fe is attributed to large amount of undissolved Fe particles reacted with the Al melt to form angular-shaped Al13Fe4, leading to the absence of soft α-Al phase. [Copyright &y& Elsevier]

Abstract: Brittle low-strength and low-toughness pressable dental ceramic can be reinforced by ductile elongated gold-particles (GP). A customized crown structure can be adequately strengthened by distributing GP only in critical sections of the crown, where high tensile stresses are experienced. In the present study, a non-uniformly structured ceramic–matrix composite with excellent interfacial bonding, twofold fracture toughness and strength at desired locations, is fabricated using pressable dental ceramic and GP. The layout pattern and sequence of different GP/ceramic powder mixtures, high-temperature flow properties of these mixtures during hot-pressing and the sample mold geometry are used to control the distribution and locations of GP for selective toughening and strengthening. Nano-crystalline structures of the pressable ceramic–matrix and the nano-scaled interfacial region around GP have been revealed by high-magnification field-emission scanning electron microscopy. Toughening and strengthening mechanisms of the elongated GP including residual stresses from composite processing and ductile fracture of GP are discussed together with SEM observations. Bulk flexural strength and local micro-indentation fracture and deformation characteristics of the selective-toughened ceramic/metal composite have been compared to those of the monolithic pressable ceramic to validate the toughening and strengthening mechanisms. [Copyright &y& Elsevier]

Abstract: Constrained groove pressing (CGP) process is one of the favorable severe plastic deformation methods to fabricate ultra-fine grain sheet materials. In this paper, commercial pure aluminum sheet was subjected to CGP process using conventional and covered sheet casing (CSC) methods. Three dies with different groove lengths and angles (t =1mm and θ =45°; t =1.8mm and θ =45°; t =1mm and θ =53°) were designed and manufactured. Mechanical properties including strength, elongation to failure and hardness are obtained and discussed. The influences of these two methods and die groove angle on specimen strain behavior and required force value are analyzed by the finite element method in plain strain condition. The results show that the CSC method leads to better hardness distribution, uniform strain dispersal and finer surface quality as compared to conventional method. Moreover, the new method is a good candidate for substituting the conventional CGP process. [Copyright &y& Elsevier]

Abstract: A high strength bulk nano-crystalline Al–1Fe–(0-1)Zr alloys were processed by mechanical alloying and spark plasma sintering. The high yield strength of ∼850MPa was achieved in an Al–1Fe–0.5Zr alloy with 19% strain in compression. The high strength was attributed to the nanosized Al–Fe grains and the dispersion of fine Al6Fe particles. High temperature compressive yield strength of 455MPa and elongation of 16.7% was achieved at 250°C. [Copyright &y& Elsevier]

Abstract: The millimetre scale W balls reinforced Zr-base bulk metallic glass composite was prepared using infiltration technique. The compressive deformation behaviours of the composite at two strain rates of 1×10−4 s−1 and 1×10−2 s−1 were investigated. The enhanced plasticity is attributed to the effect of W balls, which constrain shear bands propagation and promote the formation of multiple shear bands. The properties of the composite depend evidently on the strain rates. The high strain rate leads to the higher strength and larger plasticity. [Copyright &y& Elsevier]

Abstract: The effects of age-forming on microstructures and mechanical properties of 7050 Al alloy were investigated in this work. The alloy was subjected to age-forming as well as stress-free ageing at 160°C for 6, 12, 18 and 24h, and its microstructures were characterized by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). It was shown that creep might lead to grain elongation during age-forming, and the applied stress induces the coarsening of precipitates in 7050 Al alloy. The texture in the alloy was also influenced by age-forming. Consequently, the differences in microstructures result in differences in mechanical properties of age-forming versus traditional stress-free ageing. The ultimate tensile strength of age-formed samples were slightly lower than that of stress-free aged samples, while the yield strength of age-formed samples were apparently lower than that of stress-free aged samples. Specifically, the elongation of samples age-formed displays apparently decrease. [Copyright &y& Elsevier]

Abstract: Compression tests of the 50vol% TiC x /Al and the 50vol% TiB2/Al composites have been performed in the strain rate range from 1×10−4 s−1 to 1×10−1 s−1, and the effect of strain rate on the compressive behavior and the work-hardening capacity (H c) of them have been investigated. The yield strength () and the H c of both the composites are strain rate sensitive, while the ductility is insensitive to the strain rate. The ultimate compressive strength () of the TiB2/Al composites is not significantly influenced by the strain rate, while that of the TiC x /Al composite increases from 615MPa to 826MPa with the strain rate increasing. Under various strain rates, the of the TiB2/Al composite is significantly better than that of the TiC x /Al composite, while its ductility and H c are lower than that of the TiC x /Al composite. At the strain rate of 1×10−4 s−1, the of the TiB2/Al composite and the TiC x /Al composite are 1071MPa and 615MPa, respectively; while the H c of them are 0.87 and 0.99, respectively. [Copyright &y& Elsevier]

Abstract: 55vol.% B4C/Al composites reinforced with 3μm and 20μm particles respectively were prepared by squeeze casting. Effect of particle size on quasi-static and dynamic compression behaviors of the composites was investigated. The results show that the quasi-static compression strength of the composite with 3μm particles is almost twice that of the composite with 20μm particles. The reason is that the connectivity architectures of the composite with small particles are more stable than that of the composite with big particles due to the enhancement of in situ matrix by geometrically necessary dislocation. In addition, the dynamic compression strength and the fracture strain first increase with the increasing impact velocity and then decrease when the impact velocity is increased from 28m/s to 33m/s for the composite with 20μm particles, and no similar phenomenon occurs for the composite with 3μm particles. The size-dependent architecture stability of the connected reinforcement is the main contribution to the different dynamic deformation behaviors of the composites during adiabatic compression. [Copyright &y& Elsevier]

Abstract: Tensile fracture behaviors and mechanisms of 2D C/SiC-BC x composite were investigated. Results showed that the 2D C/SiC-BC x composite exhibited similar tensile strength (about 250MPa), but different fracture mode and damage evolution compared to that of 2D C/SiC composite under similar monotonic tensile stress condition at room temperature. The initial modulus and fracture modulus of 2D C/SiC-BC x composite obtained under tensile cyclic loading at RT were about 98.6GPa and 54.5GPa, respectively. At high temperatures, the tensile strengths were lower than that at room temperature and behaved as a step-like stress–strain curve. [Copyright &y& Elsevier]

Abstract: Mg–5.25wt.% Zn–0.6wt.% Ca alloy was subjected to equal channel angular pressing (ECAP) at 250°C using route A, Bc and C, respectively, in order to investigate the effect of shearing strain path on microstructure, texture and mechanical properties. Route Bc was the most effective in grain refinement and generating high angle grain boundaries, while route A was the least effective. Different textures were developed when a different ECAP route was applied. After ECAP using route A, the tensile yield strength (TYS) got much higher than those of the as-extruded alloy, which was primarily resulted from the strengthening due to the grain refinement. Because the Schmid factor for basal slip system was changed only a little, the texture soften effect was not obvious. After ECAP using route C, TYS of the Mg alloy was significantly decreased although the grain size was significantly refined, because the texture softening effect was dominant over the strengthening due to the grain refinement. [Copyright &y& Elsevier]

Abstract: Bulk ultrafine-grained nickel specimens having grain sizes in the range of 0.25–5μm were processed by a spark plasma sintering method. The resulting microstructures were characterized by electron backscattering diffraction, transmission electron microscopy and X-ray diffraction analysis. Compression tests were carried out at room temperature and at a strain rate of 1.6×10−4 s−1. It was found that the fine-grained microstructure and the presence of NiO phase were the main strengthening factors in the as-processed bulk materials. The contribution of the oxide phase to strengthening was even more pronounced for lower grain sizes. This contribution was calculated as the difference between the measured strength and the value obtained from a Hall–Petch plot of oxide-free samples, and this yielded a flow stress increment of about 635MPa for the lowest grain size studied here. In addition, a transition from work-hardening to -softening occurred for materials having a mean grain size smaller than about 300nm and having boundaries that could have been weakened by the presence of a high amount of NiO phase. [Copyright &y& Elsevier]

Abstract: An experimental investigation into the effect of microstructural changes, which occur during post-extrusion annealing of a Mg based AZ21 alloy, on tensile and fatigue properties is conducted. Mechanical properties in the as-cast, as-extruded, and microstructural states that correspond to recovery, recrystallization and grain growth stages of annealing are compared. Results show that these microstructural changes do not alter the yield strength of the alloy markedly whereas significant differences were noted in the ultimate tensile strength as well as ductility. The initiation of abnormal grain growth (or secondary recrystallization) renders the tensile stress–strain response elastic perfectly plastic and results in a large drop in ductility, as high as ∼60% during intermediate stages of abnormal grain growth, vis-à-vis the ductility of the as-extruded alloy. While the fatigue performance of all the wrought alloys is far superior to that of the as-cast alloy, as expected, abnormal grain growth leads to a marked decrease in the endurance limit. Possible microscopic origins of these are discussed. [Copyright &y& Elsevier]

Abstract: An Al–Cu–Li alloy has been severely deformed by rolling over a wide range of temperatures and the evolution of deformation substructure and precipitation examined. A high dislocation density is retained at all temperatures, with dislocations forming cell and subgrain arrangements. There is a greater extent of recovery and coarsening at the higher temperatures. Much finer precipitate particles are seen after rolling than after simple ageing, and grain boundary precipitation is much less extensive. Particle size is reduced both by extensive precipitation on dislocations and by the breakage of previously formed precipitates by the subsequent high strain. Material strength is increased by the presence of the deformation substructure and the fine precipitates, while ductility is improved only when extensive recovery has taken place. More severe deformation, controlling the extent of precipitation, is necessary to refine structures further. [Copyright &y& Elsevier]

Abstract: The anisotropic compressive behavior of closed-cell Al–Si alloy foams prepared by melt foaming process has been investigated. The results show that the stress drop ratio measured in the longitudinal direction is higher than that in the transverse direction. Moreover, the stress drop ratio decreases with increasing relative density of foamed specimen. The shape anisotropy ratio R increases firstly and decreases afterward with increasing relative density. The Young''s modulus ratio and yield stress ratio for closed-cell Al–Si alloy foams were investigated and compared with G&A model. A linear relationship between Young''s modulus ratio, yield strength ratio and anisotropy ratio has been obtained. The Young''s modulus is more sensitive to cell shape than yield stress. [Copyright &y& Elsevier]