Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2022"

High-strength titanium alloys for aerospace engineering applications: A review on melting-forging process

Abstract: As a crucial branch for titanium industry, high-strength titanium alloys (HS-TAs, with UTS ≥ 1100 MPa) are indispensable structural materials for advanced engineering applications such as aerospace and marine fields. Along with the expansion of HS-TAs’ market, achieving satisfying synergies of high strength, high ductility (elongation ≥ 6%) and high toughness (KIC ≥ 50 MPa⋅m1/2) has been identified as the uppermost technical bottleneck for their research and development. To overcome the challenge, two primary strategies have been initiated by the titanium community, developing novel alloys and innovating processing technologies. For the former, a dozen of newly-developed alloys were reported to exhibit excellent strength-ductility-toughness combinations, including Ti-5553, BT22, TC21 and Ti-1300, for which the ideal mechanical performances were based on specific microstructures realized by low impurity rate (e.g. oxygen content ≤ 0.15 wt%), complicated processing and complex heat treatment. For the latter, several innovatory forging and heat treatment technologies were originated for the mature alloys to optimize their balanced property by extraordinary microstructural characteristics. In this review, we provide a comprehensive overview over the research status, processing and heat treatment technologies, phase transformation, processing-microstructure-property correlation and strengthening-toughening mechanism of HS-TAs for aerospace engineering applications manufactured via melting-forging process. Finally, the prospects and recommendations for further investigation and development are proposed based on this review.

Interface formation and deformation behaviors of an additively manufactured nickel-aluminum-bronze/15-5 PH multimaterial via laser-powder directed energy deposition

Abstract: Additive manufacturing (AM) of a nickel-aluminum-bronze (NAB)/15-5 PH multimaterial by laser-powder directed energy deposition (LP-DED) accomplished a combination of excellent mechanical performance and high corrosion resistance. An NAB/15-5 PH interface without cracks and lack of fusion was achieved, which was characterized with an interlayer of FexAl dendrites. The formation of the interfacial characteristics was attributed to a synthetic effect of liquid phase separation, Marangoni convection, and atom diffusion. A miscibility gap was generated by a high degree of supercooling in the melt pool, and 15-5 PH solidified prior to NAB to form a dendritic interlayer. Marangoni convection occurred to promote the Al atom diffusion from NAB to 15-5 PH, contributing to the formation of the FexAl phase at the interface. The multimaterial sample possessed higher ultimate tensile strength of 754.64 MPa in the transverse direction and 854.57 MPa in the longitudinal direction as compared to that of copper/steel counterparts fabricated by AM. The multimaterial printed by LP-DED exhibited different deformation mechanisms in the transverse and longitudinal directions. In the transverse direction, NAB contributed more deformation than 15-5 PH and determined the improved ductility of the multimaterial; in the longitudinal direction, the brittle FexAl dendrites constrained the deformation of NAB and 15-5 PH, which resulted in the early failure of the multimaterial. The multimaterial tended to undergo cracking at the interface of the FexAl and Cu phases under stress concentration, which was induced by their crystal incoherence.

Electron beam freeform fabrication of NiTi shape memory alloys: Crystallography, martensitic transformation, and functional response

Abstract: In this work, NiTi shape memory alloys parts were additively manufactured using electron beam freeform fabrication (EBF 3 ) under different processing parameters, including the beam current, travel speed, and wire feeding speed. The forming quality, phase composition, microstructure change, crystallography, martensitic transformation, shape memory and superelastic responses were systematically investigated. All deposits mainly consisted of B2-austenite at room temperature, and a handful of B19′-martensite and submicron-sized Ti 4 Ni 2 O x precipitates were also detected. The martensitic transformation of NiTi alloys prepared by EBF 3 -technique possessed an individual reversible path between B2 and B19′ upon heating/cooling. The optimized deposit possessed the best comprehensive properties, where the values of the relative density, shape memory recovery and superelastic recovery ratios were 99.6%, 98.95%, and 55.78%, respectively. Furthermore, the dependence of the martensitic transformation behavior on the thermomechanical condition and the relationship between plastic deformation and phase transformation during superelastic deformation are discussed in detail. Our work details that the EBF 3 provides a suitable way for the complex fabrication of large-scaled parts based on shape memory alloys. • NiTi shape memory alloys parts were additively manufactured using electron beam freeform fabrication under different processing parameters. • The fabricated NiTi alloys presented a single reversible martensitic transformation (B2 ↔ B19’) upon heating/cooling. • The optimized deposit had shape memory recovery and superelastic recovery ratios of 98.95%, and 55.78%, respectively. • The dependence of the martensitic transformation behavior on the thermomechanical condition is discussed.

Effects of cryogenic treatment on microstructure and mechanical properties of AZ31 magnesium alloy rolled at different paths

Abstract: In this study, the AZ31 Mg alloy sheets were fabricated by the traditional process of air cooling after rolling and a new process of cryogenic treatment after rolling. The effect of cryogenic treatment after rolling on the mechanical properties, texture and microstructure of AZ31 Mg alloys were studied. The results showed that the {10–12} extension twin, {10–11} contraction twin and {10–12}-{10–11} double twin were observed in the samples of cryogenic treatment after rolling. All samples mainly exhibited the {0001} <11–20> basal texture. Compared with the sample of air cooling after rolling, the basal texture intensity of cryogenic treatment after rolling samples was significantly weakened. The mechanical properties of the samples of cryogenic treatment after rolling were remarkably enhanced with respect to the air cooling after rolling sample. The sample of cryogenic treatment after the rolling that was rotated by 45° along the rolling direction (RD) exhibits the highest ultimate tensile strength (∼283 MPa) and increases by 18.4% compared to the air cooling after rolling sample. This is attributed to twin strengthening, fine-grained strengthening, precipitation strengthening, grain boundary strengthening and dislocation strengthening. The elongation of the sample of cryogenic treatment after the rolling that was rotated by 45° along the RD increased by 87.0% by contrast to the air cooling after rolling sample, reaching ∼25.8%, attributing to grain refinement, the weakening of texture and the activation of pyramidal slip.

Interface formation and deformation behaviors of an additively manufactured nickel-aluminum-bronze/15-5 PH multimaterial via laser-powder directed energy deposition

Abstract: Additive manufacturing (AM) of a nickel-aluminum-bronze (NAB)/15-5 PH multimaterial by laser-powder directed energy deposition (LP-DED) accomplished a combination of excellent mechanical performance and high corrosion resistance. An NAB/15-5 PH interface without cracks and lack of fusion was achieved, which was characterized with an interlayer of FexAl dendrites. The formation of the interfacial characteristics was attributed to a synthetic effect of liquid phase separation, Marangoni convection, and atom diffusion. A miscibility gap was generated by a high degree of supercooling in the melt pool, and 15-5 PH solidified prior to NAB to form a dendritic interlayer. Marangoni convection occurred to promote the Al atom diffusion from NAB to 15-5 PH, contributing to the formation of the FexAl phase at the interface. The multimaterial sample possessed higher ultimate tensile strength of 754.64 MPa in the transverse direction and 854.57 MPa in the longitudinal direction as compared to that of copper/steel counterparts fabricated by AM. The multimaterial printed by LP-DED exhibited different deformation mechanisms in the transverse and longitudinal directions. In the transverse direction, NAB contributed more deformation than 15-5 PH and determined the improved ductility of the multimaterial; in the longitudinal direction, the brittle FexAl dendrites constrained the deformation of NAB and 15-5 PH, which resulted in the early failure of the multimaterial. The multimaterial tended to undergo cracking at the interface of the FexAl and Cu phases under stress concentration, which was induced by their crystal incoherence.

Laser-based directed energy deposition (DED-LB) of advanced materials

Abstract: Directed energy deposition (DED) has matured into an essential additive manufacturing (AM) branch. DED has been broadly implemented in the design and fabrication of novel materials. These include metals, ceramics, and composites. Successful DED operation requires a good understanding of many critical phenomena, including laser-material interactions, fundamentals of casting and solidification of alloys, welding metallurgy and joining interfaces, along with microstructure-mechanical properties relations. Also critical are powder flowability, heat transfer, and various machine-related parameters. Several review articles have been published in recent years on metal AM via powder bed fusion (PBF) and DED, focusing on either a specific material system, mapping the recent technologies for AM, or issues related to the deposition process or material properties. Yet, no recent review is dedicated to a comprehensive presentation of material systems, design, fabrication, challenges, and the relationship between microstructures and mechanical properties of various DED'ed material families. Since the DED-based approach is becoming popular to manufacture bimetallic and multi-material structures, repair high-value structures, and alloy design, this comprehensive review focuses on materials design via DED, including a survey of a variety of monolithic and multi-material compositions. Finally, the critical challenges and opportunities in this area are highlighted.

Correlation between tensile properties, microstructure, and processing routes of an Al–Cu–Mg–Ag–TiB2 (A205) alloy: Additive manufacturing and casting

Abstract: This paper aims at assessing microstructure/processing route/mechanical property correlation of a high-strength TiB2/Al–Cu–Mg–Ag aluminum alloy, namely A205 alloy, additively manufactured via laser powder bed fusion (LPBF) versus the cast counter material. To this end, high magnification advanced microstructural characterization in connection with the tensile flow and strain hardening (i.e., work hardening) behavior of the materials were studied. Ambient temperature uniaxial tensile tests, based on ASTM E8/E8M, were conducted on the LPBF and cast as well as post-heat treated (T7: overaged and stabilized) materials were performed systematically. Results show a pronounced discontinuous yielding for the LPBF material along with extended ductility. The tensile curve of the LPBF T7 heat-treated material showed some plastic instabilities (Portevin–Le Chatelier effect) in the inelastic region. However, none of these phenomena were observed in the cast material, while ductility is limited. These responses were attributed to the very high cooling/solidification rates experienced by the LPBF materials, which result in a very fine equiaxed and supersaturated aluminum grain structure as compared with the coarse-grained structure of the cast counter material.

Understanding the nanostructure evolution and the mechanical strengthening of the M50 bearing steel during ultrasonic shot peening

Abstract: Ultrasonic shot peening (USP) is a surface engineering technology used to enhance the mechanical properties of the components during manufacturing. M50 steel is one of the commonly used materials for aerospace bearings. In this study, surface nanocrystallization of the high-strength M50 bearing steel is performed at room temperature via USP technology. The materials characterizations show that the thickness of the lath martensite in the M50 bearing steel has been refined down to 10 nm. The extremely fine nanostructured M50 martensite increases the mechanical strength significantly at the nanoscale. Nanoindentation tests show that the nanohardness of the nanostructured M50 is 12.43 GPa, which is 38% higher than that of the as-received matrix materials with a value of 9.03 GPa. Additionally, the microstructure evolution of the M50 during the USP process is investigated and the grain refinement mechanism for M50 is revealed. EBSD characterization results confirm the transformation of the low angle grain boundaries to high angle grain boundaries and the formation of the equiaxed ultrafine grains. The decomposition of the carbides in the M50 during grain refinement is observed. This indicates that in addition to the diffusion of C, the decomposition of the carbides is also influenced by carbide-forming elements. This work deepens the current understanding of the grain refinement of the M50 bearing steel during the USP process and its mechanical strengthening at the nanoscale.

Effect of micron-Ti particles on microstructure and mechanical properties of Mg–3Al–1Zn based composites

Abstract: In the field of metal matrix composites, it is a great challenge to simultaneously improve the strength, ductility and elastic modulus of magnesium matrix composites (MMCs). In this work, Mg–3Al–1Zn composites reinforced with micron-Ti particles were prepared by powder metallurgy. The results show that the yield strength, elongation and elastic modulus simultaneously were increased with increasing content of Ti particles. Through diffusion, Ti and Al formed TiAl phase at the edge of Ti particle. A nanoscale layer of MgO formed between TiAl and magnesium (Mg) matrix. The 9Ti/Mg–3Al–1Zn composite obtained the best comprehensive mechanical properties with yield strength, ultimate tensile strength and elongation of 264 MPa, 294 MPa and 8%, respectively. The increased strength is mainly due to the grain refinement and strong interfacial bonding between Ti particles and Mg matrix. The improved ductility is the result of refined grains, weakened texture and collaborative deformation of Ti particles.

Grain refinement and strengthening of 316L stainless steel through addition of TiC nanoparticles and selective laser melting

Abstract: In this work, strengthening of 316L stainless steel was achieved through addition of 1 and 3 wt% TiC nanoparticles. The TiC nanoparticles and 316L powders were mixed using low energy ball milling and then processed by selective laser melting. The grains are significantly refined from 16.8 μm to 6.9 μm with the addition of 3 wt% TiC nanoparticles. Addition of 1 and 3 wt% TiC nanoparticles remarkably increased yield strength (694 MPa and 773 MPa) and ultimate tensile strength (888 MPa and 988 MPa). Meanwhile, the tensile elongation was kept at a high level (47% and 32%). The strength enhancement primarily results from Orowan strengthening by the nanoparticles, and the reduction of ductility is related to the suppression of twinning induced plasticity.

Microstructure and anisotropic mechanical properties of selective laser melted Ti6Al4V alloy under different scanning strategies

Abstract: Apparent anisotropies have been found in the microstructure and mechanical properties of selective laser melted (SLM) Ti–6Al–4V alloy, which will significantly influence the comprehensive performance of the SLM parts. In this study, the effect of scanning strategies on material anisotropy, including surface morphology, microstructure, microhardness, quasi-static and dynamic mechanical properties, were comprehensively investigated with 0°, 67.5°, and 90° scanning strategies. The SLM Ti6Al4V alloy prepared by the 0° scanning strategy exhibited the most pronounced anisotropy. The size of the primary columnar crystals on the front surface (158–173 μm) is approximately 1.8–3.2 times larger than that of the top surface (54–88 μm), and the microhardness at the top surface is ∼30% higher than that of the front surface. In addition, the modified Johnson-Cook constitutive model for the SLM Ti6Al4V alloy is developed based on the results of quasi-static compression and dynamic compression with the average absolute relative error controlled within 10%. This provides a reliable solution and an effective material constitutive model for modelling and simulation of the machining SLM parts with higher accuracy and enhanced physical meaning.

Laser additive manufacturing of nano-TiC particles reinforced CoCrFeMnNi high-entropy alloy matrix composites with high strength and ductility

Abstract: CoCrFeMnNi high-entropy alloy (HEA) matrix composites reinforced with nano-sized TiC particles were successfully fabricated by laser powder bed fusion (LPBF), which is widely used laser additive manufacturing technology. The microstructural evolution and mechanical properties of the HEA composites fabricated at various processing conditions, were investigated. The LPBF-fabricated HEA composites showed typical hierarchical microstructure characteristics including epitaxially grown columnar grains, submicron-sized cellular substructures, nano-sized precipitates, and a high density of dislocations. The rapid solidification process inherent to LPBF resulted in micro segregations of Mn and Ni along the cell walls, which contributed to a pronounced impurity-drag effect toward the present dislocations. TiC 1-x with a size of 35–100 nm uniformly precipitated along the cell boundaries as well as inside the cells. Entangled dislocations easily formed around the cell walls and the nano-TiC 1-x precipitates leading to a high dislocation density in the LPBF-fabricated HEA composites. The HEA composites with 1 wt% nano-TiC addition exhibited a promising mechanical performance readily characterized by a yield strength of 779 MPa, tensile strength of 940 MPa and an elongation of 30%. The advantageous combination between strength and ductility can be attributed to the prevailing subgrain boundaries, solid solution strengthening, grain refinement, nano-precipitation hardening, dislocation hardening, plasticity and storage. This work demonstrates the importance of hierarchical microstructures of HEA materials via incorporation of nano-sized TiC particles for imparting exceptional mechanical properties.

Microstructure and anisotropic mechanical properties of selective laser melted Ti6Al4V alloy under different scanning strategies

Abstract: Apparent anisotropies have been found in the microstructure and mechanical properties of selective laser melted (SLM) Ti–6Al–4V alloy, which will significantly influence the comprehensive performance of the SLM parts. In this study, the effect of scanning strategies on material anisotropy, including surface morphology, microstructure, microhardness, quasi-static and dynamic mechanical properties, were comprehensively investigated with 0°, 67.5°, and 90° scanning strategies. The SLM Ti6Al4V alloy prepared by the 0° scanning strategy exhibited the most pronounced anisotropy. The size of the primary columnar crystals on the front surface (158–173 μm) is approximately 1.8–3.2 times larger than that of the top surface (54–88 μm), and the microhardness at the top surface is ∼30% higher than that of the front surface. In addition, the modified Johnson-Cook constitutive model for the SLM Ti6Al4V alloy was developed based on the results of quasi-static compression and dynamic compression with the average absolute relative error controlled within 10%. This provides a reliable solution and an effective material constitutive model for modeling and simulation of the machining SLM parts with higher accuracy and enhanced physical meaning.

Effects of heat treatment before extrusion on dynamic recrystallization behavior, texture and mechanical properties of as-extruded Mg-Gd-Y-Zn-Zr alloy

Abstract: The effects of heat treatment before extrusion on the dynamic recrystallization (DRX) behavior, texture, and mechanical properties of the extruded Mg-9.8Gd-3.5Y-2.0Zn-0.4Zr (wt. %) alloy were comparatively investigated in this work. A large number of dendritic microstructures in as-homogenized alloy and intergranular block-shaped LPSO phases in as-aged alloy were found to promote the operation of particle-stimulated nucleation (PSN) and discontinuous dynamic recrystallization (DDRX) mechanisms, which further accelerate growth of dynamic recrystallized grains due to the lack of solute drag and dynamic precipitation. Also, we determined the nucleation of dynamic recrystallization affects little to the formation of abnormal texture, whereas the shear stress results from the flow velocity gradient and the pyramidal-2 slip during the growth of the dynamic recrystallized grains leading the formation of abnormal texture. However, the solution treatment before extrusion could effectively eliminate the dendritic microstructures and increase the solid solubility of the matrix, which would facilitate the occurrence of continuous dynamic recrystallization (CDRX) and dynamic precipitation during the hot extrusion process. Meanwhile, CDRX and dynamic precipitation co-contribute to the formation of a bimodal microstructure that composed of coarse deformed grains with basal orientation and fine dynamic recrystallized grains with random orientation. The observed bimodal microstructure, fine dynamic precipitations, strong fiber texture, and substructure thus well explained the improved strength and elongation of samples extruded with the solution treated materials.

Experimental investigation on the effect of gas tungsten arc welding current modes upon the microstructure, mechanical, and fractography properties of welded joints of two grades of AISI 316L and AISI310S alloy metal sheets

Abstract: In this investigation, dissimilar welded joints of AISI 316 L and AISI 310S stainless steels were produced using continuous and pulsed modes current of the gas tungsten arc welding process. A filler metal type ER309L was used to strengthen the welded joints. The fracture mode of the tensile and Charpy impact test samples was studied using a field emission scanning electron microscope (FE-SEM). Results showed that the welded joints were broken in the 316 L steel side during the tensile test due to the presence of lower alloying elements in this steel compared with the AISI 310S stainless steel. As well, microhardness and Charpy impact tests results showed that changing the welding current from continuous to the pulsed one increased the values of these two mentioned attributes. Fractography analysis, performed on the fracture surfaces of both joints, showed a completely ductile fracture under both tensile and Charpy impact tests. Moreover, microstructural observations showed that the weld metal (WM) structure was austenitic-ferritic (AF) and contained columnar and equiaxed dendrites. Changing the welding current from the continuous to the pulsed one led to the transformation of the columnar dendrites to the very fine equiaxed dendrites. This welding current variation reduced the dendrite size of the WM and decreased the area of the unmixed zone (UMZ). Finally, XRD results indicated that austenite was the predominant phase in the welded joints.

Crack-resistant σ/FCC interfaces in the Fe40Mn40Co10Cr10 high entropy alloy with the dispersed σ-phase

Abstract: We report the role of morphology and size of the σ-phase on mechanical properties of the Fe40Mn40Co10Cr10 non-equiatomic high entropy alloy. The dispersed and fine σ-precipitates formed after cold-rolling and annealing at 700 °C for 1 h lead to an increase in strength without severe ductility loss compared to the material without the σ-phase. However, the coarsened and connected σ-phase formed after prolonged annealing at 700 °C for 100 h results in early failure due to the activation of microcracks along σ/σ interfaces. The σ/FCC interface could act as strong obstacles for dislocation motion and could effectively relieve the stress concentration by activating deformation twins or dislocations. The σ/FCC interfaces are excellent in terms of hardening, accommodation of plastic deformation, and stress relaxation leading to the observed crack resistance. Therefore, we suggest that increasing σ/FCC interfaces by controlling size and dispersion of the σ-phase is essential to develop HEAs with an excellent strength-ductility combination and damage tolerance.

Effect of the aging process and pre-deformation on the precipitated phase and mechanical properties of 2195 Al–Li alloy

Abstract: Third-generation Al–Li alloys, represented by 2195 Al–Li alloy, have considerable application opportunities in lightweight aerospace structures. This work investigates the effect of the aging temperature, single/double aging strategies and pre-deformation on the precipitation behaviors of 2195 Al–Li alloy. The influence and correlation between the precipitated phase and mechanical properties, especially the tensile strength, ductility and failure fracture mode, were further studied. The results indicate that increasing the aging temperature can significantly improve the aging kinetics, thereby precipitating the large size T1 phase and decreasing the number density accordingly. Double aging is beneficial to the uniform size and distribution of the precipitated phase, which reduces the unevenness of micro-deformation. The pre-deformation increases the number of nucleation sites of the T1 phase, thereby significantly reducing the average size and increasing the number density. Pre-deformation can directly increase the strain hardening by generating dislocations while indirectly increasing the precipitation strengthening by refining the size and increasing the number density of the T1 phase. The mechanical properties and fracture mode of materials are related to the dislocation density and the type, size, distribution and number density of the precipitated phase.

Effect of the aging process and pre-deformation on the precipitated phase and mechanical properties of 2195 Al–Li alloy

Abstract: Third-generation Al–Li alloys, represented by 2195 Al–Li alloy, have considerable application opportunities in lightweight aerospace structures. This work investigates the effect of the aging temperature, single/double aging strategies and pre-deformation on the precipitation behaviors of 2195 Al–Li alloy. The influence and correlation between the precipitated phase and mechanical properties, especially the tensile strength, ductility and failure fracture mode, were further studied. The results indicate that increasing the aging temperature can significantly improve the aging kinetics, thereby precipitating the large size T1 phase and decreasing the number density accordingly. Double aging is beneficial to the uniform size and distribution of the precipitated phase, which reduces the unevenness of micro-deformation. The pre-deformation increases the number of nucleation sites of the T1 phase, thereby significantly reducing the average size and increasing the number density. Pre-deformation can directly increase the strain hardening by generating dislocations while indirectly increasing the precipitation strengthening by refining the size and increasing the number density of the T1 phase. The mechanical properties and fracture mode of materials are related to the dislocation density and the type, size, distribution and number density of the precipitated phase.

Synergistic effect of Nb and Mo alloying on the microstructure and mechanical properties of CoCrFeNi high entropy alloy

Abstract: Improving the strength of face-centered cubic high entropy alloy, CoCrFeNi, has been a research focus in recent years. In this paper, to further enhance its strength for engineering applications, a series of CoCrFeNi-(Nb, Mo) high entropy alloys were prepared using vacuum arc-melting. The synergistic effect of alloying by Nb and Mo on microstructure and mechanical properties of CoCrFeNi-based high entropy alloy were investigated. The criterion Md‾ was used to evaluate the phase evolution for current alloy system. Through synergistic alloying with Nb and Mo, a new kind of Laves phase with hexagonal close-packed structure was formed. The Laves phase is semi-coherent with the face-centered cubic matrix. Furthermore, through adjusting the ratio of Nb and Mo, fine lamellar structure Laves phase was obtained. The synergistic precipitation strengthening and solid-solution effect lead to high yield and fracture strength of 426 MPa and 714 MPa, maintaining high elongation of 17.4%. Fracture observation shows that the face-centered cubic matrix of the alloy system is ductile fracture, and the Laves phase is cleavage fracture. The results of this research would provide a basis for strengthening face-centered cubic high entropy alloy through synergistic alloying effect.

Synergistic effect of Nb and Mo alloying on the microstructure and mechanical properties of CoCrFeNi high entropy alloy

Abstract: Improving the strength of face-centered cubic high entropy alloy, CoCrFeNi, has been a research focus in recent years. In this paper, to further enhance its strength for engineering applications, a series of CoCrFeNi-(Nb, Mo) high entropy alloys were prepared using vacuum arc-melting. The synergistic effect of alloying by Nb and Mo on microstructure and mechanical properties of CoCrFeNi-based high entropy alloy were investigated. The criterion M d ‾ was used to evaluate the phase evolution for current alloy system. Through synergistic alloying with Nb and Mo, a new kind of Laves phase with hexagonal close-packed structure was formed. The Laves phase is semi-coherent with the face-centered cubic matrix. Furthermore, through adjusting the ratio of Nb and Mo, fine lamellar structure Laves phase was obtained. The synergistic precipitation strengthening and solid-solution effect lead to high yield and fracture strength of 426 MPa and 714 MPa, maintaining high elongation of 17.4%. Fracture observation shows that the face-centered cubic matrix of the alloy system is ductile fracture, and the Laves phase is cleavage fracture. The results of this research would provide a basis for strengthening face-centered cubic high entropy alloy through synergistic alloying effect.

Evolution of microstructure and mechanical properties in 2205 duplex stainless steels during additive manufacturing and heat treatment

Abstract: Metal additive manufacturing (AM) offers exceptional design freedom, but its high thermal gradients often generate non-equilibrium microstructures with chemical and interfacial instabilities. Steels that solidify as δ-ferrite often experience a further solid-state phase transformation to austenite during AM. The detailed nature of this phase transformation during AM is yet to be fully understood. Duplex stainless steel, which is known for its unique combination of high corrosion resistance and mechanical properties, is a suitable alloy to further study this phase transformation. The current study aims to gain novel insights into solid-state phase transformations and mechanical properties of duplex stainless steels during laser powder-bed fusion (LPBF). As-printed microstructures exhibit significant deviations when compared to conventionally manufactured counterparts in terms of phase balance and morphology, elemental partitioning, and interface character distribution. During LPBF, only a small fraction of austenite forms, mostly at the ferrite-ferrite grain boundaries, via a phase transformation accompanied by diffusion of interstitials. Austenite/ferrite boundaries are shown to terminate on {100}F//{111}A planes. This is due to the character of parent ferrite-ferrite boundaries which is dictated by the sharp <100> texture and geometry of austenite grains induced by directional solidification and epitaxial growth of ferrite. Benchmarking mechanical properties against a wrought counterpart demonstrates that AM offers high strength but relatively low ductility and impact toughness. A short heat treatment reverts the microstructure back to its equilibrium state resulting in balanced tensile and toughness properties, comparable to or even better than those of wrought counterparts.

Rapidly improved tensile strength of 6N01 Al alloy FSW joints by electropulsing and artificial aging treatment

Abstract: The tensile strength of 6N01 Al alloy friction stir welded joints is obviously improved in a short time by combining electropulsing and artificial aging treatment, which is attributed to the transformation of precipitates from rod-like β' to needle-like β''. Benefiting from the athermal effect of electropulsing, rapid solid solution of β' is achieved in 3 s below 110°C.

Simultaneously enhanced strength-ductility of AlCoCrFeNi2.1 eutectic high-entropy alloy via additive manufacturing

Abstract: The negative effects of thermal cycles in the process of additive manufacture present a challenge for the control of microstructure so as to fabricate the products with improved properties compared with conventional casting technique. In this work, AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) was prepared by laser metal deposition (LMD). Compared with conventionally cast EHEA samples, the LMD-fabricated EHEA samples showed significantly enhanced tensile strength (by 19.7%) and increased tensile ductility (by 56.4%). Such enhancement in tensile properties was attributed to the refinement of the uniformly distributed eutectic-structure, which improved the strain hardening/dislocation accumulation capability of the EHEA. The present work provides a new strategy to utilize both the high cooling rates of LMD and the eutectic-structure characteristics for forming refined homogeneous structures and thus achieving superior mechanical properties to those prepared by traditional processing techniques.

Fatigue behaviour of notched laser powder bed fusion AlSi10Mg after thermal and mechanical surface post-processing

Abstract: Laser powder bed fusion (LPBF) as an additive manufacturing technology offers high potential to fabricate parts with complex geometries layer-by-layer. However, these parts have inhomogeneous microstructure and very poor surface quality in their as-built condition. The presence of high surface irregularities especially in the downskin surfaces is a challenging issue that can directly influence their mechanical performance especially under fatigue loading conditions. Hence, applying post-treatments to modulate these imperfections can play a critical role. In this study, the individual and hybrid effects of different post-treatments including T6 heat treatment and shot peening on microstructure, mechanical properties and fatigue behaviour of LPBF V-notched AlSi10Mg specimens were investigated. Two different shot peening processes were applied on both as-built and heat treated specimens using steel and ceramic shots with different Almen intensity, shot diameter and shot hardness. The specimens were comprehensively characterized in terms of microstructural features, surface morphology and surface roughness. Mechanical properties including microhardness and residual stresses were measured and fatigue behaviour of the specimens was determined using a stair-case method; fracture surfaces were also critically analyzed. The results of the analysis performed both on the smooth section and the notched section (including notch root, up and down skin areas) indicated the importance of the choice of shot peening parameters with respect to the target geometry and its material properties. In this case, the shot peening treatment with smaller media and lower intensity was more efficient in terms of surface modification and homogenization especially in the downskin surfaces leading to higher fatigue strength. The significant finding of this study is that by pairing the heat treatment and shot peening, the effect of the presence of the notch can be masked obtaining almost the same fatigue strength for the notched specimens as the un-notched counterparts.

Fatigue behaviour of notched laser powder bed fusion AlSi10Mg after thermal and mechanical surface post-processing

Abstract: Laser powder bed fusion (LPBF) as an additive manufacturing technology offers high potential to fabricate parts with complex geometries layer-by-layer. However, these parts have inhomogeneous microstructure and very poor surface quality in their as-built condition. The presence of high surface irregularities especially in the downskin surfaces is a challenging issue that can directly influence their mechanical performance especially under fatigue loading conditions. Hence, applying post-treatments to modulate these imperfections can play a critical role. In this study, the individual and hybrid effects of different post-treatments including T6 heat treatment and shot peening on microstructure, mechanical properties and fatigue behaviour of LPBF V-notched AlSi10Mg specimens were investigated. Two different shot peening processes were applied on both as-built and heat treated specimens using steel and ceramic shots with different Almen intensity, shot diameter and shot hardness. The specimens were comprehensively characterized in terms of microstructural features, surface morphology and surface roughness. Mechanical properties including microhardness and residual stresses were measured and fatigue behaviour of the specimens was determined using a stair-case method; fracture surfaces were also critically analyzed. The results of the analysis performed both on the smooth section and the notched section (including notch root, up and down skin areas) indicated the importance of the choice of shot peening parameters with respect to the target geometry and its material properties. In this case, the shot peening treatment with smaller media and lower intensity was more efficient in terms of surface modification and homogenization especially in the downskin surfaces leading to higher fatigue strength. The significant finding of this study is that by pairing the heat treatment and shot peening, the effect of the presence of the notch can be masked obtaining almost the same fatigue strength for the notched specimens as the un-notched counterparts.

Effect of heat treatment on microstructure evolution and mechanical properties of selective laser melted Mg-11Gd-2Zn-0.4Zr alloy

Abstract: The Mg-11Gd-2Zn-0.4Zr (GZ112K, wt.%) alloys fabricated by selective laser melting (SLM) still contain some hard and brittle β-(Mg,Zn)3Gd eutectic phase with an area fraction of 4.86% despite the ultra-fast cooling rate of SLM process, so post heat treatment is necessary. The effects of different solution conditions and aging heat treatment on microstructure and mechanical properties of the SLMed GZ112K alloys were systematically analyzed. The results show that the optimized solution condition (400 °C × 12 h) for the SLMed GZ112K alloys can not only transform hard and brittle β-(Mg,Zn)3Gd eutectic phase into relatively softer lamellar 14H type long period stacking order (LPSO) structure leading to the improved elongation (EL) of the SLM-T4 alloys, but also retain the fine grains of the SLMed GZ112K alloys to a large extent with a small average grain size of 3.1 μm in the SLM-T4 state contributing to the high yield strength (YS) of the SLM-T4 and SLM-T6 GZ112K alloys. After peak aging heat treatment, basal γ′ precipitates or lamellar 14H-LPSO structure and dense prismatic elliptical β′ precipitates coexist in the SLM-T6 GZ112K alloys. As a result, the SLM-T6 GZ112K alloys exhibit YS of 343 ± 9 MPa, ultimate tensile strength (UTS) of 371 ± 4 MPa and EL of 4.0 ± 0.05%, while those of the SLMed GZ112K alloys are 332 ± 3 MPa, 351 ± 5 MPa and 8.6 ± 1.0%. The ultra-high YS of SLM-T6 GZ112K alloys results from the fine grain strengthening, the precipitation strengthening from β′ aging precipitates, the secondary phase strengthening from LPSO structure and X phase and the extra composite strengthening from the coexistence of basal and prismatic precipitates. The findings indicate that appropriate post heat treatment is an effective scheme for modifying microstructure and mechanical properties of the SLMed alloys.

Fabrication of exceptionally high-strength Mg-4Sm-0.6Zn-0.4Zr alloy via low-temperature extrusion

Abstract: Ultrahigh strength is achieved in low-cost Mg-4Sm-0.6Zn-0.4Zr alloy by a traditional extrusion process, with tensile yield strength over 450 MPa, which exceeds the majority of heavy rare-earth (RE) alloyed Mg-RE extrusion alloys. The alloy showed a typical bimodal microstructure, consisting of fine recrystallized grains with random orientations and coarse hot-worked grains with strong basal texture. And profuse Mg3Sm particles were dynamically precipitated in recrystallized grains boundary as well as hot-worked grains interior during extrusion. The ultrahigh yield strength of the alloy is closely related to the combined effect of fine recrystallized grains, highly textural hot-worked grains, and numerous nanoscale particles, independent of the accepted age-hardening in those heavy RE containing Mg alloys. Low cost, high strength, and simple extrusion process make the present alloy gets more fantastic potential in industrial applications than other Mg-RE extrusion alloys.

Laser powder bed fusion of Zr-modified Al–Cu–Mg alloy: Crack-inhibiting, grain refinement, and mechanical properties

Abstract: The application of laser powder bed fusion (L-PBF) in conventional high-strength wrought aluminum alloys is a significant challenge due to its high cracking sensitivity. In this study, 2024Al and 1.3-wt% Zr-modified 2024Al alloy deposits were prepared by L-PBF using pre-alloyed powder. The solidification cracking susceptibility of the 1.3-wt% Zr-modified 2024Al alloy was reduced to ∼0.4 of that of the 2024Al alloy. Compared to the complete columnar α-Al grain microstructure in the 2024Al deposit, the crack-free Zr-modified 2024Al deposit exhibited a heterogeneous grain structure having columnar α-Al grains in the inner region of the molten pool and ultrafine α-Al equiaxed grains at the fusion boundary. Based on the time-dependent nucleation theory and constitutional undercooling analysis ahead of solidification interface, the precipitation behavior of primary Al3Zr nanoparticles is analyzed, and it is deduced that the high intensity of effective nucleation sites by nanosized primary Al3Zr at the fusion boundary of molten pool leads to the formation of equiaxed grains at the beginning of the solidification of molten pool. The as-deposited Zr-modified 2024Al exhibited a yield strength (YS) of ∼376 ± 7 MPa and an ultimate tensile strength (UTS) of 441 ± 7 MPa with an elongation of 14.1 ± 1.6%. The YS and UTS increased to 402 ± 9 MPa and 483 ± 37 MPa after T6 heat treatment, respectively. Both the tensile properties of the as-deposited and T6-treated Zr-modified 2024Al were comparable to the wrought 2024-T651 alloy and higher than most of the previous L-PBFed Al–Cu–Mg alloys.

Effect of martensite morphology and volume fraction on the low-temperature impact toughness of dual-phase steels

Abstract: The influence of martensite morphology and volume fraction on low-temperature impact toughness of dual-phase (DP) steels was investigated by impact tests, nanoindentation tests, atom probe tomography (APT) and microstructural examination. The APT results of medium-C martensite show that many small-sized C clusters are formed at the twin grain boundaries inside the lath, which increase nanohardness and decrease impact toughness. The impact test results indicate that the low-temperature impact toughness of lath martensite is significantly superior to ferrite. Refining the lath martensite substructure can improve the low-temperature impact toughness of DP steels. However, martensite twins obviously reduce the low-temperature impact toughness of DP steels. It is further found that the impact toughness of DP steel is affected by the toughness of ferrite and martensite, and conforms to rule of mixtures. Finally, the low-temperature impact fracture mechanism of DP steels was observed, and it is found that the large-size ferrite grains in DP steels preferentially occur cleavage fracture. As the impact temperature decreases, the fracture mode of DP steel changes from dimple plus quasi-cleavage fracture to brittle cleavage fracture.

High-strength and high-conductivity in situ Cu–TiB2 nanocomposites

Abstract: It is of significant interests to enhance the strength of Cu alloys while retaining their good electrical conductivity for broad applications. In situ incorporation of nano-phase into Cu alloys is a promising method for this objective. However, little success has been reported. In this study, we successfully fabricated in situ Cu–TiB2 nanocomposites by a molten salt processing and systematically studied their mechanical and electrical properties. During the processing, Al was used to react and introduce nano- and submicron-size TiB2 efficiently into the copper melt. Immiscible Fe was later added to recover the electrical conductivity through sequential solutionizing, hot rolling, and electrical conductivity-recovering. Detailed microstructure, phase, and electrical studies validate the feasibility of the proposed procedures to achieve high-strength and high-conductivity Cu(–Fe)-TiB2 nanocomposites. Cu-1 wt.% Fe-5 vol% TiB2 offers 554.3 MPa in ultimate tensile strength, 4.4% in ductility, and 62.4% IACS in electrical conductivity. The new method paves an effective way to synthesize and incorporate nano-phase into copper melt for high-performance Cu nanocomposites.