A novel Be-Al-Ag-Co-Ge (BA-A) alloy was prepared to address the limited understanding of strain rate sensitivity and fracture mechanisms in multi-element modified Be-Al systems, and investigated tensile properties within strain rate range of 0.001 to 0.05 s-1. BA-A alloy exhibits typical dual-phase microstructure consisting of equiaxed Be particles embedded in continuous Al matrix. Experimental results demonstrate a significant strain-rate strengthening effect: ultimate tensile strength (UTS) increases from 217.0 MPa to 237.4 MPa with increasing strain rate, while elongation decreases from 1.52% to 1.28%. Strain rate sensitivity factor shows non-monotonic trend, initially decreasing and then increasing. Although cleavage fracture of Be phase dominates across the entire strain rate range, the Al phase exhibits more pronounced ductile features at lower strain rates. As the strain rate increases, the cleavage fracture becomes increasingly dominant. This synchronized variation in macroscopic mechanical behavior and fracture characteristics originates from the inherent deformation incompatibility between the soft Al phase and hard Be phase. This incompatibility evolves with strain rate and governs the macroscopic response by regulating the density and distribution of dislocations at Be/Al interface. The study clarifies the role of deformation incompatibility as a critical bridge connecting microscopic mechanisms with macroscopic rate-sensitive performance, providing theoretical guidance for design of dynamic-loading-resistant Be-Al-based alloys.

This study examines the microstructural evolution and tensile response of a cold-rolled C17200 Cu–Be–Co alloy strip after short-time annealing at 550 and 610 °C. X-ray diffraction (XRD), electron backscatter diffraction (EBSD), kernel average misorientation (KAM) analysis, transmission electron microscopy (TEM), and room-temperature tensile testing were used to compare phase constitution, grain-boundary character, recrystallization behavior, texture evolution, dislocation substructure, and tensile properties. The results show that both annealed samples mainly consisted of an α-Cu matrix and a small amount of BeCu-related precipitates. After annealing at 550 °C, the alloy retained a recovery-dominated partially recrystallized microstructure, with an average grain size of 1.47 μm, a low-angle grain boundary fraction of 8.4%, a recrystallized fraction of 18.06%, and evident residual dislocation substructures. This condition exhibited a yield strength of 365.5 MPa. After annealing at 610 °C, recrystallization was substantially promoted, the average grain size increased to 1.92 μm, the high-angle grain boundary fraction increased to 97.5%, and the recrystallized fraction reached 82.76%. Meanwhile, the yield strength decreased to 276.6 MPa because of the reduced contribution from dislocation strengthening. These results indicate that, under the two investigated short-time annealing conditions, the strength difference in the alloy is mainly associated with the transition of microstructural evolution from recovery-dominated partial recrystallization to a recrystallization-dominated microstructure.

To address weldability challenges and achieve excellent tensile performance of low-density austenitic steel in friction stir welding (FSW), two low heat input parameters were used to join Fe-30Mn-9Al-0.85C steel. The results indicate that low heat input suppresses the dissolution of kappa-carbides in the stir zone, while promoting the growth of grains and carbides and increasing annealing twin boundaries (ATBs) in the heat-affected zone (HAZ). The welded joints exhibited a joint efficiency of 103%. This study uncovers a mechanism - during tensile deformation in the HAZ, ATBs transformed into high-angle grain boundaries, leading to grain refinement and giving rise to the dynamic Hall-Petch effect - that enables a strategy for achieving the excellent tensile performance of FSW joints in low-density austenitic steel.

In this study, the microstructures and mechanical properties of Fe-30Mn-9Al-0.85C lightweight steel after friction stir processing (FSP) and post-aging treatment were systemically investigated. The processing zones (PZs) exhibited no macroscopic defects at rotational speeds of 400 rpm, 500 rpm, and 600 rpm with a processing speed of 40 mm/min. Grain refinement occurred in the PZs, and the initial x-carbides in the base metal dissolved after FSP. Post-aging treatment at 500 degrees C and 600 degrees C promoted the precipitation of x-carbides without inducing grain growth in the PZs. The aged microstructure of the PZs exhibited an incompletely recrystallized state with numerous residue deformation dislocations. Grain boundary and dislocation strengthening had a limited contribution to yield strength, while the precipitation strengthening of x-carbides played a significant role in controlling the tensile properties of the PZs before and after aging treatment. The optimal tensile properties were achieved using a processing parameter of 500 rpm and 40 mm/min, followed by post-aging treatment at 600 degrees C for 1 h.

Friction stir processing (FSP) has been applied to improve the microstructure and tensile performance of as-cast Mg-3Zn-0.5Zr magnesium alloy. By FSP, grains within the stir zone (SZ) demonstrate noticeable refinement and the varied processing parameters can lead to the formation of both strong and weak textures within the SZ. Second-phase particles are broken down, dispersed, and partially re-dissolved. In addition, optimal processing parameters lead to enhanced uniformity in refined grains and increased dispersion of secondary-phase particles, resulting in improved both in microhardness and tensile properties. The sample processed at 800-200 (rpm/mm.min(-1)) demonstrates an overall improvement in tensile properties compared to the base material, with yield strength, ultimate strength, and uniform elongation of 93.7 MPa, 215.0 MPa, and 25.4%, respectively.

The microstructures and cryogenic mechanical properties of dissimilar friction stir welding (FSW) joints between nitrogen-alloyed CoCrFeMnNi high-entropy alloys (HEAs) and Fe–32.1Mn–7.5Cr–0.6Mo–1.2N steel were investigated. The results reveal that defect-free dissimilar joints can be achieved through FSW. Furthermore, the grains of nitrogen-alloyed CoCrFeMnNi HEAs in the stir zone of the dissimilar joint are significantly more refined than those of Fe–32.1Mn–7.5Cr–0.6Mo–1.2N steel. Joint efficiency at room and low temperature both exceed 90% of the base metal. Moreover, the cryogenic yield and ultimate strength of the dissimilar joints are higher than those recorded at room temperature. The fracture position is at the heat-affected zone of HEAs under two temperature conditions.

The effect of high-temperature deformation twinning on the work hardening behaviors of Fe-38Mn alloy during hot shear-compression deformation was investigated. The discovery of micro-shear bands and deformation twinning is significant for continuous work hardening, and this represents an important step toward gaining a complete understanding of the effect of deformation twinning on work hardening behaviors. Deformation twinning is widely acknowledged to accommodate plastic strain under cold deformation, even under severe plastic deformation. At present, the equivalent stress vs. strain curves for hot shear-compression deformation of Fe-38Mn alloy exhibit the characteristics of continuous work hardening. In addition, continuous work hardening is classified into five stages when considering high-temperature deformation twinning.

The microstructures and hot tensile behaviors of ZK30 alloys subjected to single- and multi-pass friction stir processing (FSP) were systematically investigated. Following single-pass FSP (S-FSP), coarse grains underwent refinement to 1-2 mu m, with a distinct basal texture emerging in the stir zone (SZ). Additionally, second-phase particles were fragmented, dispersed, and partially dissolved. Multi-pass FSP (M-FSP) further enhanced the homogeneity of the microstructure, reduced texture intensity differences, and decreased the fraction of second-phase particles by 50%. Both S-FSP and M-FSP SZs demonstrated superplasticity at strain rates below 1x10-3 s -1 and at temperatures of 250-350 degrees C. The S-FSP SZ exhibited an elongation of 390% at 250 degrees C and 1x10-4s-1, while the M-FSP SZ achieved an elongation of 406% at 350 degrees C and 1x10-3s-1. The superplastic deformation of SZ was co-dominated by grain boundary sliding (GBS) and the solute-drag mechanism in S-FSP and mainly by GBS in M-FSP.

The objective of this study is to investigate the effects of prestrain-assisted friction stir processing and subsequent aging heat treatment on the microstructural evolution and performance of copper-chromium-zirconium alloys. The findings indicated that prestrain, generated through cold rolling before friction stir processing, considerably affected grain refinement and enhanced the mechanical properties and electrical conductivity in the processing zone. The prestrain not only decreased the activation energy of dynamic restoration and precipitation in the processing zone, but also strengthened the “constraint effect” of the base metals to the processing zone. This effectively reduced the width of micro-bands and promoted nanocrystalline formation in the processing zone. Furthermore, dynamic precipitation resulted in higher thermal stability of the microstructure in the processing zone during subsequent aging heat treatment. Thus, the synergistic improvement of the mechanical properties and electrical conductivity of copper-chromium-zirconium alloys was successfully achieved.

The effect of deformation twinning on microstructure refinement was investigated for Fe-38Mn alloy during friction stir processing. It was proved that the friction stir processing microstructure of Fe-38Mn alloy was gradient distribution. The stir zone contained both recrystallized grains and fine deformed grains with average grain size of about 15 mu m, and the proportion of twin boundaries in the deformed grain zone was up to 35%. Moreover, the cyclic multidirectional shear induced micro-shear bands and sub-grains with high density dislocations, as well as plentiful deformation twins. The microstructure refinement of Fe-38Mn alloy during friction stir processing resulted from the dynamic recrystallization mechanism and twinning mechanism. In addition, the cyclic multidirectional shear deformation enhanced the deformation twinning effect, so that the microstructure refinement process was completed quickly and effectively. The above works indicate a dynamic mechanism of microstructure refinement in term of the deformation twinning during friction stir processing.(c) 2022 Elsevier B.V. All rights reserved.

The dynamic microstructural evolution and tensile performances of CoCrFeMnNi high-entropy alloys (HEAs) with and without nitrogen alloying subjected to multi-pass friction stir processing (FSP) were comparatively investigated. Results showed that the strength and ductility of nitrogen-alloyed CoCrFeMnNi HEAs were synergistically enhanced by multi-pass FSP. Compared to free-nitrogen CoCrFeMnNi HEAs, the gains in the processed zone of the nitrogen-alloyed CoCrFeMnNi HEAs exhibited not only more refinement，but also a monotonic decrease in average gain size (AGS) with the increase in FSP pass. In addition, nitrogen-alloyed CoCrFeMnNi HEAs had higher fraction of low angle boundaries (LABs) than free-nitrogen CoCrFeMnNi HEAs after multi-pass FSP. The interaction between nitrogen atoms and LABs during multi-pass FSP was indirectly proved by the comparison of microstructural thermal-stability between two groups FSP HEAs. Meanwhile, the mechanism of synergistical enhancement in strength and ductility of nitrogen-alloyed CoCrFeMnNi HEAs was discussed.

A counterintuitive strain-induced alpha -> beta dynamic transformation of commercial pure titanium deformed at a temperature below the beta-transus was investigated. It was revealed that the serrated-shaped beta phase first formed at grain boundaries and then grew into a grains. Finally, the initial equiaxed a grain was transformed into a (alpha + beta) lamellar structure. The development of beta phase laths was controlled by moving dislocations in the a phase, and both were affected by the diffusion of beta-stabilizing elements. Furthermore, the redistribution of Fe in CP-Ti resulted in a special reversal transformation of beta -> alpha in the isothermal holding period after deformation. (C) 2020 Elsevier B.V. All rights reserved.

Equiatomic CoCrFeMnNi high-entropy alloys (HEAs) with and without nitrogen alloying were produced by vacuum induction melting and then subjected to friction stir processing. The effects of nitrogen alloying and friction stir processing on the microstructures and mechanical properties of these alloys were systematically investigated. The results suggested that nitrogen alloying effectively increases the yield strength and ultimate tensile strength of CoCrFeMnNi HEAs in the as-cast condition. Friction stir processing considerably broke down the coarse dendritic structures of the as-cast HEAs and caused grain refinement and composition homogenization. More significantly, nitrogen alloying promoted grain refinement by increasing the nucleation rate and retarding grain growth during dynamic recrystallization over the course of friction stir processing. The yield strength, ultimate tensile strength, and uniform elongation of CoCrFeMnNi HEAs treated by nitrogen alloying and friction stir processing reached 493 MPa, 832 MPa, and 32.6%, respectively. These improvements could be attributed to solid solution strengthening through nitrogen alloying and grain boundary strengthening by friction stir processing. This study provides an alternative technical route to enhance the mechanical properties of CoCrFeMnNi HEAs. (C) 2019 Elsevier B.V. All rights reserved.

The influences of nitrogen alloying on the microstructural evolution and tensile properties of CoCrFeMnNi high-entropy alloys (HEAs), which were subjected to cold rolling and subsequent annealing at 773-1173 K, were systematically investigated. The results show cold rolling-induced microbands in the nitrogen-alloyed HEAs instead of the deformation twins and shear bands found in nitrogen-free HEAs. During annealing at 773-873 K, the cold-rolled nitrogen-free HEAs experience a partial annihilation of the deformation twins, while the cold-rolled nitrogen-alloyed HEAs exhibited higher microstructural stability. When the annealing temperature exceeds 973 K, a large number of Cr2N precipitates form in the recrystallized regions of the nitrogen-alloyed HEAs, and the tensile strength of the cold-rolled nitrogen-free HEAs decreases with increasing annealing temperature. However, the tensile strength of the nitrogen-alloyed HEAs annealed at 773 and 873 K experiences an abnormal increase compared to that of the cold-rolled sample. The best combination of strength and ductility are achieved in the nitrogen-alloyed HEAs treated by cold-rolling and annealing at 973 K. When the annealing temperature increases to 1073-1173 K, the nitrogen-alloyed and -free HEAs exhibit completely recrystallized structures with relatively low strength and excellent ductility. The influences of nitrogen alloying on the microstructural evolution are related to the interaction between nitrogen atoms and dislocations, and the multi-mechanism (including nitrogen solid-solution strengthening, precipitation strengthening, and grain refinement strengthening) accounts for the improved tensile properties of the nitrogen-alloyed HEAs treated by cold-rolling and subsequent annealing at 973 K.

A bulk Cu-0.55Cr-0.2Zr alloy with structure of ultra-fined grains (UFGs), nanograins (NGs) and nano precipitates in grain boundaries (GBs) was fabricated by cryogenic friction stir processing (CFSP) followed with annealing treatment. A combination of high strength, pronounced electrical conductivity, and good microstructure thermal and mechanical stability was achieved.

Friction stir welding was used to join two AA2060-T8 plates, and then the effect of precipitate evolution on microstructure and corrosion behavior of the joint was investigated. The evolution of precipitates on the top surface of the joint was characterized by scanning electron microscopy and transmission electron microscopy. The corrosion behaviors of different regions in the joint were investigated by an electrochemistry method and an alternating salt spray exposure. The corrosion was mainly dependent on the nature of precipitates in each region of the joint. The shoulder affected zone had the worst corrosion resistance as a result of the re-dissolved of θ′(Al2Cu), T1(Al2CuLi) and δ′(Al3Li) phases, the formation of intergranular precipitates and precipitate-free zones. However, the thermomechanically affected zone had a slightly improved corrosion resistance because it had no intergranular precipitates. The heat affected zone and base metal had the best corrosion resistance.

The microstructural stability during long-term storage and tensile deformation have been investigated for nanograined (NG) pure Cu and pure Zr produced by severe plastic deformation. Abnormal and uniform grain growth behaviour is observed in the NG Cu after long-term storage and tensile deformation at different strain rates, respectively. The abnormal grain growth during long-term storage is related to the relaxation of the fluctuating microstrain in the deformed microstructure and can be accelerated by increasing the temperature. The uniform grain growth during tensile deformation is dominated by grain boundary (GB) migration due to the absorption of dislocations. The degree of GB migration in the case of a low strain rate is relatively large owing to the dynamic equilibrium between the formation and disappearance of dislocations. However, the GB migration at a high strain rate is suppressed by the piled-up dislocations. In comparison, the NG Zr exhibits relatively high microstructural stability with no obvious grain growth during long-term storage and tensile deformation. This is attributed to the difference in the crystal structure between Cu and Zr.

Pure copper with different fraction of nonequilibrium grain boundaries were achieved by friction stir processing (FSP) under air, water and liquid nitrogen cooling conditions. Tensile behaviors at room temperature exhibited significant difference for above three cases involving different fraction of nonequilibrium grain boundaries. The case with nitrogen cooling showed better combination of strength and elongation for the largest fraction of high energy nonequilibrium boundaries, which contribute to emit dislocations from grain boundaries and suppress grain boundary sliding. Fully relaxed grain boundaries in air cooling samples can suppress the grain boundary sliding and dislocation emission causing high stress and very low elongation. However, appropriate relaxed grain boundaries in the water cooling samples will promote grain boundary sliding and the increase of elongation. The grain coarsening during tensile deformation was observed in those samples with nonequilibrium grain boundaries and will increase elongation and cause work softening behavior of these samples.

The interactions between twins and dislocations were investigated at different stage of dynamic microstructure evolution during hot shear-compression deformation of the Fe-38Mn austenitic steel. It was proved that the step-like structure located at the boundaries of pre-existing annealing twins was induced by the interactions of twins and dislocations in the case of low strain level. These steps provided favorable nucleating positions for the new grains. Benefited from the large strain and high strain rate during the hot shear-compression deformation, the deformation twins in different scales appeared with further increasing the strain. Moreover, the interactions between the deformation twins and dislocations were frequently found. As a result, the nano-grain was induced by the migration of incoherent twin boundaries in the case of large strain. The above works indicate a dynamic mechanism of grain refinement in term of the interactions between twins and dislocations during hot deformation with large strain and high strain rate.