Alloy design based on single-principal-element systems has approached its limit for performance enhancements. A substantial increase in strength up to gigapascal levels typically causes the premature failure of materials with reduced ductility. Here, we report a strategy to break this trade-off by controllably introducing high-density ductile multicomponent intermetallic nanoparticles (MCINPs) in complex alloy systems. Distinct from the intermetallic-induced embrittlement under conventional wisdom, such MCINP-strengthened alloys exhibit superior strengths of 1.5 gigapascals and ductility as high as 50% in tension at ambient temperature. The plastic instability, a major concern for high-strength materials, can be completely eliminated by generating a distinctive multistage work-hardening behavior, resulting from pronounced dislocation activities and deformation-induced microbands. This MCINP strategy offers a paradigm to develop next-generation materials for structural applications.

Multicomponent intermetallic nanoparticles and superb mechanical behaviors of complex alloys

Friction stir welding/processing of metals and alloys: A comprehensive review on microstructural evolution

Friction Stir Welding(摩擦撹拌接合)法の熱工学的研究

Review on friction stir welding of magnesium alloys

Microstructure evolution during friction-stir welding of AZ31 magnesium alloy

Three kinds of AZ31 plates with different initial grain orientations were friction stir processed (FSP). Detailed microstructure and texture evolutions of the FSP plates were obtained. Fracture behavior and non-uniform plastic deformation were focused on. It was found that initial grain orientation had little effect on grain size and texture evolution in stir zone (SZ), but it greatly affected the texture transition at thermal–mechanical affected zone (TMAZ)/SZ interface. Non-uniform plastic deformation was observed in all the three kinds of FSP plates along the tensile direction. The strength difference of the FSP plates was largely reduced compared with the initial plates. Fracture consistently initiated in transition region on advancing side even for the 90° specimen, in which no sudden texture transition was present at TMAZ/SZ interface. As revealed in electron backscatter diffraction (EBSD) maps, SZ-side could be divided into two micro-regions. One was adjacent to TMAZ/SZ interface, named as the region of easy to activate extension twinning (EAET region), where the c -axis was almost parallel with transverse direction (TD). The other was slightly away from TMAZ/SZ interface, named as the region of easy to activate basal slip (EABS region), where the c -axis was tilted about 45° towards TD. The activation of different deformation mechanisms in SZ-side may cause plastic deformation incompatibility and initiate fracture. An “embossed” phenomenon appeared in SZ-center during the transverse tensile tests of all the three kinds of FSP plates, which was related to the distribution of basal plane and caused by the different deformation between SZ-center and SZ-side. The activation of basal slip in EABS region was considered as a major role in the formation of the “embossed phenomenon”. The findings of this study extend understanding the mechanism of fracture and inhomogeneous deformation behaviors of FSP Mg alloys.

Mechanisms of fracture and inhomogeneous deformation on transverse tensile test of friction-stir-processed AZ31 Mg alloy

Dissimilar friction stir welding (FSW) of ZK60 and AZ31 magnesium alloys was studied. Two kinds of material arrangements, with ZK60 or AZ31 in advancing side (AS), were applied. The results showed that the dissimilar plates with both material arrangements were joined successfully. No obvious defect was observed. Microstructure and texture variation across the joints were examined by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The grains in stir zone (SZ) were significantly refined during FSW (∼8.7 μm in AZ31 side and ∼2.7 μm in ZK60 side). The second phases in ZK60 side were broken up, which contributed to grain refinement. Two-dimensional (2D) hardness maps across the normal direction (ND)—transverse direction (TD) sections in the joints were obtained. Clear boundary was observed between SZ and AZ31 base metal, but it was less clear between SZ and ZK60 base metal. The mixture of materials, grain refinement and broken up of particles were responsible for the sharp increasing of hardness between SZ and AZ31 base metal. The transverse ultimate tensile strength (UTS) of the joints was 78–82% of AZ31 base metal. The joints consistently fractured in the transition region of the AZ31 side. The sudden hardness decrease from SZ to AZ31 base metal might result in deformation incompatibility at the interface, which is considered as the main cause for the failing in the AZ31 side. In addition, the activation of twinning in SZ-side (with c -axis parallel to TD) and slip in the region adjacent to the SZ-side (with c -axis at ∼45° angle to TD) aggregated the deformation incompatibility in the transition region of the AZ31 side. As a result, the break of the specimens started at the boundary of transition zone (TZ)/SZ. Many extension twins were observed in AZ31–SZ-side after fracture. Although the ZK60–SZ-side presented the similar texture with the AZ31–SZ-side, much less extension twins were observed in the former because of the hindering effects by the refined grains and secondary particles.

Microstructure and mechanical properties of friction stir welded dissimilar Mg alloys of ZK60–AZ31

A strong and complicated microtexture usually forms in stir zone (SZ) of friction stir welded (FSW) Mg alloys, which significantly influences the tensile properties of the joint due to the strong anisotropic mechanical properties of Mg alloys. This study aims to study the evolution of microstructure and texture in SZ during post-rolling of FSW AZ31 alloy and its effect on the tensile properties and fracture of the FSW joint. A detailed characterization of microstructure and texture in SZ and especially in SZ/base material (BM) interface was conducted on the FSW-undeformed and FSW-rolled specimens. Schmid factor (SF) changes for extension twinning and basal slip for transverse tensile tests were analyzed and discussed. It confirmed that the c-axis in SZ-center was parallel to welding direction (WD), but tilted towards transverse direction (TD) with the area moving to SZ-side. After subsequent rolling (by ∼2.5% strain), a large number of extension twins were found in SZ-side, but it was less in SZ-center. The production of extension twins rotated the grains in SZ-side and affected the transverse tensile properties significantly. The electron backscatter diffraction (EBSD) analyses showed that the SF of the twins was decreased compared with the matrix for both extension twinning and basal slip for the transverse tensile tests. So, the extension twins generated by post-rolling could cause texture strengthening for the transverse tensile tests. Moreover, the introduction of twin boundaries subdivided the grains, which also increased the tensile properties of the FSW-rolled specimens. The effect of post-rolling on the tensile properties and fracture was studied. It showed that post-rolling increased the transverse yield strength (YS) of the FSW AZ31 alloy, and the YS was increased further with the increase of rolling strain. The YS was raised from ∼87 MPa of the FSW-undeformed specimen to ∼115 MPa for the FSW-R-2.5 and ∼171 MPa for the FSW-R-7 specimens. Because most extension twins were generated in SZ-side, the strength increase was most significant in SZ-side, which reduced the plastic inhomogeneity between SZ-side and SZ-center of the post-rolled specimens. Fracture occurred in SZ-side of the FSW-undeformed specimens because most grains in there were favorably oriented for extension twinning and basal slip. But it occurred in BM for all the FSW-rolled specimens. This was mainly attributed to the texture strengthening and grain subdivision strengthening by the extension twins generated in SZ-side during post-rolling of the FSW specimens. So, it suggested that subsequent rolling was effective to increase the joint strength of FSW AZ31 alloy.

Dejia Liu

Papers

Mechanisms of fracture and inhomogeneous deformation on transverse tensile test of friction-stir-processed AZ31 Mg alloy

Microstructure and mechanical properties of friction stir welded dissimilar Mg alloys of ZK60–AZ31

Changes in texture and microstructure of friction stir welded Mg alloy during post-rolling and their effects on mechanical properties

Microstructure and texture evolution of an Mg–Gd–Y–Nd–Zr alloy during friction stir processing

Influence of welding parameter on texture distribution and plastic deformation behavior of as-rolled AZ31 Mg alloys