中国科学院金属研究所机构知识库

Ultrafine grained (UFG) and nanostructured (NS) materials processed by severe plastic deformation (SPD) have received considerable attentions. The physical fundamentals of this processing technique are refining the coarse grains into UFG range and nanoscale through a series of complicated dislocations or twinning activities induced by plastic deformation. According to the conventional crystal plasticity theory, stacking fault energy (SFE) is one of the most crucial factors to govern the choice of two essential deformation mechanisms: dislocation slip and deformation twinning. In the present work, using two most popular SPD methods-Equal Channel Angular Pressing (ECAP) and High-Pressure Torsion (HPT), pure Cu and Cu-Al alloys were selected as model materials to systematically explore the effects of SFE on the deformation mechanism, microstructure evolution, grain refinement and the related tensile properties and fatigue strengths. To decode the underlying roles of SFE during SPD can provide us not only with an excellent opportunity to profoundly comprehend the nature of grain refinement mechanism and to extend our understanding of the structure-properties relationship of UFG/NS materials processed by SPD, but also present an attractive potential to prepare UFG/NS materials with excellent mechanical properties for technological applications. The main results in the present investigation are as follows:

(1) After 1 pass of ECAP processing, with decreasing the SFE in Cu and Cu-Al alloys, the dominated deformation mechanism was transformed from dislocation slipping to deformation twinning and the slip modes were also transformed from wavy slip to planar slip, meanwhile a transition of microscale shear bands from “copper” type to “brass” type was simultaneously found and they play increasingly essential roles in accommodating the intense shear strain. Moreover, with the decrease of SFE, a transition of the grain refinement mechanism from dislocation subdivision to twin fragmentation was analyzed and with this transition, the grain size can be further refined from UFG range to nanoscale. Furthermore, combined with the previous investigations, it is demonstrated that the deformation mechanisms and microstructures of the Cu and Cu-Al alloys with medium SFEs are highly sensitive to the external SPD loading conditions. 

(2) Due to the inhomogeneous deformation mode of HPT, lower hardness values in the central regions of the disks of Cu and Cu-Al alloys were observed and this inhomogeneous distribution of hardness decreased with increasing numbers of turns. Moreover, the homogeneity factor, a , and the equivalent saturation strain, , increased and then decreased with increasing Al content and therefore with decreasing the SFE. The results show homogeneity is achieved more readily in materials with either high or low SFE than in materials with medium SFE due to a transition in the mechanism for microstructural evolution from recovery processes in high- and medium- SFE materials to twin fragmentation in materials having low SFE. Furthermore, the limiting minimum grain size of materials subjected to SPD processing decreases with lowering the SFE, however, the dependence of the minimum grain size on SFE decreases with increasing severity under the external loading conditions. In addition, after 5-turn HPT, a fivefold deformation twin formed in an ultrafine grain of Cu-16at.% Al alloy and the hypothesis that transformation from multiple-fold deformation twins can be transformed into fivefold twins via the emission of partials from multiple-fold nodes was experimentally substantiated, both of which imply that SFE is still one of the most crucial factors influencing the deformation mechanisms even at nanoscale. 

(3) With lowering the SFE, the tensile strengths of Cu and Cu-Al alloys after various passes of ECAP and HPT processing significantly increased; meanwhile, their tensile strengths can be further improved with increasing the severity of external loading conditions, and at intermediate SFE as in NS Cu and Cu-Al alloys with lower Al contents, their strengths are especially sensitive to the processing conditions. Moreover, the uniform elongations of Cu-Al alloys processed by ECAP are slightly improved with increasing the passes of ECAP and decreasing the SFE, however, for the materials processed by HPT, where the loading conditions are more severe, the ductility increases with decreasing SFE but ultimately there is a reversal at the lowest SFE where the elongation decreases due to the formation of the extremely fine grains in the Cu-16at.%Al, which is unable to maintain a significant uniform deformation. Furthermore, the NS Cu and Cu-Al alloys processed by ECAP and HPT were annealed and the thermal stability is improved with the decrease of SFE in NS Cu and Cu-Al alloys with uniform microstructures. More significantly, the strength-ductility synergy of Cu and Cu-Al alloys is prominently enhanced with decreasing the SFE, which presents an efficient pathway to fabricate materials with superior mechanical properties for technological utilizations. 

(4) The decreased grain sizes of pure Cu and Cu-Al alloys with lowering the SFE render the increased tensile strength and then enhance the fatigue strength coefficency  in the Basquin law due to its close relationship with monotonic strength. Meanwhile, the shear banding and instability of the microstructures, which are two fundamental fatigue damage mechanisms for NS metal and alloys, are also remarkably improved with the reduction of the SFE and then leads to the increase of fatigue strength exponent b in the Basquin equation. Thus, the fatigue strengths of NS Cu and Cu-Al alloys are remarkably upgraded due to the enhanced fatigue strength coefficient and fatigue strength exponent  in the Basquin relationship. Moreover, although the monotonic strengths in the NS Cu with high purity and NS Cu-Al alloys with high Al contents can be further apparently increased by enhancing the severity of the SPD methods, the corresponding further preeminent promotion of their fatigue strengths was not found. This may be ascribed to the microstructure instability and the easy formation of the extremely fine grain readily initiating grain boundary activities during cyclic deformation, respectively, both of which limit the betterment of their fatigue endurance to a greater extent.