Effect Of Temperature On The Plastic Deformation And Damage Behavior Of Utrafine-Grained Pure Metals

Various mechanical behaviors of ultrafine-grained (UFG) materials produced by equal channel angular pressing (ECAP) generally exhibit distinctive characteristics from those of conventional grained materials, so that the relevant studies have received sustained attention in recent years. However, the serious accumulation of strains caused by severe plastic deformation (SPD) makes their microstructures to be actually in a metastable non-equilibrium state, so that their microstructures are easily changed by dynamic recovery and recrystallization during their practical services at a certain temperature (even at room temperature), leading to the degradation of their mechanical properties. Up to now, the investigation on the temperature-dependent mechanical properties and structural stabilities of such UFG materials is not well understood. Therefore, in the present work, three kinds of UFG pure metals with different crystalline structures are selected as the target materials, such as face-centered cubic (fcc) UFG Cu, hexagonal close-packed (hcp) UFG Ti, and body-centered cubic (bcc) UFG Fe, are prepared by the ECAP technique. The thermal stability and high-temperature compressive deformation and damage behavior of these materials are studied, focusing on the joint effect of applied load and temperature on the microstructures, mechanical properties as well as deformation and damage characteristics.After three kinds of pure metals are subjected to ECAP, their grains can be effectively refined to a submicron scale. The differential scanning calorimetry (DSC) curve of fcc UFG Cu shows a sharp recrystallization exothermic peak, but the DSC curves of hep UFG Ti and bcc UFG Fe exhibit a slow exothermic process rather than a sharp exothermic peak. This indicates that hcp UFG Ti and bcc UFG Fe possess a better thermal stability than fcc UFG Cu.After ECAP treatment, the compressive yield stress and steady flow stress of pure metals are greatly enhanced as compared with its original state. At the same time, all the metals processed by ECAP show the lack of strain hardening. They undergo a fast hardening only in the initial stages of deformation, and then enter into a steady flow state along with various degrees of softening. The compressive yield stress and steady-state flow stress of the UFG materials generally decrease with increasing temperature. However, the compression yield strength of bcc UFG Fe exhibit a irregular change, as the temperature is higher than its recrystallization starting temperature, resulting from a preferential growth of grains during deformation. The hcp UFG Ti and fcc UFG Cu are materials with a positive strain rate sensitivity of the yield stress, while bcc UFG Fe shows a negative strain rate sensitivity at high temperatures. Besides, the deformation constitutive equation of UFG Ti can be described by an Arrhenius equation modified with hyperbolic sine, and its deformation activation energy is decreased to be about294.4kJ/mol as compared with347.6kJ/mol of conventional-grained commercially pure Ti.Temperature has different effects on the surface deformation characteristics of UFG pure metals with different crystalline structures. For fcc UFG Cu, when temperature is below recrystallization, the most prominent damage feature is the formation of small-and large-scale cracks along the shear bands. When the temperature is higher than recrystallization temperature, surface shear bands disappear completely, and some micro-cracks nucleate along the grain boundary (GB) of coarsened grains. For hcp UFG Ti, as the temperature is below recrystallization, the damage feature is dominated by shear bands. With increasing temperature, the shear bands become finer. At this temperature range, the surface damage of hcp UFG Ti is much less than that of fcc UFG Cu. As the temperature is higher than the recrystallization temperature, only extrusions and intrusions comprising coarsened grains are observable, showing a good co-deformation capability. For bcc UFG Fe, within the range of temperatures below recrystallization, it shows good GB coordination deformation. A plenty of secondary shear lines are formed within the flow shear bands, which formed during ECAP. As the temperature is above recrystallization, the capacity of GB co-deformation becomes weakened. The number of secondary shear lines reduces largely and the cracks appear along the shear bands.Temperature also has different affects on the microstructure changes in uniaxially compressed UFG metals with different crystalline structure. For the fcc UFG Cu compressed at temperatures below recrystallization, the microstructure does not change seriously, and only few grains are coarsened slightly. As the testing temperature is near recrystallization, the grains undergo significant coarsening, and the dislocation density is obviously reduced. As temperature is above recrystallization, the grains are coarsened seriously and the dislocation density further reduced greatly. Only few dislocation lines and dislocation tangles exist in the coarsened grains. Concerning the hcp UFG Ti compressed at temperatures below recrystallization, a large number of dislocations exist at GBs and grains, and GBs are too vague to be identified. Few dislocation dipoles are found in some grains. When the temperature rises to be near the recrystallization, the dislocation density decreases significantly and few grains coarsened. The grain boundaries become clear. When the temperature is above recrystallization, the grains are coarsened severely, and some interlaced dislocations or some dislocation tangles are found in some coarsened grains. In contrast, the bcc UFG Fe shows unusual structural changes. For instance, as the temperature is below recrystallization, the flow shear bands formed during ECAP still exist, and some dislocation cells and subgrains are arranged along the shear bands. As the testing temperature is close to the recrystallization temperature, the dislocation density reduces significantly and flow shear bands disappear. Most of grains are UFG grains with clear boundaries, which are formed by dynamic recrystallization. Above the recrystallization temperature, an obvious preferential growth of grains occurs in bcc UFG Fe, and high density dislocations are still remained at GBs and grain interiors.The grain coarsening behavior of UFG Cu under high-temperature compression is closely related to the strain rate, i.e., the higher the strain rate, more remarkable the localization of grain coarsening becomes; the lower the strain rate, more grains are coarsened integrally. Comparatively speaking, the grain coarsening induced by high-temperature cyclic deformation takes place more notably and uniformly, and some typical dislocation arrangements, like dislocation walls and dislocation cells etc., can be observed in some coarsened grains. In general, grain coarsening becomes more non-uniform with increasing temperature under various applied loading, but the non-uniformity of grain coarsening changes little with temperature under cyclic loading. A comprehensive analysis indicates that the deformation mechanism varies with the grain size and grain shape for the annealed and un-annealed UFG materials. For UFG materials with equiaxed grains (e.g., the present ECAPed Cu), an appropriate short-term annealing treatment at temperatures below recrystallization could improve their strength and ductility together (e.g., hand-in-hand improvement of compressive yield stress and plastic deformation ability of the present UFG Cu).