Investigations On Fatigue Dislocation Structures And Their Thermal Stability Of FCC Metallic Crystals With Different Stacking Fault Energies

Although the effects of stacking fault energy (SFE) on fatigue dislocation structures of face-centered cubic (FCC) metallic crystals have been studied extensively, there is less report about the effects of SFE on the thermal stability of the fatigue dislocation structures. In the present work, three kinds of FCC metallic crystals (Al, Cu and Cu-16at.%Al) with obvious different SFEs were selected for push-pull fatigue tests with constant strain amplitude control, and then all samples were annealed at different temperatures. The electron channeling contrast (ECC) technique in scanning electron microscope (SEM) and transmission electron microscopy (TEM) were adopted to observe the fatigue microstructures before and after annealing, in order for revealing effects of SFE on the fatigue dislocations and their thermal stability, which could offer a substantial experimental basis for further developing the Low Energy Dislocation Structures (LEDS) theory.With increasing SFE, the fatigue dislocation structures of FCC metallic crystals was changed from typical planar slip to wavy slip structures:(1) The microstructures after cyclic deformation of Cu-16at.%Al(Low-SFE) alloy consist of primary dislocation arrays, planar slip bands and numerous stacking faults. With increasing total-strain-amplitude (Aεt/2), the planar slip bands are gradually developed into persistent Liider’s bands (PLBs), and the width of stacking faults decreases, but the volume fraction of stacking faults increases;(2) The dislocation structures of fatigued [112] Cu single crystals (Medium-SFE) are strongly dependent upon the applied plastic strain amplitude (yp1). When the sample is fatigued at the strain amplitude yp1of3.7×10-4(below the plateau region in the cyclic stress-strain (CSS) curve), the main structure is composed of veins, but a few persistent slip band (PSB) ladders are also found at this strain amplitude, indicating that the formation of PSB ladder structures is a necessary but not sufficient condition for the appearance of plateau region in the CSS curve. When yp1are within the plateau region, the microstructures consist of PSB ladder structures and matrix veins, namely two-phase structures. The matrix veins will transform into dislocation walls and even a few dislocation cells as yp1increases to a certain value, meanwhile, the volume fraction of PSB increases; When yp1are beyond the plateau region, numerous deformation bands (DBs) appear, and the dislocation structures in DBs will transform from dislocation walls into elongated cells, and finally into equiaxial cells, with increasing yp1, meanwhile, the cell size decreases in this process for bearing a higher accumulated plastic strain;(3) The dislocation structures of fatigued pure Al (High-SFE) with coarse grains are mainly featured by dislocation cells. With increasing Aεt/2, the average cell size decreases, cell walls become denser, and the density of dislocations in cells decreases.Investigations on the thermal stability of fatigue dislocation structures have been conducted through the isothermal annealing treatments at300℃,500℃and800℃for30min on the cyclically deformed Cu-16at.%Al alloy. The results show that no obvious recovery occurs for the samples fatigued at different Aεt/2to a low accumulated plastic strain after annealing at300℃and500℃, but a violent recovery of fatigue dislocation structures occurs when all samples were annealed at as high as800℃and the density of dislocations decreases seriously. In contrast, for the samples fatigued at high Aεt/2to a high accumulated plastic strain and then annealed at300℃, a visible recovery of fatigue dislocation structures had been detected, and interestingly, the numerous recovery twins with small size were observed to occur in the samples, which are strongly dependent upon the disappearance of stacking faults involved in fatigue microstructures after annealing at300℃. However, no recrystallization phenomenon was observed to occur in all fatigue samples of Cu-16at.%Al alloy after annealing at different temperatures. The results mentioned above show that the planar slip structures are relatively stable, and the thermal stability are closely related to the applied plastic strain amplitude and accumulated plastic strain, and therefore the temperature for recovery phenomenon will decrease as the plastic strain amplitude and accumulated plastic strain are getting higher.With increasing SFE, the dislocation cross slip and climb are becoming easier, and the thermal stability of fatigue dislocation structures thus decreases gradually. For example, investigations on the thermal stability of fatigue dislocation structures have been conducted through the isothermal annealing treatments at250℃and450℃for30min on the cyclically saturated [112] oriented copper single crystals with medium-SFE. The results show that a slight recovery occurs only at higher strain amplitude at250℃, but the recrystallization is not detected in all samples. When the annealing temperature is raised to450℃, a clear dislocation recovery is found in all samples, but the recrystallization is still not observed at lower strain amplitudes until the strain amplitude increases to γp1=4.9x10-3or yp1=7.2x10-3. In addition, more or less incomplete annealing twins are formed at γp1=12x10"3. Concerning the pure Al (High-SFE) with coarse grains fatigued and then annealed at200℃,330℃and450℃for30min, a clear recovery of fatigue dislocation structures occurs for all samples in all annealing temperatures. The only difference exists in the recovery mechanism of the sample fatigued at different strain amplitudes and then annealed at a lower temperature of200℃, for instance, the recovery of samples fatigued at lower strain amplitude is just dependent upon the disappearance of dislocation vacancies, while the polygonization phenomenon occurs at higher strain amplitudes.