Investigation On The Thermal Stability Of Fatigue Dislocation Structures Of Face-centered Cubic Metal Crystals

In the past few decades,many studies have been focused on the cyclic deformation behavior of various face-centered cubic(FCC)metallic materials.The relevant research findings about cyclic deformation behavior and dislocation structures have been achieved in succession by investigators.Nevertheless,systematic work on the thermal stability of dislocation structures induced by fatigue deformation still remains rather limited.In the present work,Cu single crystals with different orientations and FCC polycrystalline alloys with different stacking fault energies have been selected as the target materials,and the microstructures of fatigued and annealed samples were examined using electronic channel contrast technique in scanning electron microscopy(SEM-ECC)and transmission electron microscopy(TEM).The major influencing factors on the thermal stability of dislocation structures have been revealed.Based on the studies on the Cu single crystals,it was found that fatigue saturation dislocation structures,e.g.,persistent slip band(PSB)ladder structure,PSB cells,veins,labyrinth,walls and cells,have undergone an obvious process of recovery after annealing treatment at 300?.After being annealed at 500? and 800?,those dislocation structures basically disappear,recrystallization phenomenon occurs,and plenty of annealing twins occur in the fatigued samples.The formation of annealing twins and recrystallization are strongly dependent on the annealing temperature,the applied plastic strain amplitude and the accumulated plastic strain.With increasing plastic strain amplitude and accumulated plastic strain,the degree of strain concentration increases significantly,which provides the required local strain energy for the initiation of twins and recrystallization.The recrystallization becomes more evident,and the number and the size of annealing twins increase.The nucleation of recrystallization occurs in the form of rotational polymerization of sub-grains.The annealing twins are developed strictly along the activated slip planes after fatigue.The formation of annealing twins is also closely related to the occurrence of stacking faults.The mistake of atomic arrangement at grain boundaries leads to the appearance of stacking faults,and they are further coalesced to grow up,eventually evolving into twins.The higher the degree of slip deformation after fatigue is,the more the twin formed after annealing is.However,the high annealing temperature will accelerate the migration rate of grain boundary,and inhibit the formation of twin.The level of thermal stability of the dislocation structure in Cu single crystals with different orientations from higher to lower is arranged in order:dislocation veins? PSB ladder structure?labyrinth?dislocation wall/cell structure.Based on the studies of FCC metallic materials with different stacking fault energies(SFE),it is found that,only planar-slip dislocation structures are induced in fatigued Cu-16at.%Al alloy with a very low SFE,mainly including planar slip bands,persistent Luder's bands(PLBs),stacking faults and dislocation arrays.After being annealed at 300?and 500?,the original fatigue dislocation structures have undergone an obvious process of recovery,and the dislocation density decreases,but those structures are still clearly visible.However,there are only few dislocation arrays and dislocation lines are survived after annealing treatment at 800?.For Cu polycrystals,various wavy-slip dislocation structures are formed after cyclic deformation,like veins,irregular cell-walls,walls and cells.After being annealed at 300?,the dislocation density decreases,but the original dislocation structures still exist.After annealing at the higher temperatures,e.g.,500 and 800?,some dislocation structures become disappeared,and there only exists a few loose cells and dislocation lines.For coarse-grained pure Al,the dislocation structures induced by fatigue deformation are mainly featured by dislocation cells.The polygonization recovery happens at higher strain amplitudes underan annealing treatment at 200?,while the dislocation cells become looser at 330? and 450? annealing treatments.Based on the studies of Cu-16at.%Al alloy and Cu polycrystals,it is found that,under the same heat treatment conditions,the higher the accumulated plastic strain is,the more remarkable the recovery becomes.According to the above experimental results,the thermal stability of the dislocation structures of FCC metals is jointly affected by many factors including annealing temperature,stacking fault energy,plastic strain amplitude and accumulated plastic strain.For a fixed material,the higher the annealing temperature and strain amplitude are,the worse the thermal stability of the dislocation structures is;however,such a difference is not obvious.With the accumulated plastic strain increasing,particularly as the accumulated plastic strainis very high,the thermal stability of the dislocation structures would reduce greatly.For the metallic materials with different SFEs,the fatigue dislocation structures(e.g.,planar-slip dislocation structures)are more stable in low SFE materials;however,with increasing SFE,wavy-slip dislocation structures tend to form,and the relevant thermal stability of dislocation structure gradually decreases.