In Situ Investigation On Plastic Deformation And Cracking Of Metals Under TEM/SEM

In the present work, techniques of in situ tension under transmission electron microscope (TEM) and scanning electron microscope (SEM) are used to study the processes of plastic deformation and cracking in 304L stainless steel, high manganese austenitic steel, brass, Al-alloy ZL101 and 15 steel; techniques of synchronized analysis at the same position of specimens by TEM / SAD (selected area electron diffraction) or SEM / TEM are also used.Detailed studies on formation and characteristics of dislocation-free zones (DFZ) in the high manganese austenitic steel, 304L steel and brass with face-center-cubic structure have been carried out. Under tension, dislocation-free-zones (DFZ) in ellipsoidal shapes can be often observed in all three metals investigated. The DFZs are surrounded by dense arrays of anti-pile-up dislocations. Rather high elastic distortion of crystal has been detected in the DFZs, the maximum lattice distortion in which is about 14%. The half axis ratio of the DFZ formed under the same condition in the high manganese austenitic steel and brass are different, the average value is 1.74 for the former and 3.85 for the latter. It is believed that the half axis ratio of the DFZ is related to the slip ability of dislocations in the crystals. Synchronized analyses on the process of crack propagation and the lattice plane distortion changing in the DFZ were carried out by using the in situ technique under TEM-SAD. From diffraction patterns it can be concluded that the distortion of lattice occurs in the forms of compression, incline and local bending.The cracking processes in the single phase FCC metals were also studied. Crack initiations were often observed in the DFZ and did not be found in the area of anti-pile-up dislocation arrays. There are two different modes for crack propagation: continuous mode and zig-zag mode, which are determined by the relationship of crystal orientation with tension direction. Continuous crack propagation occurs when the shearing stress component on the {111} plane and the normal stress component on the {100} plane are located at the same side of the tension axis; while zig-zag propagations occurs when these stress components are located on the different sides of the tension axis. When the crack propagates with the continuous mode, its tip is blunted at first in the DFZ formed previously, then a new crack forms in the DFZ and, finally, it connects withinthe original crack. There is no fixed crystallographic direction being observed during the continuous crack propagation. When the crack propagates with the zig-zag mode, twins are always observed in the DFZ at first. Then, a new crack initiates at a stress concentration point where one of the twin boundaries intersects a (100) plane; subsequently, it propagates dramatically along the (100) plane and stops at the opposite twin boundary. Furthermore, another crack initiates in the grain on the other side of the twin boundary, along the (100) plane. The formed cracks broaden and connect with each other due to the gliding of crack along the twin boundaries. As a result, the propagated crack shows the zig-zag characteristic. The continuous and the zig-zag modes of crack propagation can be observed in all three FCC metals (high manganese austenitic steel, austenitic steel, aluminum and brass) studied, which indicates that the crack propagation modes are determined mainly by the crystallographic factorsStudies about the role of grain boundary in plastic deformation and cracking processes provides direct evidences for relative concepts of dislocation theory. The plastic deformation behaviors related to grain boundary are grain boundary movement, dislocation piling-up at the grain boundary, the emission of dislocation from the gain boundary and the activation of slip systems in neighbouring grains. Grain boundary and annealing twin boundary may be bent and moved under the action of high stress near the DFZ. Steps have been often observed at the common grain boundaries, which act as dislocation sources and emit dislocat...