Dislocation Loop Formation in Refractory Metal under Nanoindentation Studied by Molecular Dynamics

In virtue of their high melting points and excellent high-temperature mechanical properties, refractory metals, such as Ta, Mo, and W are important potential structural materials for space reactors and fusion reactors, but their embrittlement is one of the limiting issues for their application. It is wellknown that the mechanical properties of materials are often related to their defect behaviors, especially dislocations. Indentation is one of the most commonly used techniques to measure the mechanical properties of materials, therefore, in order to understand the defect behavior in the metals, nanoindentations of these three refractory metals were studied by molecular dynamics (MD) in this paper. Nanoindentation processes of these three metals by choosing different potential functions at room temperature were simulated, 100 and 111 crystal orientations were selected to conduct the simulation. The hardness and reduced elastic modulus of these three metals were calculated and the length of the dislocations in the metals was counted. Initially, dislocations in the metal nucleate and grow under the action of the indenter, so the length of dislocations generally presents an upward trend, and then, when the indenter begins to return to its initial position, no new dislocation is generated in the metal and the continuous reaction between existing dislocations leads to the reduction of the total dislocation length, it is also found that the length of 〈111〉 dislocation is the longest in all cases. At the same time, the formation of independent dislocation loops is observed in the three refractory metals, but the simulation results of different crystal orientations are different. In 111 crystal orientation, it is found that independent dislocation loops are formed in all three refractory metals, and the formation mechanisms of dislocation loops can be summarized in two types. In Ta and Mo, as the edge dislocation moves forward continuously, the screw dislocations connecting the two ends of the edge dislocation get closer and closer, and eventually shear each other to form independent dislocation loop. In W, the edge dislocations move forward which driving the screw dislocation on both sides to move forward together, causing the connection between screw dislocation on both sides and the edge dislocation in the upper to be disconnected, forming an isolated dislocation segment, and then the dislocation segments are connected end to end to form a closed dislocation loop. However, in 100 crystal orientation, independent dislocation loops are observed only in Mo which is formed in the same way as in 111 crystal orientation, while no independent dislocation loops are found in W and Ta.