Deformation mechanisms of advanced structural alloys : from single-phase alloys to metal-matrix composite

Abstract

In recent years, there has been an increasing demand for high-performance structural metals and alloys for lightweight applications in transportation and construction industries. High-performance metals and alloys in general, require simultaneously ultra-high strength as well as good ductility. In order to provide guidelines for the design of new high-performance metals and alloys with high strength and good ductility, one needs to understand their deformation mechanisms that determine the strength and ductility of metals and alloys.

This thesis aims at providing new scientific understanding of deformation mechanisms of various metals and alloys by carrying out systematic theoretical, experimental and numerical studies. In the first part, fundamental questions related to the rate-dependent deformation mechanisms of single-phase metals and alloys are addressed. For coarse-grained metals, two types of rate-controlling flow rules were unified into a simple crystal plasticity model and strain-rate sensitivity (SRS) was found to have explicit physical meaning. For ultrafine-grained or nano-grained metals, grain boundary (GB)-mediated plasticity was incorporated into the crystal plasticity model. GB strengthening, stress saturation and evolution of SRS were all predicted by this model. For carbon-added alloys such as carbon-added twinning-induced plasticity (TWIP) steel, the roles of carbon atoms on the rate-controlling plastic deformation mechanisms and on the SRS were investigated in details, showing the fundamental effects of carbon on both the thermally activated dislocation behaviors at yielding and on the dislocation multiplication. In the second part, the damage mechanisms of TiB2 reinforced steel matrix composite were investigated, indicating that the damage process of this type of material is a sequential process of primary-particle cracking, the fracture of surrounding eutectic particles and finally the growth and coalescence of void in ferrite matrix. The deformation behavior and shear strength of the interface between TiB2 and matrix were probed, showing that the interfacial shear strength is as high as ~700 MPa. This high interfacial strength promotes high load transfer rate between matrix and reinforcements, and explains why particle fracture rather than interface debonding was observed.