SHEAR LOCALIZATION AND CHEMICAL-REACTION IN TI-SI AND NB-SI POWDER MIXTURES - THERMOCHEMICAL ANALYSIS

Chemical reactions in Ti-Si and Nb-Si powder mixtures were initiated i n regions of high plastic strain induced by high-strain-rate deformati on. These regions of high localized plastic strain had thicknesses of 10-25 mu m and a characteristic spacing of 600-1000 mu m. Scaling up o f the experiments revealed a shear-band spacing that is constant and d ictated by material and deformation parameters. The generation of heat due to plastic deformation and chemical reaction is treated in a one- dimensional calculation. The calculations are in qualitative agreement with experimental results: shear bands can serve as ignition regions for the propagation of the reaction throughout the entire specimen for Ti-Si, whereas in the Nb-Si system (that has a much lower enthalpy of reaction), the reaction is always localized in the shear bands. These results enable the estimation of a reaction time in the Ti-Si mixture (similar to 10 ms) and of a critical global strain required for the c omplete reaction to take place (epsilon(eff)= 0.38). This is a new reg ime of reaction, intermediate between combustion synthesis and shock c ompression synthesis. (C) 1998 American Institute of Physics.