CONSOLIDATION OF MECHANICALLY ALLOYED AMORPHOUS CONBZR POWDER BY HIP

A novel HIP processing of mechanically alloyed (MA) amorphous powder is developed, based on the densification via viscous flow mechanism, with which it is possible to obtain a fully dense amorphous compact of large scale. This study describes the evaluation of viscous flow in a thermal mechanical analyzer (TMA) and the optimization of HIP variables, time, temperature, pressure and heating rate for a nearly full densification of MA amorphous Co79.5Nb15Zr5.5 powder. The pre-compacted compressive sample of amorphous Co79.5Nb15Zr5.5 shows a drastic plastic displacement after thermal shrinkage in a constant heating rate experiment far below the temperature at the onset of crystallization (T(x)). An analysis of the TMA curve for a porous amorphous compact permits us to derive the displacement rate and the viscosity. The temperature dependence of the newtonian viscosity (eta) is fairly well expressed by an Arrhenius typed relation of eta = eta(0) exp (221 kJ.mol-1/kT) within the range of the experiment in this study. The well-defined glass temperature (T(g)) shows a constancy for heating rates above 1.7 x 10(-1) K.s-1, but a great increase due to structural relaxation below 8.3 x 10(-2) K.s-1. HIP cmpaction of MA amorphous powder in vacuumed can in a temperature range, T(g) < T < T(x) under a pressure of 196 MPa makes it possible to obtain a high density amorphous product with a diameter of 24 mm at the nearly maximum in our laboratory HIP apparatus. An increase in holding time and/or temperature leads to an increase in relative density of the amorphous HIP compact, approaching to theoretical one. These increases are in good agreement with a HIP map which is constructed based on newtonian viscous flow mechanism with an Arrhenius equation with an increased eta(0) for MA amorphous powder subjected to extensive structural relaxation. The compressive strength (sigma(F)) for the amorphous HIP compact of Co79.5Nb15Zr5.5 greatly increases with decreasing porosity (P(o)). A power law of compressive strength is derived as sigma(F) = BP(o)-0.5.