X-ray characterization of the microstructure of α-CuTi alloys by Rietveld's method

The lattice-defect related features of the microstructures and the properties as evoluted in Cu-1.0, 3.0, 4.5 and 6.0 wt.%Ti alloys have been studied using Rietveld's whole X-ray profile fitting method incorporating Popa model for crystallite (domain) size and microstrain (root mean square, r.m.s. strain) and the preferred orientation of the crystallites. Direct observations of the microstructure have been effected through the optical and scanning electron microscopy studies and the mechanical property has been measured from microhardness studies. The high Ti content alloys are found to decompose into two phases, alpha -CuTi and beta -C3Ti, through spinodal decomposition which is intensified on prolonged aging at 673 K. In the annealed powders aged at 673 K the crystallite sizes of beta -Cu3Ti phase are found to be smaller whereas the microstrains and dislocation densities are higher compared to those of the alpha -CuTi phase. In the alpha -matrix of the aged alloy powders the microstrain and the dislocation density increase and the crystallite size decreases with the increase in Ti concentration. The transformation behaviour of the aged bulk reveals beta -Cu3Ti phase has very high values of microstrains and dislocation densities (similar to 10(12) cm/cm(3)) compared to those of alpha -CuTi. Profuse twinning is found in the alpha -CuTi phase of both the annealed powders and bulk aged at 673 K. Severe cold working has little effect on the microstructure of beta -Cu3Ti phase but the deformation effect on the alpha -CuTi phase results in the generation of low densities of deformation stacking faults, high densities of deformation twins and high dislocation densities (similar to 10(11) cm/cm(3)) with increasing Ti concentration. With increasing volume fraction of beta -Cu3Ti phase in aged bulk Cu-6.0 wt.%Ti alloy the microhardness value increases almost 2-fold (from 190 to 490 kg/mm(2)) imparting considerable improvement in its mechanical property. (C) 2000 Elsevier Science B.V. All rights reserved.