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

Abstract

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, α-CuTi and β-Cu 3Ti, through spinodal decomposition which is intensified on prolonged aging at 673 K. In the annealed powders aged at 673 K the crystallite sizes of β-Cu 3Ti phase are found to be smaller whereas the microstrains and dislocation densities are higher compared to those of the α-CuTi phase. In the α-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 β-Cu 3Ti phase has very high values of microstrains and dislocation densities (∼10 12 cm/cm 3) compared to those of α-CuTi. Profuse twinning is found in the α-CuTi phase of both the annealed powders and bulk aged at 673 K. Severe cold working has little effect on the microstructure of β-Cu 3Ti phase but the deformation effect on the α-CuTi phase results in the generation of low densities of deformation stacking faults, high densities of deformation twins and high dislocation densities (∼10 11 cm/cm 3) with increasing Ti concentration. With increasing volume fraction of β-Cu 3Ti phase in aged bulk Cu–6.0 wt.%Ti alloy the microhardness value increases almost 2-fold (from 290 to 490 kg/mm 2) imparting considerable improvement in its mechanical property.