Quantitative phase analysis using the Rietveld method for samples in the Ti-Cr binary systems

TiCiappears in three allotropic phases, cubic R.T. phase and two hexagonal H.T. phases. In practice it is difficult to obtain single phase materials at room temperature and quantitative phase analysis should be performed in order to determine the relative phase amounts. The diffractograms of samples from different sources showed that the major contribution to the intensities came from the cubic phase (C15) CuzMg type. A second phase was identified as the hexagonal (C36) MgNi2 type. The two phases yield complicated diffracograms in which most lines are overlapping. Thus, the quantitative phase analysis aided by the Rietveld method was adopted. The results for the TiCr2 powders showed that the weight fraction of the hexagonal was much above the contribution in integrated intensities. The conflict between the intensity contribution of the diffractograms and the actual weight fraction of the phases, comes from different structure factors and multiplicities. Introduction There are some interesting aspects in the thermodynamic system of TiCr2-I$, for example, low critical temperature, high absorption capacity and low hysteresis [ 11. According to the equilibrium phase diagram [2], there is only one intermediate phase compound TiCr2, formed concurrently at 1370°C from Ti-Cr cI2 solid solution. This intermetallic compound exists in three intermediate allotrope phases. The crystal structures of the three allotropic phases of TiCr2 are known [3,4]. The room temperature phase (a) is cubic (C15) CuzMg type, S.G. Fd3m, stable up to l%O”C, and two hexagonal high temperatures phases, p (C36) MgNiz type and y (C14) MgZn, type, S.G. P6&nmc for both. There is a wide temperature range (SOO-1220°C) in which the a and p phases are overlapped. Thus, due to the slow kinetics, the as cast material is formed mainly as p phase, and the a phase is obtained after heat treatment. In practice it is difficult to obtain pure a phase at room temperature, and special prolonged heat treatments are required to reduce the amounts of the unwanted phases in these mixtures [ 11. The two phases, a and p, have the same densities and compositions, the metallography shows no contrast between the phases, thus, Quantitative X-Ray Powder Diffraction (QXRPD) is required for accomplishing complete phase analysis. Yet, the two phases yield complicated diffractograms in which most lines are overlapping. Hence, the quantitative phase analysis aided by the Rietveld method was adopted. Copyright (C) JCPDS-International Centre for Diffraction Data 1999