Phase transformations in metastable Ti-V alloys induced by high pressure treatment
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I. INTRODUCTION W H E N alloying titanium with transition elements the metastable phases can be formed by quenching. It is of considerable interest to understand the physical principles of phase stability of such alloys for different pressures, compositions and temperatures. The alloying of titanium with VB-VIII elements is always characterized by the decrease of an alloy mean atomic radius and by the increase of the number of valence electrons per atom. The application of high pressure to these alloys causes a similar effect: the density of valence electrons increases and the mean atomic radius decreases. However, the effect of composition and the effect of high pressure treatment are not completely analogous. The weight of an alloy per mole is not changed at high pressure whilst the composition change does affect the molar weight and, consequently, the lattice dynamics and electron-phonon interaction. Besides, high pressure influences the inherent lattice properties to a lesser degree than does the composition. Therefore, it was of direct interest to study systematically the phase stability and structural changes induced by high pressure in titanium alloys and to compare the data obtained with the composition effect. In our recent work ~we have investigated the Ti-Mo system. In the present study experiments with high pressure treatment of Ti-V alloys were carried out. Only the metastable structures created by quenching were pressurized and high pressure was applied at room temperature. Therefore all the observed phase transformations were regarded as completely diffusionless. It should be noted that in previous publications 3-5 the phase transformations in metastable Ti-alloys were diffusion-controlled. The calculated temperature-composition T-X (at P-- 1 atm) and pressure-composition P-X (at T = 300 ~ diagrams for Ti-V alloys are presented in Fig. 1.2 CH. LEIBOVITCH, formerly Graduate Student, is now Research Fellow, and A. RABINKIN is Associate Professor, Materials Engineering Department, Technion - Israel Institute of Technology, Haifa, Israel. M. TALIANKER is Lecturer, Materials Engineering Department, Ben Gurion University of the Negev, Beer Sheva, Israel. Manuscript submitted May 16, 1980. METALLURGICAL TRANSACTIONS A
These diagrams were calculated by using data which we obtained, and a thermodynamical regular solutions approach. The following metastable phases should be observed in as-quenched Ti-V alloy: a) alpha-prime (a') martensitic phase which occurs at --0- 8atpctV. b) ~'+fl+r phases which occur at ~ 8 - 14 at pet V. c) fl+r phases at ~ 1 4 - 2 5 at pet. d) fl metastable phase, when V-content is more than 25 at pct. The shaded zone in the (T-x) diagram marks the area where the o~-phase appears. II. EXPERIMENTAL PROCEDURE The alloys were arc melted from titanium (~99.9 wt pct analytical purity) with vanadium (spectroscopical purity) on a water-cooled cold hearth and remelted 5 times to insure homogeneity. A protective atmosphere of purified argon and a titanium getter were used. Weight change after m
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