Diffusion in the titanium-nickel system: I. occurrence and growth of the various intermetallic compounds
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THIS work deals with interdiffusion phenomena in the Ti-Ni system. One of the objects was to establish the constitution of the Ti-Nt equilibrium diagram, especially in the region of the equiatomic composition TiNi. To this end a diffusion couple technique was chosen in which diffusion couples, consisting of combinations of the pure metals and intermetallic compounds, were subjected to heat treatment at various temperatures and subsequently examined. Using this technique, it was possible at the same t i m e to collect important diffusion data which govern the interdiffusion behavior in the Tt-Nl system. Such a study seemed worthwhile in the light of the growing interest in the intermeta!ltc compound TiNi which shows some remarkable properties, e . g . , the shape m e m o r y effect. Part I of this work will be devoted to the growth kinetics of the various intermetallic compounds and will have a phenomenological character. Part II deals with calculations of diffusion coefficients and relative mobiltties of the diffusing species. I. EXPERIMENTAL P R O C E D U R E A. Materials Used For a first exploration of the Ti-Ni system use was made of Tl-sheet (99.5 wt pot) and Nt-sheet (99.4 wt pot), both supplied by Drijfhout (Holland). Ti rod (99.5 wt pct), supplied by V.D.M. (Germany) and Ni rod (99.997 wt pct), supplied by Halewood Chemicals (England), were used in the more quantitative stage of the investigation. Finally, all results were checked using v e r y pure metals: Ti rod (99.97 wt pct) and Ni rod (99.99 wt pot), both supplied by M.R.C. (U.S.A.). G. F. BASTINand G. D. RIECKare Research Scientist and Professor of PhysicalChemistry, respectively, University of Technology, Eindhoven, Netherlands. This paper is based on a Thesis submitted by G. F. BASTINin fulfillment or requirements for the degree of Doctor in TechnologicalSciences. Manuscript submitted July 23, 1973. METALLURGICALTRANSACTIONS
Alloys for diffusion couples were prepared by repeated a r g o n - a r c melting. The weight losses during melting were never more than 0.1 wt pct. B. Preparation of Diffusion Couples Diffusion couples were prepared by welding 2-ram thick alloy discs together. Before welding, the discs were ground on silicon carbide paper through 600 grit. No better results were obtained by polishing or etching the starting materials. For the preparation of diffusion couPles three methods have been used: a) Hot pressing in a vice. With this method, which has mainly been used in the explorative stage of the investigation, a titanium disc was sandwiched between two nickel discs and clamped in a vice. The whole a s sembly was then heated under argon. Times required for a good weld varied between 15 min at 800~ and 2 h at 700~ It turned out, however, that the argon atmosphere did not protect the titanium sufficiently against contamination by oxygen and nitrogen. Therefore, the next method was preferred. b) Hot pressing under controlled p r e s s u r e . A special furnace x was designed in which diffusion couples could be prepared at temperatures up
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