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Phase transformations in titanium-rich alloys with iron and nickel Polonis, Douglas Hugh

Abstract

Phase transformations have been studied in titanium-rich binary alloys with iron and nickel. Particular attention has been given to the formation and decomposition of metastable phases in powder specimens. All alloys were prepared by a levitation melting technique and precautions were taken throughout the experimental work to minimize contamination. In the Ti=Fe system martensitic α'is produced when powder specimens containing up to 12% iron are quenched-from 1000⁰C. The hardness of hypoeutectoid specimens increases with iron content to a maximum at 12% Fe The eutectoid temperature for the system has been reassessed at 625 ± 10⁰C. During tempering the decomposition rates of retained β phase are slow but the appearance of FeTi is accompanied by an increase in slope of the β /log time curve. The hardness of tempered alloys increases as the FeTi content increases. Contrary to the results of other Investigators Ti₂Fe has been found to exist in sensibly oxygen-free -alloys. This phase forms at 1000⁰C in crushed powder specimens but decomposes below the euteetoid temperature. In the Ti-Ni system the .constitution of quenched alloys is found to depend on both composition and cooling rate from the β range. An 'inverse stabilization' of the β phase has been observed and the 100% β phase exhibits two types of substructures which have been attributed to polygonization and stacking faults. The hardness of quenched alloys is higher for higher nickel contents and for faster cooling rates. Orientation relationships were observed between β and α¹ and a shear mechanism suggested by Burgers for Zr is proposed for this system. Decomposition studies have shown that α¹ breaks down by a growth-controlled process similar to that described by Johnson and Mehl. An activation energy of 84000 cal/mole has been determined and a model has been proposed which involves planar interfaces of Ti₂Ni advancing into α regions to produce a Widmanstatten-type microstructure. The self-diffusion of titanium is believed to be the growth -controlling factor. Hardness values decrease with longer tempering times and higher tempering temperature. Retained β decomposes on tempering by a two stage process: β →α" →α + Ti₂Ni X-ray diffraction data indicate that α" has the same crystal structure as α'. The β-α" reaction appears to be a diffusion process although reaction curves are similar to those observed for isothermal martensite formation in steels. During the first stage of the reaction (β-α") the hardnesses and x-ray diffraction line breadths initially show a sharp increase, probably due to coherence between β and α¹¹ The reaction α" →α + Ti₂Ni proceeds In a similar way to the decomposition of α ; but with a shorter induction period for Ti2Ni formation. Further, the. activation energy for the α" →α + Ti₂Ni growth process (71000 cal/mole) is lower than that for α' decomposition. These observations suggest that Ni-rich regions exist in the α" phase and accelerate the nucleation and growth processes.

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