Influence of test temperature and microstructure on the tensile properties of titanium alloys
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I.
INTRODUCTION
THEapplication of titanium alloys at elevated temperatures is limited to approximately 500 ~ which generally is considered as an upper value for a safe service temperature region. With regard to the high melting temperature of titanium (about 1667 ~ this temperature range is rather small. The reasons for the restricted application of titanium alloys as high temperature materials are the rather low creep resistance, l severe oxidation embrittlement at elevated temperatures, 2 and a substantial loss in yield stress with increasing test temperature. 3 For example, the Ti-6A1-4V alloy exhibits a yield stress of about 1000 MNm -2 if tested at room temperature, while this value can be as low as 450 MNm -2 at test temperatures of about 500 ~ Most commercial titanium alloys contain rather high concentrations of interstitial oxygen atoms in order to reach the high yield stress values at room temperature required especially in aircraft specifications. It is well known, however, from studies on pure titanium and single phase a-titanium alloys that the interstitial atoms are responsible for the strong temperature dependence of the yield stress/ On the other hand, high temperature alloys, for example, Ni superalloys, are known to retain a rather high yield stress up to test temperatures of about 800 ~ 5 The superior high temperature behavior of nickel alloys results from the presence of a high-volume fraction of ordered and coherent Ni3AI particles. 5'6 These ordered particles are reported to be responsible for the observed inverse temperature dependence of the yield stress in nickel alloys, 5'6'7 an effect which also has been found in other fcc alloys containing ordered phases. 8'9 Titanium alloys with aluminum as an alloying element in excess of about 6 wt pct in the a-phase also can be heat treated in order to precipitate ordered and coherent Ti3A1 particles. 1~ However, in most commercial titanium alloys the volume fraction of Ti3A1 is rather low due to the low aluminum content.
A. GYSLER is Research Scientist, German Aerospace Research Establishment (DFVLR), 5000 Cologne 90, Germany. G. LUTJERING is Professor, Technische Universitht Hamburg-Harburg, 2100 Hamburg 90, Germany. Manuscript submitted January 28, 1981. METALLURGICALTRANSACTIONS A
The purpose of the present work was to study the influence of some selected microstructural parameters on tensile properties of titanium alloys in the region between room temperature and 500 ~ The parameters included in the study are the degree of age-hardening, grain size, phase morphology, and phase dimensions of (a + /3) microstructures, texture, and concentration of interstitial atoms. Ti-6A1-4V and Ti-AI alloys were chosen for this study.
II.
EXPERIMENTAL PROCEDURE
The tests were performed on two binary Ti-Al alloys with 8.6 and 10 wt pct A1 and on three Ti-6AI-4V alloys. The chemical compositions of these alloys are listed in Table I. The Ti-A1 alloys were solution heat treated and hot rolled in the a-phase field to obtain equiaxed grain structures. Differen
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