Comparative Creep Properties of Single-Phase Intermetallics of the Ti-Al System
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COMPARATIVE CREEP PROPERTIES OF SINGLE-PHASE INTERMETALLICS OF THE Ti-Al SYSTEM Kouichi MARUYAMA and Hiroshi OIKAWA Department of Materials Science, Tohoku University, Sendai, 980, Japan
ABSTRACT
Creep characteristics of polycrystalline single-phase intermetallics, a2-Ti 3AI and ,-TiAI, at 1100 K have been summarized to evaluate their relative strength at elevated temperature at three strain-rate levels, 10-1, 10-4 and 10"8s-1, and are compared with those of a Ti-Al terminal solid solution alloys. The strength of these intermetallics is obviously higher than that of (x,random solid solutions, especially at low strainrates. However, the difference in high-temperature long-term strength among intermetallic alloys is not significant at any strain-rate level. INTRODUCTION Alloys based on the intermetallic phases of the Ti-Al system have been studied by many investigators as promising high-temperature materials. One of the essential properties in materials for high-temperature use is their creep feature. However, systematic studies of creep in intermetallics of the Ti-Al system are still limited. Although the evaluation of the long-term strength of matrices will give a sound basis for alloy design, the relative strength among basic phases in the Ti-Al system has not been investigated systematically. In this study, the high-temperature strength of three phases inthe Ti-Al system has been compared. Most experimental data used here have been obtained on single-phase polycrystalline materials inthe authors' laboratory during recent years. The strength of these single-phase materials gives a guaranteed lower-bound elevated-temperature strength of intermetallic alloys based on these matrices. In this report, the minimum creep rates rmare taken as the measure of creep strength and the effects of material conditions, such as the composition N (the mole fraction of solute) and the initial grain size do, and external conditions, such as stress a and temperature T, on the minimum creep rate are analyzed according to the following equation,
e im-
(1 +X)m d0P (Y" exp(-Q/R7).
(1)
Here, X is the off-stoichiometric factor which is related with N by X=(N/No-1) (NO: the
stoichiometric composition), R and Thave the usual meanings. Parameters, m, p, n and 0, are numerical constants which are determined experimentally. CREEP CHARACTERISTICS OF THE y-PHASE A fair amount of data has been reported on the creep behavior of the -TiAI, for example, see ref.l. Typical examples of the • -a relationship in three single-phase polycrystalline y-phase materials is shown in r.1 [2], inwhich three regions can be recognized clearly inevery material. Creep in each region has unique characteristics. The values of parameters in eq.(1) for creep of single-phase ,-alloys are cited in Table I. These values are based mostly on ref.1. Creep characteristics inthe low stress region are not yet determined firmly, but tentative values, based on preliminary experiments [3,4], are cited with Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Society
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