Effect of phase composition and hydrogen level on the deformation behavior of titanium-hydrogen alloys
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I.
INTRODUCTION
HYDROGEN is a unique alloying element in titanium alloys because, unlike other elements, it can readily be added and removed without melting. The addition of hydrogen as an alloying element has a beneficial effect on hot workability,tl.2J because it leads to a significant increase in the ductility and a decrease in the strength at elevated temperatures, t3,4,51 Thus, operations involving hot processing, such as forging,t3,61hot pressing,tt.4~ superplastic forming,t7,8~ powder consolidation,t2] and composite processing,t9] are all facilitated. The extent of the hydrogen improved workability (HIW) depends markedly on the temperature, strain rate, and hydrogen concentration,t4,51 as it is sensitive to phase composition and microstructure. As a rule, the maximum extent of HIW occurs in the two-phase (a + /3) region.[lOl Because hydrogen is a/3 stabilizer in titanium alloys, it has been suggestedv,3,~l that HIW is a result of the increased proportion of the more-workable bcc /3 phase. This case has not been convincingly made, however, because most of these investigations have dealt with complex alloys in which there is redistribution of the other alloying elements between the a and/3 phases, and this also affects the mechanical properties. The phase diagram of the Ti-H system is shown in Figure 1. Below the eutectoid temperature (300 ~ the solubility of hydrogen in the a phase is low, and when this is exceeded, hydride precipitates (6 phase) form. At temperatures above the eutectoid, the addition of hydrogen causes the a phase to transform into the /3 phase. The solubility of hydrogen in the/3 phase is much greater than in the a phase. In the two-phase region, the hydrogen concentrations in the a and/3 phases do not change with increasing hydrogen content at a given temperature; only the phase proportion changes. It is therefore possible to compare the properties of the two-phase alloys with the properties of the single-phase c~ and /3 alloys and to study the effects of phase configuration on the HIW.
O.N. SENKOV, Visiting Scientist, and J.J. JONAS, Professor, are with the Department of Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2A7. Manuscript submitted June 29, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
In this article, the deformation properties of Ti-H alloys in the single-phase c~ and/3 fields are presented. These are then compared with the properties of two-phase (a + /3) Ti-H alloys. The validity of the rule of mixtures in the (a + /3) region is then examined.
II.
E X P E R I M E N T A L MATERIALS AND PROCEDURE
Cylindrical specimens 8 mm in diameter and 12 mm in height were prepared from rolled rods of a titanium of technical purity.* The specimens were hydrogenated by holding *Chemical composition in at. pct: AI 0.426, V 0.255, Fe 0.058, O 0.016, C 0,067, N 0.001, and H 0.047.
at 800 ~ in a pure hydrogen environment for 2.5 hours. A powdered high-purity titanium hydride was the source of hydrogen for this purpose. Specimens to be hydrogenated were placed
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