Creep of Hydrogen-Charged Ti-5 AI-2.5 Sn at room temperature
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Creep of Hydrogen-Charged Ti-5 AI-2.5 Sn at Room Temperature 10-3
NEIL E. PATON and ANTHONY W. THOMPSON Previous work by Thompson and Odegard ~ has established the extent of creep in Ti-5 A1-2.5 Sn (hereafter Ti-5-2.5) at room temperature for a variety of stress levels and thermomechanical histories. Among the conclusions of that work were that room-temperature creep is entirely primary or "transient" creep; that dislocation source availability is critical to the total creep strain at a given time; and that dislocation overcoming of interstitial solute atoms 2 was the process governing the rate of dislocation motion. It was thus of interest to consider how additions of hydrogen might affect creep behavior. Interstitial hydrogen would be expected to behave like other interstitials 2 in titanium (oxygen, carbon, and nitrogen) and act as a barrier to dislocations, thereby reducing creep strain at a given time; the same would be predicted by analogy with detailed knowledge of hydrogen in iron.3 On the other hand, if enough hydrogen were present to permit formation of stress-assisted hydrides, the dislocations punched out 4,5 in the process could be available to assist creep strain. Specimens were prepared from the alpha-forged Ti-5-2.5 material used previously; analysis of the bar stock used for forging billets showed 12 ppm by wt hydrogen, l It was intended to creep-test these specimens at 295 K in a temperature-controlled room, using established procedures 1'6 with a strain precision I of ---2 • 10 -6, at a stress of 737 MPa (107 ksi), or 80 pct of the yield strength. The specimens were prepared by vacuum annealing at 1200 K (1700 ~ and then adding various amounts of hydrogen in a Sieverts apparatus. The three conditions tested then were: 111 ppm H, 186 ppm, and 303 ppm H (ppm by wt). It was planned to compare the results to data for as-annealed specimens tested previously. 1 Tests for 1000 hours on the 111 and 186 ppm specimens gave the creep strains shown in Figure 1. The 111 ppm specimen results were in good agreement with the prior results ~ on as-annealed specimens containing a nominal 12 ppm H, suggesting that as much as 100 ppm H is not sufficient to alter creep rates in this material. On the other hand, the 186-ppm specimen initially exhibited accelerated creep (Figure 1), which evidently would converge with or intersect the Thompson and Odegard data at about 7000 hours. In terms of the equation best fitting these data,1 e = A t a, whereA anda are constants, a < 1, the 186 ppm data have a lower a value. The 303 ppm specimen similarly showed enhanced early creep (higher A) but apparently with the lowest a value, with a tendency to converge with or intersect the data for lower hydrogen content. NEIL E. PATON is with Corporate Engineering, Rockwell International, 600 Grant Street, Pittsburgh, PA 15219. ANTHONY W. THOMPSON, formerly with Rockwell International, is now Professor, Department of Metallurgical Engineering and Materials Science, CarnegieMellon University, Pittsburgh,
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