Effect of metallurgical condition on the creep behaviour of Ti-13V-11Cr-3Al

Ti -13V-11Cr -3A1 was the first of the beta titanium alloys to be developed commercially having its initial development in the early to mid-1950s [1]. Despite melting, processing and reproducibility problems [2], and the conservatism of designers, T i 13V11Cr3A1 found quite extensive application in the Lockheed SR-71 Blackbird Mach 3+ surveillance airplane [3]. Because of the large additions of the beta-stabilizing elements vanadium and chromium, normal air cooling of the alloy from the annealing temperature above the /3-transus temperature of 650 to 700°C [4] retains the b c c/3-phase structure, and thus annealing is synonymous with solution treating. The//-phase retained at room temperature from this annealing or solution treating is metastable and will decompose by thermal treatment (in situ operation) or ageing to give the h c p e-phase and an intermetallic compound TiCr 2. Although the alloy is very formable in the beta-phase condition, the formation of the e or TiCr2 phases causes mechanical property changes and can lead to embrittlement of the alloy [4, 5-10]. As part of a wider investigation into the micromechanisms of creep [11], the Ti 13V l l C r 3A1 alloy was creep tested at 650°C in two metallurgical conditions, all-3 and // + e + TiCr2, and both the shape of the primary creep curve and the steady-state creep rates were determined. Given a melting temperature of approximately 2000K [7], the creep test temperature of 650 ° C is approximately 0.46T,,, where Tm is the melting temperature (K). The results are compared with those found by Oikawa et al. [12-14] for pure titanium in both the 3-phase (high temperature) and e-phase (low temperature) forms. The Ti 13V-11Cr 3A1 alloy was in the form of bar stock. Cylindrical tensile creep specimens were machined with a gauge length of 25.4 mm and a crosssectional area of 10 mm 2. Two different heat-treatment schedules were applied to the creep specimens; (i) 1 h at 800 ° C (in vacuum) followed by water quench. This produced an all-beta phase material (see T 7 T diagram in Fig. 1); (ii) as for (i) plus 100h at 500 ° C. This produced/3 + e + TiCr 2 structure (Fig. 1). Sections of the creep specimens prior to creep testing were mounted, polished, etched in a solution of 1 part HNO3, 1 part HF; 2 parts glycerin, and the grain size was determined by the linear intercept method using optical microscopy at a magnification of x 100. The grain sizes were determined as 68 _ 22 #m for the //-material and 99 + 40/~m for the 3 + e + 7 0 0