Diffusion Research in BCC Ti-Al-Mo Ternary Alloys
- PDF / 1,659,437 Bytes
- 6 Pages / 593.972 x 792 pts Page_size
- 65 Downloads / 221 Views
trength reached by b-Ti alloys, along with the extraordinary corrosion resistance, boosts them with widespread application in aerospace and automotive industries.[1–3] b-Ti alloys typically exhibit complex microstructure largely resulting from recovery, recrystallization, grain growth, and precipitation, while much of these processes are governed by diffusion interaction during thermomechanical processing, heat treatment, and service. The a-stabilizing Al addition remarkably increases the b transus temperature that in turn provides Ti alloys with enhanced tensile and creep strength and high elastic modulus whereas the bstabilizers, like Mo, V, Cr, and Fe, greatly contribute to superb high strength.[4] There are some studies focused on diffusion of alloying elements in binary[5–10] and ternary (including Ti-Al-Co,[7] Ti-Al-Fe,[8] Ti-Al-V,[9] and Ti-Al-Cr[10]) Ti-based b phase alloys. Surprisingly, the diffusion in the Ti-Al-Mo ternary, the archetypal base system of many important b-Ti alloys like Ti-5553 and b-21S remains unknown, which is therefore the objective of the present work.
Six binary and two ternary Ti-based alloys in Table I, with their nominal compositions placed in the b solid solution area of Ti-Al-Mo at 1523 K (1250 °C) according to accepted phase diagrams,[11–13] were prepared from 99.9 pct sponge Ti, 99.99 pct Al, and 99.9 pct Mo (mass pct) by arc melting in an argon atmosphere. The arc melting was repeated ten times to attain a homogeneous composition. The ingots were solid-solutioned at 1473 K (1200 °C) for 8 hours under vacuum followed by water quenching that resulted in the alloys with average grain size larger than several millimeters such that the effect of grain boundary diffusion can be ignored. Small cylinder samples for diffusion couple were cut from the ingots by a size of B15 9 5 mm by using wire-electrode cutting. Small disks were prepared by polishing one surface of the cylinder samples and annealed pure Ti to mirror-like quality. The wellcontacted diffusion couples were assembled with appropriate pairs of the small disks under vacuum at 1173 K (900 °C) for 90 minutes with a load of 10 MPa on a vacuum diffusion bonding machine. The diffusion couples were sealed into evacuated and argon-back-flushed quartz capsules, and finally were subjected to diffusion annealing at 1523 K (1250 °C) for 12 hours followed by quenching in ice water. The diffusion couples were then sectioned parallel to the diffusion direction which suffered no oxidation and evaporation of elements, mounted, and polished by standard metallographic techniques. The microstructure of diffusion zone was observed by scanning electron microscopy (SEM) and the local composition was analyzed by electron microprobe analysis (EPMA) on JEOL JXA 8900. To eliminate the need for positioning a Matano plane without relating to the drifting of the Matano plane among the different profiles (evidenced in Figure 1 for the G3 couple and Figure 3 for the couple H2), the Whittle and Green (W–G) method was utilized that introduces a normalized concentration var
Data Loading...
