Calorimetric Studies of Superconductive Proximity Effects in a Two-Phase Ti—Fe (7.5 at. %) Alloy

Abstract

Transition metal binary alloys based on group IV elements such as Ti exist in various structural states depending on composition and heat treatment. In alloys for which the valence-electron-to-atom ratio z has a value between 4.0 and approximately 4.3, the high-temperature stable bcc (β) phase often undergoes structural change or decomposition during quenching. The transformations have been the subject of numerous studies in the metallurgical literature. For the present purpose the situation can be adequately summarized as follows. In almost every system the resulting structures are successively (a) martensitic (a′) for 4.0 ≤ z ≲ 4.1; (b) two-phase (β + ω) for 4.1 ≲ z ≲ 4.3; and (c) single-phase bcc for z ≳ 4.3. In other words, the low-temperature instability of the bcc lattice in the low-z range results in transformations to the hexagonal structured α′ or ω phase. The ω phase forms as a submicroscopic precipitate within the bœ matrix. Although there is a tendency for the precipitation to take place within 4.1 ≲ z ≲ 4.3, it is only at¹ z = 4.1₂ ± 0.0₆ that the transformation occurs spontaneously and reversibly through a diffusionless process. The resulting “athermal” ω-phase particles have the same composition as the matrix. For larger values of z the precipitation is accompanied by diffusion. The occurrence, abundance, and composition of this “thermal” ω phase therefore depend on the quenching rate and on the temperature and duration of any subsequent heat treatment.²

Supported by the U.S. Air Force Materials Laboratory under Contract AF33 (615) 69-C-1594 and the U.S. Air Force Office of Scientific Research under Grant 71-2084.