Influence of β-Stabilizers on the α-Ti→ω-Ti Transformation in Ti-Based Alloys

The development of next generation Ti-based alloys demand completely new processes and approaches. In particular, the Ti-alloys of next generation will contain not only α-Ti and β-Ti phases, but also small amounts of ω-phase and intermetallic compounds. The β→ω phase transformation induced by high-pressure torsion (HPT) has been studied in detail recently. In this work, we investigated the HPT-induced α→ω phase transformation. For this purpose, we added various β-stabilizers into α-Ti matrix of studied Ti-alloys. Ti-alloys with 4% Fe, 2% Cr, 3% Ni, and 4% Co (wt. %) have been annealed at the temperatures below their point of eutectoid decomposition, from β-Ti to α-Ti, and respective intermetallics (TiFe, Ti2Co, Ti2Ni, TiCr2). Volume fraction of HPT-driven ω-phase (from ≤5 up to ~80%) depended on the amount of alloying element dissolved in the α-matrix. Evaluation of lattice parameters revealed accelerated mass transfer during HPT at room temperature corresponding to bulk diffusion in α-Ti at ~600 °С.

Experimental Continuous Casting of Nitinol

Commercially available nitinol is currently manufactured using classic casting methods that produce blocks, the processing of which is difficult and time consuming. By continuous casting, wherein molten metal solidifies directly into a semi-finished product, the casting and processing of ingots can be avoided, which saves time and expense. However, no reports on continuous casting of nitinol could be found in the literature. In this work, Φ 12 mm nitinol strands were continuously cast. Using a graphite crucible, smelting of pure Ni and Ti in a medium frequency induction furnace is difficult, because it is hard to prevent a stormy reaction between Ni and Ti and to reach a homogeneous melt without a prolonged long holding time. Using a clay-graphite crucible, the stormy reaction is easily controlled, while effective stirring assures a homogeneous melt within minutes. Strands of nearly equiatomic chemical compositions were obtained with acceptable surface quality. The microstructure of strands containing over 50 at. % Ni, consisted of Ti2Ni and cubic NiTi, whereas the microstructure of strands containing less than 50 at. % Ni consisted of TiNi3 and cubic NiTi. This is consonant with the results of some other authors, and indicates that the eutectoid decomposition NiTi → Ti2Ni + TiNi3 does not take place.