Effect of the deviation of the chemical composition from the stoichiometric composition on the structural and phase transformations and properties of rapidly quenched Ti50 + x Ni25 − x Cu25 alloys

Effect of deviations of composition from the quasi-binary section TiNi-TiCu on structural and phase transformations in rapidly quenched alloys

“…It is obvious that the slowing down of the rate of crystal growth up to their mutual joining is mainly caused by the effect of the concentration gradients of those chemical elements in which the alloy is weakly supersaturated relative to Ti 50 Ni 25 Cu 25 ; their supersat uration is partly eliminated inside B2 crystallites, whereas, around these crystallites, the concentration of these elements is excessive. Therefore, unlike the case of massive crystallization, the development of this mechanism, which is called "primary crystallization" and occurs with a change in the chemical composition [23,24], is slowed down somewhat and, since the number of nuclei will increase upon heat treatment, in the final account, the crystallization is completed by an increase in the dispersity of the grain structure. The continuation of heat treatment after the exhaustion of both polymorphous and primary crystallization can lead to the subsequent preferentially heterogeneous decomposition.…”

“…4). The orientation relationships (ORs) between the B19 mar tensite and B2 austenite are close to Bain ORs [23] (1) Analysis has shown that the average size of the ini tial B2 austenite grains that arise as a result of the above heat treatment is 1.0-1.5 μm. A noticeable decrease in the dimensions of grains of the B2 austenite is caused by deviations from stoichiometry in titanium and cop per at a fixed content of nickel (25 at %).…”

“…Figure 9 presents bright field and dark field images of the nanostructure and the corresponding SAED pat terns of this alloy after annealing at 723 K for 10 min. The calculations of the electron diffraction patterns showed that, after such heat treatment in the nanoc rystalline alloy Ti 52 Ni 25 Cu 23 , two phases should be present at room temperature, i.e., the matrix marten site (B19) and a second phase Ti 2 Ni. Thus, this exper iment additionally confirms the formation of second phases in nonstoichiometric alloys upon their nanoc rystallization from the amorphous state.…”

“…Based on the results of [23], it was assumed that this treatment would reveal the effect of the chem ical composition (deviations from the stoichiometric composition) on the process of the devitrification and its main structural and morphological characteristics. As a consequence, the revealed regularities and, first of all, the structural mechanism of the crystallization of amorphous RQM ribbons would make it possible to explain the behavior of the physicomechanical prop erties and to develop principles of controlling the SME of the alloys under investigation.…”

“…In our previous work [23], we have studied the main specific features of the structure, phase and elemental composition, and properties of nonstoichiometric Ti 50 -x Ni 25 + x Cu 25 alloys (х ≤ ±1 at %) for the first time. To obtain these SME alloys in a high strength nano crystalline state in the form of thin ribbons, here, we again employed the method of free jet melt spinning.…”