Effect of True Strains in Isothermal abc Pressing on Mechanical Properties of Ti49.8Ni50.2 Alloy

Abstract

The paper analyzes the microstructure and mechanical properties of Ti49.8Ni50.2 alloy (at.%) under uniaxial tension at room temperature after isothermal abc pressing to true strains e = 0.29−8.44 at T = 723 K. The analysis shows that as the true strain e is increased, the grain–subgrain structure of the alloy is gradually refined. This leads to an increase in its yield stress σy and strain hardening coefficient θ = dσ/dε at linear stage III of its tensile stress–strain curve according to the Hall–Petch relation. However, the ultimate tensile strength remains invariant to such refinement. The possible mechanism is proposed to explain why the ultimate tensile strength can remain invariant to the average grains size (dav). It is assumed that the sharp increase of the ultimate tensile strength σUTS begins when (dav) is less than the critical average grain size (dav)cr. In our opinion, for the investigated alloy (dav)cr ≈ 0.5 µm. In our study, the attained average grain size is larger the critical one. The main idea of the mechanism is next. In alloys with an average grain size (dav) less than the critical one, a higher external stress is required for the nucleation and propagation of the main crack.

Effect of abc Pressing at 573 K on the Microstructure and Martensite Transformation Temperatures in Ti49.8Ni50.2 (at%)

“…The test material was Ti 49.8 Ni 50.2 (at%) supplied by MATEK-SMA Ltd. (Moscow, Russia). Its specimens were made as described in detail elsewhere [22]. The initial specimens, which were shaped as a cube with a side of 20 mm via a single abc pressing cycle at 1073 K, were placed in dies, aged at 573 K for 10 min, and compressed on a hydraulic press with a rate of 0.16-0.18 s −1 .…”

“…After abc pressing to e = 8.44 at 723 K [19][20][21][22], this type of alloy acquires an inhomogeneous grain-subgrain structure with an average grain size of approximately 1 µm that comprises fine-grained, submicrocrystalline, and nanocrystalline fractions. It is noted that intense recovery processes develop in the alloy abc pressed at 723 K.…”

Microstructure and Mechanical Properties of Titanium Alloys

“…It was found that in the temperature range studied, the maximum inelastic strain that could be obtained in the coarse-grained samples subjected to torsion could be completely returned due to the superelasticity effect of 4-7%. The effect of true strains on the microstructure and mechanical properties of a Ti 49.8 Ni 50.2 alloy subjected to isothermal abc pressing was analysed in [3]. It was shown that the grain structure of the alloy was refined by increasing the true strain, which increased its yield stress and strain-hardening coefficient.…”

ВЛИЯНИЕ ВЕЛИЧИНЫ ДЕФОРМАЦИИ ПРИ ABC-ПРЕССОВАНИИ НА ТЕМПЕРАТУРЫ МАРТЕНСИТНЫХ ПРЕВРАЩЕНИЙ И ПАРАМЕТРЫ КРИСТАЛЛИЧЕСКИХ СТРУКТУР СПЛАВА Ti49.8Ni50.2

“…Исследования проведены на сплаве Ti 49.8 Ni 50.2 (ат.%) производства ООО «Промышленный центр МАТЭК-СПФ», который в состоянии поставки имел структуру моноклинной мартенситной фазы B19'. Последовательность изготовления образцов для исследования описана в работе [4]. Образцы с различной величиной истинной деформации е (1.84; 3.60; 5.40; 7.43; 9.55) получали изотермическим abc-прессованием при 300°С.…”