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

• Published in: Metals (Basel ) 2020-10, Vol.10 (10), p.1313
• Main Authors: Kashin, Oleg, Krukovskii, Konstantin, Lotkov, Aleksandr, Grishkov, Victor
• Format: Article
• Language: English
• Subjects: Aging; Alloys; average grain–subgrain size; Crack propagation; Crystal lattices; Grain size; Intermetallic compounds; Invariants; isothermal abc pressing; Mechanical properties; Nickel base alloys; Nucleation; Pressing; Room temperature; Strain hardening; Stress propagation; Stress-strain curves; Tensile strength; Tensile stress; TiNi; True strain; Ultimate tensile strength; Yield stress
• ISSN: 2075-4701; 2075-4701
• DOI: 10.3390/met10101313

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.