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

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.

Yield Stress and Reversible Strain in Titanium Nickelide Alloys after Warm Abc Pressing

The results of the position analysis of the yield stress τ0.3 on the "stress–strain" (τ–γ) dependences, received at the torsion of specimens of Ti49.8Ni50.2 (at%) alloy are presented. The critical stress τ0.3 (IV), corresponding to the end of linear stage III and the beginning of the intensive development of plastic strain at stage IV, preceding the fracture of the specimens, were obtained as well. The structure of the specimens was transformed from coarse-grained to microcrystalline as a result of warm (723 K) abc pressing with a true deformation e of 8.4. The regularities of the development of reversible inelastic strain (superelasticity, SE, and shape memory effect, SME) and plastic strain γpl after isothermal (295 K) loading of specimens up to τ ≤ τ0.3(IV), unloading, and their subsequent heating up to 500 K are studied. From the joint analysis of the “τ–γ” dependences obtained at 295 K and "plastic strain–total strain" dependences the yield stress τ0.3 corresponding to the development of 0.3% of the plastic strain under loading of the specimens was determined. Critical stress τ0.3(IV) was determined as equal to the stress corresponding to a deviation of 0.3% from the linear “τ–γ”dependence at stage III. It is shown that the yield stress τ0.3 for all specimens is localized at the beginning of stage III for all specimens. The ratio τ0.3(IV)/τ0.3 is from 2.3 to 3.8. The accumulation of plastic strain at stage III (after loading with τ from τ0.3 to τ0.3(IV)) is from 2.4% to 4.7% (depending on the true deformation of the specimens during warm abc pressing). Thus, stage III is the stage of deformation hardening of specimens under torsion. On the basis of the results of this and previous works it is shown that, in alloys with thermoelastic martensitic transformations and with thermomechanical memory, the ratio τ0.3(IV)/τ0.3 can vary in a wide range: in reinforced specimens τ0.3 can be close to τ0.3(IV), and in more ductile specimens τ0.3 can be significantly less than τ0.3(IV). However, in order to correctly determine the yield stress of τ0.3 and the corresponding strain γt(0.3), it is necessary to carry out a joint analysis of “τ–γ” and "plastic strain–total strain" dependencies.

Hydrogen-Assisted Fracture Mechanisms in Ultrafine-Grained CrNi Austenitic Stainless Steels with Different Initial Microstructures

We investigated the effect of electrolytic hydrogen-charging on regularities of plastic flow, strength and fracture mechanisms of AISI 316L and 321 austenitic stainless steels. In the steels, an ultrafine-grained structure of various morphologies was formed using methods of warm abc-pressing and thermomechanical treatment (cold rolling and annealing). Hydrogen-charging of ultrafine-grained steels reduces their yield strength and elongation. The high dislocation density and low-angle boundaries inhibit the effects of hydrogen embrittlement in 316L and 321 steels.