Vol 2018, No 4 (2018)

The fragmentation of metals during megaplastic (severe) deformation is described in terms of a two-defect model. The process kinetics has been studied qualitatively. A general expression for the Lyapunov indices, which determine the stability of limiting (stationary) structures, is derived. Critical conditions for controlling parameters are found, and the diagrams that determine the stability of stationary states are constructed.

A model is proposed to describe the phase-structural deformation of shape memory alloys with allowance for the nonuniform strain hardening of the martensite part of representative volume. A scheme is developed to determine the volume fraction of martensite undergoing structural transformation during proportional nonreversible loading. The problem of reactive-stress generation in experiments on orientational transformation with constrained deformation after unloading is resolved.

The stress relaxation in titanium nickelide specimens subjected to strain controlled uniaxial tension under martensitic anelasticity conditions at a given rate and the limited creep effect observed at the stages of stress controlled steplike loading are studied. The rheonomic properties of a shape memory alloy are found to depend on the type of state of stress, and they are more pronounced in tension than in compression. A simple model is develop to describe the detected effects.

The mechanical properties and the fracture of the joints between a PT3V titanium alloy and 12Cr18Ni10Ti stainless steel formed by diffusion welding through ultrafine-grained layers of nickel and a Cr2Ni98 alloy are studied. The highest strength of the joint (about 390 MPa) is achieved upon welding at T = 700°C, 20 min irrespective of the interlayer material. However, in the case of the nickel interlayer, fracture occurs in the regions adjacent to the TiNi layer; in the case of the Cr2Ni98 alloy, along the alloy/steel interface.

Combined studies of hard magnetic Nd₂Fe₁₄B/α-Fe nanocomposites are performed. They were prepared by mechanical alloying of melt-quenched Nd_7.4Pr_2.0Fe_76.6Co_4.2Zr_3.4B_6.4 and Nd_5.8Fe₈₀Co_4.9Ti_1.5Si_2.5B_5.3 alloys taken in mass proportions of 90/10 and 70/30. It is found that, after mechanical alloying, an amorphous–crystalline structure is formed; it consists of the hard magnetic Nd₂Fe₁₄B and soft magnetic (amorphous and α-Fe) phases. Subsequent annealing at ~500°C initiates the decomposition of the amorphous phase and the formation of the nanocrystalline Nd₂Fe₁₄B and α-Fe phases. This leads to an increase in the coercivity and the residual magnetization-to-saturation magnetization ratio (σ_r/σ_s ≥ 0.5). It is assumed that the magnetic hardening of powders is due to the formation of an exchange-coupled state, which results from the exchange interaction between α-Fe nanocrystals and the Nd₂Fe₁₄B phase.

The influence of the removal of a surface layer with a high strain-induced martensite content on the mechanical properties and the shape of stress–strain curves of austenitic–martensitic VNS9-Sh TRIP steel is studied by room-temperature tests. The removal of a surface layer 5–20 μm thick by electropolishing is shown not to decrease the mechanical properties of this steel and not to change the shape of its stress–strain curves, which have a developed yield plateau. This effect can be related to the presence of a long (up to 1%) stage of microyield in this steel. The existence of a yield plateau in the stress–strain curves of VNS9-Sh steel in the initial state and after the removal of a surface layer can also be explained by the simultaneous operation of three plastic deformation mechanisms, namely, slip, twinning, and martensitic transformation, during deformation.

A layered hybrid material is prepared by high-pressure torsion of a three-layer (vanadium alloy–zirconium alloy–vanadium alloy) billet. A mixed nanostructure with a highly uniform distribution of zirconium and vanadium alloy regions forms under the chosen severe plastic deformation conditions in the middle layer of the material. The microhardness of this layer increases by a factor of about 2.5 as compared to the initial state.

The plastic deformation of a three-layer bimetallic composite, namely, structural low-carbon St3sp steel clad by an austenitic corrosion-resistant 12Kh18N9T steel on both sides is studied. The laws of Chernov–Lüders band propagation during uniaxial tension and the load that corresponds to the yield strength are considered.

The properties of austenitic–martensitic VNS9-Sh (23Kh15N5AM3-Sh) sheet TRIP steel during static and cyclic loading are studied. The specific features of the mechanical behavior of the steel during static tension that are related to shearing, twinning, and martensite formation processes are detected. The static stress–strain curve of the steel has a developed microyield stage, a long yield plateau, and a serrated stage of strain hardening (Portevin–Le Chatelier effect). The shear mechanisms at the initial stages of cyclic deformation and fatigue crack propagation mechanisms are investigated.

The results of estimating the ductile–brittle transition temperature range for structural steels are compared using direct and indirect toughness characteristics (work, force, strain, type of fracture) for different testing schemes. The critical cold-shortness thresholds determined from different toughness characteristics for the same material are shown to differ by a few hundred degrees Celsius. An approach is formulated to choose a testing scheme and an effective toughness criterion.

The changes in the ultrasonic surface and lateral wave velocities near the head running surfaces of rails having carried various weights are studied. The surface and lateral wave velocities are found to correlate with the hardness and the structure of rail steel.