Substructural and carbide transformations during plastic deformation in tempered chromium-nickel martensitic steel

The evolution of the defect and carbide subsystems has been studied during the plastic deformation of chromium nickel steel with the structure of tempered martensite. A correlation has been established between the substructural transformation and the change during plastic deformation. The nondislocation shear resistance has been estimated.Tomsk Civil Engineering Institute. Translated from Izvestlya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 25–32, December, 1992.     

Special features of the localized plastic deformation and fracture of high-chromium steel of the martensitic class

The deformation behavior of high-chromium steel (0.4%C–0.6%Si–0.55%Mn–12.5%Cr) of martensitic structure upon quenching and of sorbitic structure upon high-temperature tempering has been investigated. Each of the states is shown to be represented by a particular stress-strain curve. The stress-strain curve for the steel in the martensitic state consists of a single linear-hardening stage, whereas in the sorbitic state, it exhibits a three-stage deformation pattern. The plastic flow of the examined material in the two states has been found to be of a localized character. The evolution of localized-strain center distributions follows the law of plastic flow, i.e., it depends on the deformation stages in the stress-strain curve. The fracture process is determined by the kinetics of the localized-strain centers in the final (prefracture) deformation stage in the stress-strain curve.     

Crystalline damage growth during martensitic phase transformations

A thermomechanical model is developed within a large deformation setting in order to simulate the interactions between martensitic phase transformations and crystalline damage growth at the austenitic grain level. Subgrain information is included in the model via the crystallographic theory of martensitic transformations. The damage and transformation characteristics are dependent of the specific martensitic transformation systems activated during a loading process, which makes the model strongly anisotropic. The state of transformation for the individual transformation systems is represented by the corresponding volume fractions. The state of damage in the austenite and in the martensitic transformation systems is reflected by the corresponding damaged volume fractions. The thermodynamical forces energetically conjugated to the rate of volume fraction and the rate of damaged volume fraction are the driving forces for transformation and crystalline damage, respectively. The expressions for these driving forces follow after constructing the specific form of the Helmholtz energy for a phase-changing, damaging material. The model is used to analyze several three-dimensional boundary value problems that are representative of microstructures appearing in multiphase carbon steels containing retained austenite. The analyses show that the incorporation of damage in the model effectively limits the elastic stresses developing in the martensitic product phase, where the maximum value of the stress strongly depends on the toughness of the martensite. Furthermore, in an aggregate of randomly oriented grains of retained austenite embedded in a ferritic matrix the generation of crystalline damage delays the phase transformation process, and may arrest it if the martensitic product phase is sufficiently brittle. The response characteristics computed with the phase-changing damage model are confirmed by experimental results.     

Substructural phase transitions during intense plastic deformation of low-carbon ferrite-perlite steel

We have studied the evolution of the defect structure and phase composition of low-carbon ferrite-perlite steel subjected to intense plastic deformation using diffraction electron microscopy. It has been shown that a high degree of deformation is accompanied by disruption of the perlite columns. We have found and described two perlite decay mechanisms: decay of the carbide plates by a path of their granulation due to dislocation slip and dissolution of cementite arising from the outflow of carbon atoms from the carbide phase into ferrite crystal lattice defects. We have described the phenomenon of morphological reconstruction of the cementite-phase particles (a transition from layers to spheres) under plastic deformation conditions. Tomsk State Architectural and Construction University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 63–71, March, 1998.     

Dissipative processes during the nonequilibrium phase transformations in martensitic thermoelastic alloys

Martensitic thermoelastic transformations are considered under nonequilibrium conditions, where a system nonmonotonically tends toward a stationary process. The specific features of a phase transformation are experimentally studied on molecular models under these nonequilibrium conditions. A resonance mode of the phase transformation, which can increase the process rate by an order of magnitude without increasing the heat source temperature, is found. The dissipative processes that occur under the monotonic and resonance conditions of martensitic thermoelastic transformations are estimated. The resonance mode is shown to be accompanied by negative entropy production and to demonstrate the self-organization of the system. These results can be used to design materials and techniques for the processing of low-potential heat sources.     

Surface composition effects on martensitic phase transformations in nickel aluminium nanowires

Atomistic simulations are utilized to quantify the effects of surface composition on stress-induced B2 to body centred tetragonal (BCT) martensitic phase transformations in intermetallic nickel aluminium (NiAl) nanowires. The simulations show that the phase transformation is observed in all considered cases, regardless of the material composition of the transverse {100} surfaces of the initially B2 wires. The results indicate that, for ?100? oriented B2 wires with {100} transverse surfaces, the {100} orientation and not the material composition of the {100} surfaces is the dominant factor in controlling the ability of NiAl alloys to undergo martensitic phase transformations at nanometer scales.     

Mechanism of low-temperature plastic deformation in structural steel

It has been discovered that grade 30KhGSA steel after a high-temperature thermomechanical treatment is more plastic at –196°C than at room temperature, unlike in the case of quenched steel and uncharacteristically for metals or alloys with a bcc crystal lattice. A study has, therefore, been made to establish the temperature characteristic of the resistance to deformation and, especially, to its reversible component, both after an anneal and after a heat treatment of steel. The activation energy and volume of the thermally induced deformation were measured, whereupon the results were analyzed on the basis of the Payerls mechanism with dislocations frozen by interstitial impurities. No differentiation was made between the behavior of steel after a quench and after a high-temperature thermomechanical treatment, respectively, so that differences in the low-temperature diagrams could be related to the peculiarities of nonthermal deformation and to the resistance to brittle fracture.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 26–33, September, 1974.     

Phase transformations in crystalline Ti-Ni-Cu alloy upon severe plastic deformation

X-ray diffraction and transmission electron microscopy were used to study features of the structural and phase transformations of initially crystalline T1₅₀Ni₂₅Cu₂₅ alloy upon severe plastic deformation in a Bridgman cell. Three cycles of successive phase transitions of the crystal ⟹ amorphous state and amorphous state ⟹ crystal type were revealed under continually increasing deformation. The results are explained within the superposition of different channels of elastic energy dissipation at severe plastic deformation.     

Structural features of martensitic transformations upon the low-temperature deformation of metastable austenitic Fe-Cr-Ni alloy single crystals

The features of martensitic transformations upon the low-temperature (1.8–300 K) deformation of metastable Fe-18Cr-10Ni and Fe-18Cr-15Ni alloy single crystals with low stacking-fault energies were studied. It was shown that the γ → ɛ and γ → α martensitic transformations play the main role in increasing strength and plasticity when the deformation temperature is reduced to 4.2 K.     

Structural and phase transformations in ferromanganese alloys during deformation under pressure

Using optical metallographic, TEM, Mössbauer spectroscopy, and X-ray analysis the structural and phase transformations in Fe-(3–55) wt % Mn alloys during shear deformation under pressure were investigated. It is established that a large deformation under high pressure causes the formation of a nanocrystalline structure with grain sizes of 40–60 nm. Nanostructure increases the hysteresis of inverse (hcp-fcc) transformation and stabilizes the (hcp) ? phase in alloys containing more than 40 wt % Mn, up to normal conditions. The Fe-3 wt % Mn alloy after shear under pressure treatment became nanostructured, retaining the original bcc phase state.     

Role of point defects in martensitic transformations

The effect of point-defect complexes on martensitic phase transformations in a bcc system with low elastic moduli is studied by means of computer simulation. The interaction of strain fields generated by defects in shown to facilitate realization of a martensitic transition from the bcc to fcc or ω-like structure depending on the defect symmetry. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 58–63, June, 2000.     

Study of plastic deformation and proton irradiation influence on phase transitions in C18N9T stainless steel

Transmission and post-nuclear-reaction emission Mössbauer spectroscopy was used for investigation of radiation effects and plastic deformation influence on C18N9T steel. The rolled foil was annealed in 10^?6 Torr vacuum at 870 K during 2 hours and was irradiated by 30 MeV protons at current density of 0.8μ A up to doses of 5×10¹⁷ cm^?2 on the isochronous cyclotron at the liquid nitrogen cooling of the target. The Mössbauer spectra obtained before and after the sample irradiation in transmission geometry are identical. The emission spectrum is analogous to the spectrum of the plastically deformed sample. It is concluded that during irradiation in the steel the processes take place which are similar to those at plastic deformation and connected with redistribution of the alloy component atoms.     

Residual stress field in steel samples during plastic deformation and recovery processes

An X-ray diffraction method was applied to measure residual stresses and stored elastic energy in deformed and annealed polycrystalline ferritic and austenitic steel samples. The orientation distribution of plastic incompatibility second-order stresses created during elastoplastic deformation was determined and presented in Euler space. Using deformation models, these stresses were correlated with different types of intergranular interactions occurring in the studied materials. An important decrease of the first- and the second-order residual stresses was observed during recovery and recrystallisation processes. Diffraction peak widths, related to dislocation density, were studied and correlated with stress variation during annealing process. Differences in stress relaxation between ferritic and austenitic samples were explained by different values of the stacking fault energy, which influences dislocation climb and cross-slip.     

Quasi-equlibrium description of acoustic emission in martensitic transformations

Acoustic emission is used to study martensitic transformations in metal systems within the framework of the quasiequilibrium theory. In this approach, the dissipative part in the balance equation of driving forces for the matertensitic transformation is represented as the sum of contributions from heat dissipation and “nonchemical” energy dissipation due to acoustic emission. The acoustic contribution is defined as dynamic relaxation which in its turn is related to the transformation kinetics. Examples of events responsible for the production of acoustic radiation are the “microexplosive” nucleation-collapse of martensite crystals, single acts of their stick-slip displacement, and plastic relaxation of elastic transformation stresses. Siberian Physicotechnical Institute at Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 72–79, October, 1999.