Elasto-plastic phase-field simulation of martensitic transformation in lath martensite steels

Phase-field simulation of lath martensite formation in Fe–0.1C mass% steel was carried out based on the two types of slip deformation (TTSD) model, which is recently developed as a result of analytical solution for the martensitic transformation being composed of Bain deformation and plastic deformation but without lattice rotation matrix. The simulation results reveal that the plastic deformation along the two types of slip system is complementary. The simulation result of the relationship between the two types of slip deformation is consistent with the analytical result calculated by TTSD model, indicating the validity of TTSD model for explaining the formation of lath martensite.     

Some transport properties in martensitic iron-nickel alloys

Summary Resistivity, Hall effect and magnetoresistivity measurements in Fe−Ni alloy specimens with Ni content ranging from 29 to 32 wt.% and subjected to martensitic transformation have been performed. The results have been interpreted by considering both the mechanisms of the martensitic transformation, different in 29 and 30 wt.% Ni alloys from that in 31 and 32 wt.% Ni ones, and the theories of Smith and Berger on the Hall effect in dilute ferromagnetic alloys, which have been applied to the above-described alloys. The Hall data, analysed by means of Kohler polts, have allowed us to identify the charge carrier scattering mechanisms prevailing in the different alloys,i.e. the skew scattering in 29 and 30 wt.% Ni alloys and the side jump scattering in 31 and 32 wt.% Ni ones. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

First-principles study of martensitic transformation of IrTi alloy

Ab initio phonon calculations were performed to probe the martensitic transformation of IrTi. The details of the orthorhombic structure were obtained by the soft-phonon approach. We demonstrate that the tetragonal orthorhombic (Cmmm) transition is driven by the softening of a phonon at the R point (0 ) of the Brillouin zone. The energy landscapes between the various phases of IrTi show that the structural behaviors of IrTi alloy are from cubic to tetragonal, then to orthorhombic and thus the original thought of cubic to monoclinic transition is modified.     

First-principles study of martensitic phase transformation of TiRh alloy

Geometric structures and atomic positions were studied with plane wave pseudo-potential method based on density functional theory for cubic, tetragonal, and monoclinic phases of TiRh alloy. Their phonon dispersion curves were obtained with frozen phonon method at harmonic approximation using density-functional perturbation theory. Our calculations revealed that both B2 and L1₀ phases are thermodynamically unstable. Jahn-Teller effect triggers the occurrence of Bain transformation from cubic to tetragonal phase, and then soft-mode phonon further leads to the transition from tetragonal to monoclinic phase on cooling. The monoclinic phase was predicted to be P2/m space group through atomic vibrational movement along [001] direction of virtual frequency modes of L1₀ phase. The temperature from B2 to L1₀ and then to P2/m were predicted to be about T=1100.53 K and T=324.48 K through free energy calculations with the electronic plus vibrational energy of formation, respectively, which is in good agreement with experimental observations.     

Motion picture x-ray diffraction investigation of the martensitic transformation in nickel-titanium alloy

The thermoelastic martensitic transformation of an equiatomic nickel-titanium alloy is investigated by means of x-ray diffraction motion pictures using synchrotron radiation. The x-ray patterns are recorded by a parallel detector simultaneously in the range of angles 2=34°–51° at a wavelength of 1.4879 Å during heating and cooling with good temperature resolution. The basic parameters of the attendant B2, R, and B19 phases are determined simultaneously and compared. The variations of the parameters of the martensite B19 unit cell are traced, along with the evolution of the R phase {202}{202} reflection doublet during transformation. Volumetric transformation phenomena are determined, and it is shown that martensite originates in a more unconsolidated structure than in the equilibrium state.Institute of Physics of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 49–54, August, 1994.     

Temperature variation of the magnetic cluster structures in an iron-nickel invar alloy

Small-angle scattering of long wavelength neutrons $\text{(λ = 6.42}\text{A}\text{?}\text{)}$ from an Fe₆₅Ni₃₅ single crystal has been measured with the applied magnetic field (6.2 kG) parallel and perpendicular to the scattering vector $\text{K}$ of the elastic scattering over the temperature range from 25 to 422°C (T_c = 227°C). The scattering cross sections due to the longitudinal spin fluctuation have been analyzed by means of Guinier's approximation $\text{(dσ/dω)}{}_0\text{exp}\text{(?κ}{}^2\text{R}{}_{\mathrm{<mtext>g</mtext>}}{}^2\text{3}\text{)}$, where the forward cross section (dσ/dω)₀ is proportional to n, which is the number of atoms in a paramagnetic cluster, and R_g is the radius of gyration of the cluster. The empirical relation between n and R_g is = 0.298 × R_g^2.34 to be compared with that calculated for a simple spherical cluster model n = 1.274 R_g³.     

Crystallographic variant selection of martensite during fatigue deformation

Metastable austenitic stainless steels are prone to form deformation-induced martensite under the influence of externally applied stress. Crystallographic variant selection during martensitic transformation of metastable austenite has been investigated thoroughly with respect to the interaction between the applied uniaxial cyclic stress and the resulting accumulated plastic strain during cyclic plastic deformation. The orientation of all the Kurdjomov–Sachs (K-S) variants has been evaluated extensively and compared with the measured orientation of martensite with their corresponding interaction energies by applying the elegant transformation texture model recently developed by Kundu and Bhadeshia. Encouraging correlation between model prediction and experimental data generation for martensite pole figures at many deformed austenite grains has been observed. It has been found that both the applied uniaxial cyclic stress and the accumulated plastic strain are having strong influence on crystallographic variant selection during cyclic plastic deformation. Patel and Cohen’s classical theory can be utilized to predict the crystallographic variant selection, if it is correctly used along with the phenomenological theory of martensite crystallography.     

Study of stabilization of CuAlBe alloy during martensitic transformation by internal friction

In this study, the internal friction (IF) of a CuAlBe alloy during reverse martensitic transformation (MT) was investigated under isothermal and non-isothermal conditions. It was found that the IF decreases regularly with the aging time in the temperature range of phase transformation. It is thought that this phenomenon is caused by the defect movement. The defects diffuse to the interfaces between martensite and austenite and pin there gradually, thus leading to the IF decrease.     

Orientation relationship and the mechanism of martensite transformation in medium-carbon steel with batch martensite

Exact orientation relationship for martensite transformation in medium-carbon 37KhN3A steel with lath martensite are determined. The mechanism of deformation during the transformation of martensite in steel is described.     

Critical point of martensitic transformation under stress in an Fe-31.2Pd (at.%) shape memory alloy

We have examined the transformation strain, Δε, in the [0 0 1] direction of an Fe-31.2Pd (at.%) shape memory alloy under compressive stress applied in the same direction. When the stress is absent, the alloy exhibits a cubic to tetragonal martensitic transformation at 230 K with |Δε| of 1.4%. As the stress increases, the transformation temperature increases linearly and |Δε| decreases linearly and vanishes at 40 MPa (280 K). This point is the critical point of this transformation at which the first-order nature disappears, and the critical exponent β is evaluated to be 0.47?±?0.04.     

Magnetic indication of the stress-induced martensitic transformation in ferromagnetic Ni–Mn–Ga alloy

A quantitative study of the stress-induced martensitic transformation in Ni_49.7Mn_29.1Ga_21.2 magnetic shape memory alloy has been carried out in two different ways: the first way is based on the measurements of saturation magnetization under variable mechanical stress and the second one is founded on the quantitative theoretical treatment of experimental stress–strain loops. A functional dependence between the volume fraction of transformed martensite and applied stress has been determined from both magnetization and strain values. A quantitative agreement between the functions determined in two different ways has been observed, and hence, the effectiveness of the magnetic indication of the stress-induced martensitic transformations has been proved. This method can be used to monitor stress-induced transformations in martensitic films, needles and small specimens.     

Analysis of acoustic emission during martensitic transformation of CuZnAl alloy by measurement of forces

A special composition of sample transducer is presented by which forces corresponding to AE microevents of martensitic transformation in CuZnAl alloy can be measured.     

Crystallographic variant selection of martensite at high stress/strain

The phenomenological theory of martensitic transformation is well understood that the displacive phase transformations are mainly influenced by the externally applied stress. Martensitic transformation occurs with 24 possible Kurdjomov-Sachs (K-S) variants, where each variant shows a distinct lattice orientation. The elegant transformation texture model of Kundu and Bhadeshia for crystallographic variant selection of martensite in metastable austenite at various stress/strain levels has been assessed in this present research. The corresponding interaction energies have also been evaluated. Encouraging correlation between model prediction and experimental data generation for martensite pole figures at many deformed austenite grains has been observed at different stress/strain levels. It has been investigated that the mechanical driving force alone is able to explain the observed martensite microtextures at all stress/strain levels under uniaxial tensile deformation of metastable austenite under low temperature at a slow strain rate. The present investigation also proves that the Patel and Cohen’s classical theory can be utilized to predict the crystallographic variant selection, if it is correctly used along with the phenomenological theory of martensite crystallography.     

Concerning the two magnetic correlation lengths above T c in an Invar iron-nickel alloy

The magnetic phase transition in the Invar alloy Fe₇₀Ni₃₀ is investigated by means of small-angle neutron scattering over a wide range of momentum transfer. This method was used to measure two magnetic correlation lengths R _c 1 and R _c 2 which coexist in the alloy above the phase transition temperature T _c . The critical correlations with correlation length R _c 1 are described well by an Ornstein-Zernicke expression, and the critical correlations with the second correlation length, an order of magnitude larger than R _c 2, are described well by a squared Ornstein-Zernicke expression. The temperature dependences obtained for the correlation lengths R _c 1 and R _c 2 satisfy the power law R _c ∼((T−T _c )/T _c ) ^−υ with critical exponents υ₁=0.65±0.05 and υ₂=1.3±0.1 for the shorter and longer scales, respectively. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 53–57 (10 July 1997)     

Mössbauer study of the hydrogen induced martensitic transformation in high carbon Fe−C martensite and its ageing behaviour

Cathodic hydrogen charging at low current density at room temperature in acidic medium is carried out on an austenitic 1.9 wt% C iron alloy. This induces the martensitic transformation as checked by Mössbauer spectroscopy and the spectra observed at room temperature present all the features of an already aged martensite. Further ageing at higher temperatures up to 390 K clearly demonstrates that the ageing kinetics is stopped compared to that of uncharged martensite of a similar alloy. Further, in situ transformation of extended multiplets into ε-η carbide precipitates resumes at the same temperature in both cases.     

Heat of the martensitic transformation in CuAlMn alloy subjected to thermal cycling under constrained deformation conditions

The calorimetric study of Cu-Al-Mn alloy under constrained deformation conditions shows a 9–15% decrease in the heat of the γ′₁ ? β₁ martensitic transformation as compared to the free state, and this decrease takes place against the background of an increase in the transformation temperature. This effect is explained by a load-induced change in the symmetry of the crystal lattice.