Estimation of martensite feature size in a low-carbon alloy steel by microtexture analysis of boundaries

A methodology for classifying the hierarchy of martensite boundaries from the EBSD microtexture data of low-carbon steel is presented. Quaternion algebra has been used to calculate the ideal misorientation between product α variants for Kurdjumov–Sachs (KS) and its nearby orientation relationships, and arrive at the misorientation angle-axis set corresponding to packet (12 types), block (3 types) and sub-block boundaries. Analysis of proximity of experimental misorientation between data points from the theoretical misorientation set is found to be useful for identifying the different types of martensite boundaries. The optimal OR in the alloy system and the critical deviation threshold for identification of martensite boundaries could both be ascertained by invoking the ‘Enhancement Factor’ concept. The prior-γ grain boundaries, packet, block and sub-block boundaries could be identified reasonably well, and their average intercept lengths in a typical tempered martensite microstructure of 9Cr–1Mo–0.1C steel was estimated as 31 μm, 14 μm, 9 μm and 4 μm respectively.     

45~#碳钢激光相变硬化层磨损特性的研究

本文在相同条件下对激光相变硬化处理、淬火-回火及未经处理的45~#碳钢的磨损特性进行了对比试验研究。结果表明,激光相变硬化处理明显地提高了材料表面的抗粘着和抗擦伤能力,因而可以使其在一定负荷和往复速度下的耐磨性能提高一个数量级。往复速度(频率)、负荷对激光相变硬化层和淬火-回火试样之磨损率的影响不同:淬火-回火试样的磨损率随着负荷的增大而迅速增大,但相变硬化层在负荷低于294N时的磨损率增加十分缓慢,只有当负荷超过294N之后才迅速增大;淬火-回火试样的磨损率是随往复频率的增大而迅速降低,而相变硬化层的磨损率随着往复频率的增大却是先上升而后下降,但其量值的变化较小,当往复频率高于每分钟500次时,二者的磨损率基本相同。     

A computational model for the indentation and phase transformation of a martensitic thin film

We propose a computational model for a stress-induced martensitic phase transformation of a single-crystal thin film by indentation and its reverse transformation to austenite by heating. Our model utilizes a surface energy that allows sharp interfaces with finite energy and a penalty that forces the film to lie above the indenter and undergo a stress-induced austenite-to-martensite phase transformation. We introduce a method to nucleate the martensite-to-austenite phase transformation since in our model the film would otherwise remain in the martensitic phase in a local minimum of the energy.     

Void formation in zirconium under irradiation in the high-voltage electron microscope

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HYPERFINE INTERACTIONS, Journal devoted to research in the border region of solid state, atomic and nuclear physics. Editors: B. I. Deutch and H. de Waard. North-Holland, Amsterdam, $46.95 per year.

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Hierarchical microstructure in structural phase transitions

We consider a model in the context of martensitic materials in which hierarchical twinning near the habit plane (austenite-martensite interface) is a new and crucial ingredient. The model includes (1) a triple-well potential in local deviatoric (rectangular) strain, (2) strain gradient terms up to second order in strain and fourth order in gradient, and (3) all symmetry allowed compositional fluctuation-induced strain gradient terms. The last term favors branching of domain walls which enables communication between macroscopic and microscopic regions essential for shape memory. Below the transition temperature (T₀) we obtain the conditions under which branching of twins is energetically favorable. Above T₀ a hierarchy of branched domain walls also stabilizes tweed formation (criss-cross patterns of twins). External stress or pressure modulates (“patterns”) the spacing of domain walls. Results based on 2D time-dependent Ginzburg-Landau simulations are shown for twins, tweed and hierarchy formation.     

Comparison of microstructure and mechanical properties of ultra-narrow gap laser and gas-metal-arc welded S960 high strength steel

The microstructural characteristics and mechanical properties, including micro-hardness, tensile properties, three-point bending properties and Charpy impact toughness at different test temperatures of 8 mm thick S960 high strength steel plates were investigated following their joining by multi-pass ultra-narrow gap laser welding (NGLW) and gas metal arc welding (GMAW) techniques. It was found that the microstructure in the fusion zone (FZ) for the ultra-NGLW joint was predominantly martensite mixed with some tempered martensite, while the FZ for the GMAW joint was mainly consisted of ferrite with some martensite. The strength of the ultra-NGLW specimens was comparable to that of the base material (BM), with all welded specimens failed in the BM in the tensile tests. The tensile strength of the GMAW specimens was reduced approximately by 100 MPa when compared with the base material by a broad and soft heat affected zone (HAZ) with failure located in the soft HAZ. Both the ultra-NGLW and GMAW specimens performed well in three-point bending tests. The GMAW joints exhibited better impact toughness than the ultra-NGLW joints.     

Structural first-order transformation in La2/3Ba1/3MnO3: ESR study

We have studied by the electron-spin resonance (ESR) and static magnetic field techniques, the La_2/3Ba_1/3MnO₃ perovskite, which was previously shown to exhibit a martensitic phase transformation in the vicinity of T_s∼200 K [Physical Review B 68, 054109 (2003)], leading to its structural phase-segregated state. Resonant absorptions reveal that in the temperature interval from 100 K to 340 K the compound represents a mixture of two ferromagnetic phases possessing different magnetizations, in varying proportions depending on the temperature, and a small amount of a paramagnetic phase. The results agree well with the previous neutron diffraction study. Applied in the ESR experiments, magnetic fields (2–6 kOe) strongly affect the magnetization curves: even magnetic field as high as 700 Oe modifies the anomaly in the phase transformation region and removes the difference between the zero-field cooled and field-cooled magnetization curves, which implies that the difference in the magnetic susceptibility of the coexisting phases is small and the magnetic domain configuration can be easily changed.     

Crystal plasticity finite element modeling of mechanically induced martensitic transformation (MIMT) in metastable austenite

A new crystal plasticity model incorporating the mechanically induced martensitic transformation in metastable austenitic steel has been formulated and implemented into the finite element analysis. The kinetics of martensite transformation is modeled by taking into consideration of a nucleation-controlled phenomenon, where each potential martensitic variant based on Kurdjumov–Sachs (KS) relationship has different nucleation probability as a function of the interaction energy between externally applied stress and lattice deformation. Therefore, the transformed volume fractions are determined following selective variants given by the crystallographic orientation of austenitic matrix and applied stress in the frame of the crystal plasticity finite element. The developed finite element program is capable of considering the effect of volume change by the Bain deformation and the lattice-invariant shear during the martensitic transformation by effectively modifying the evolution of plastic deformation gradient of the conventional rate-dependent crystal plasticity finite element. The validation of the proposed model has been carried out by comparing with the experimentally measured data under simple loading conditions. Good agreements with the measurements for the stress–strain responses, transformed martensitic volume fractions and the influence of strain rate on the deformation behavior will enable the model to be promising for the future applications to the real forming process of the TRIP aided steel.     

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.     

A justification of the theory of martensitic thin films in the absence of an interfacial energy

In the framework of Nonlinear Elasticity, the asymptotic behavior of the free energy of a martensitic material is studied as the height of the sample tends to zero. The effective thin film energy is identified in terms of parametrized probability measures, which allows for a justification of the kinematic compatibility conditions proposed in Bhattacharya and James' theory of thin films of martensitic materials, in the absence of higher order interfacial energy contributions to the three-dimensional bulk energy.