Computational thermodynamic approach based on the CALPHAD method has played a key role in materials science and engineering for these several decades as a bridge between experimental observations and theoretical prediction of phase equilibria. Within the same time frame, first-principles calculation based on the electron theory has progressed extensively and has interacted with the CALPHAD method. In this paper, we overview our recent research results on the calculation of thermodynamic properties such as enthalpy of formation for binary and ternary alloys at finite temperatures using electron theory as well as lattice vibration and cluster expansion method. Some examples of integrating those theoretical values comparable with experimental uncertainties and the CALPHAD methodology for construction of thermodynamic databases are presented.
In this study, recent developments regarding the modeling of microstructural evolution during recrystallization are reviewed, and this paper highlights a unified theory for continuous and discontinuous annealing phenomena by means of the subgrain growth mechanism. In Sec. 3.2, the mean field analysis based on the unified theory is reviewed. In the analysis, the possibility of abnormal subgrain and/or grain growth based on non-uniform grain boundary mobility and energy has been clearly shown. With the developments in the unified subgrain growth theory, a number of Monte Carlo, vertex, and phase-field simulations have been performed, in order to investigate the microstructural evolution during recrystallization by considering the local alignment of the subgrain structure. Then, in Sec. 3.3, the numerical simulation results based on the unified theory are outlined. Finally, in Chap. 4, the numerical simulation results of static recrystallization in two-dimensional polycrystalline structures by coupling the unified sub-grain growth theory with the phase-field methodology are reviewed. In particular, the effects of the microstructural inhomogeneities formed during the deformation state on the recrystallization kinetics are discussed.
Nowadays, most of cold rolled steel sheets are produced in continuous annealing lines. In continuous annealing lines, the recovery, recrystallization, reverse transformation and dissolution of precipitates occur in the heating and homogenizing process and the transformation and precipitation occur in the cooling and aging process. The required microstructure and mechanical properties are created by controlling these metallurgical phenomena properly. For the sophisticated control of these phenomena, the model for predicting microstructure is a useful tool and therefore, many models have been developed. In this review, the present status of those models is surveyed and their problems and future tasks are discussed.
Fundamental mechanism governing the fracture toughness of materials is reviewed in terms of a concept of the interaction between a crack and dislocations. The mechanism of brittle-to-ductile transition (BDT) is demonstrated using a simple model based on dislocation dynamics and the theory of crack-tip shielding by dislocations. The effects of dislocation mobility as well as dislocation sources on the BDT behavior are discussed, which enables us to understand the various factors such as grain refinement influencing fracture toughness.
Effects of temperature and strain rate on stress–strain curves for two types of dual-phase (DP) steel with different carbon contents were investigated from the viewpoints of tensile tests and the Kocks–Mecking (KM) model based on thermal activation theory. In the tensile tests, flow stress increased but elongation decreased with decreasing temperature or increasing strain rate. Uniform elongation for each DP steel was almost independent of deformation temperature between 123 and 373K. In the comparison of strain rate sensitivity exponent (m), the m-value increased with decreasing the volume fraction of martensite. The calculated true stress–true strain curves by using the KM model agreed with the measured ones at various temperatures and strain rates for the DP steels. In terms of the parameters for the KM model, the athermal stress and mechanical threshold stresses were different between the two types of DP steel. This seems to be associated with the difference of volume fractions of ferrite and martensite.
The stress–strain curve of the composite material has been calculated on the basis of the secant method proposed by Weng. Although the effect of inclusion shape on the stress–strain curve has been evaluated by Zhao and Weng, they have assumed the inclusion phase to be a rigid elastic medium which dose not deform plastically. In this study, we modified their secant method so as to be able to calculate the stress–strain curve of the composite material which contains the plastically deformed inclusion phase, and evaluated the effect of the inclusion shape on the deformation behavior of the stress–strain curve. The results obtained are as follows: When both phases are soft, i.e. the matrix and inclusion phases are both deformable, the stress–strain curve is determined only from the volume fraction of the inclusion phase. In the case that the inclusion phase is hard, i.e. the matrix phase is deformable but the inclusion phase is less deformable, the stress–strain curve depends not only on the volume fraction of the inclusion phase but also on the shape of the inclusion phase. The modified secant method proposed in this study will be useful for understanding the effect of inclusion shape on the mechanical property of the composite materials, comprehensively.
Elasto–plasticity behavior of a IF steel sheet was investigated by performing uniaxial tension tests in three directions (0°, 45° and 90‚ to the rolling direction of the sheet), in-plane cyclic tension–compression test and bi-axial tension test. The sheet has strong planar r-value anisotropy (r0=2.15, r45=2.12 and r90=2.89) but very weak flow stress directionality. Equi-biaxial flow stress is as large as 1.23 times of the uniaxial flow stress. These elasto–plasticity deformation characteristics, as well as the Bauschinger effect and cyclic hardening behavior, are well described by a macro–plasticity model (Yoshida–Uemori model incorpolating with the 4th-order anisotropic yield function). Further, the simulation of elasto–plasticity stress–strain responses of the sheet were conducted by two types of crystal plasticity models, i.e., Taylor hypothesis based model and CPFEM, using the crystallographic orientation distribution data measured by neutron diffraction method. The models capture most of the above-mentioned deformation characteristics qualitatively, but the predicted anisotropy and the Bauschinger effect are weaker than those of the real material. The CPFEM gives more realistic results than the Taylor model.
Crystallographic textures of hot-rolled steel sheets are thought to be inherited from the textures of austenite, which develop during hot rolling, due to the orientation relationship between austenite and product phases and the therein-involved variant selection rules. However, despite numerous investigations, there is still difficulty in the prediction of textures of ferrite or martensite in the hot-rolled steels. In this article are described the present state of such prediction of transformation textures in hot-rolled steel sheets. In particular, the way to calculate the transformation textures based on the so-called MODF method and a variant selection rule that has been recently proposed are described in detail. Combined with the proposed variant selection rule, the MODF method may be capable of quantitatively predicting the transformation texture in hot-rolled steel sheets.
Microstructual observations indicate that the recrystallization of cold rolled extralow carbon steel sheets occurs due to the abnormal growth of selected subgrains in recovered subgrain microstructures. Based on this recrystallization mechanism, a new model for predicting the recrystallization behavior of a material with idealized microstructure was proposed by Humphreys. The model cannot, however, predict the recrystallization behavior of cold rolled steel sheets in the practice because of the complexities of their microstructure. In this study, a model for predicting the recrystallization behavior of cold rolled extralow carbon steels has been developed based on the model by Humphreys. The feature of the present model is that the pinning and solute drag effects on the recrystallization behavior can be evaluated quantitatively. Using the model, the influence of Ti addition on the recrystallization in extralow carbon steel sheets was discussed. The result indicates that the retardation of the recrystallization due to the addition of Ti is attributed mainly to the retardation of the subgrain formation in the recovery process, and the effect of the pinning of precipitates and the dragging of solute Ti atoms on the progress of recystallization are relatively small.