We studied damage evolution behavior associated with ε-martensite in a Fe-28Mn alloy. Visible factors of damage evolution associated with ε-martensite are considered to be strain distribution, microstructure, micro-void and crack. Combinatorial use of replica digital image correlation, electron backscattering diffraction, and electron channeling contrast imaging enables to clarify the distributions of strain, microstructure and damage. Through quantitative damage analysis, damage evolution behavior was classified into three regimes: (i) incubation regime, (ii) nucleation regime, and (iii) growth regime. In the incubation regime, an interaction of ε/ε-martensite plates and impingement of ε-martensite plates on grain boundaries caused plastic strain localization owing to plastic accommodation. In the nucleation regime, accumulation of the plastic strain on the boundaries caused microvoid formation. The damage propagated along with the boundaries through coalescence with other micro-voids, but the propagation was arrested by crack blunting at non-transformed austenite. In the growth regime, the arrested damage grew again when a further plastic strain was provided sufficiently to initiate ε-martensite near the damage.
To obtain the fundamental information on the microstructure evolution during cooling from the β region in a Fe containing α+β type titanium alloy “Ti-5Al-1Fe”, microstructures of the specimens continuously cooled from the β region at various cooling rates were closely examined and compared with those in conventional titanium alloys Ti-6Al-4V and Ti-5Al-2.5Sn.
The kinetics of the α phase formation during cooling in Ti-5Al-1Fe, in which fast diffusive Fe is used as an alloying element, was considerably different from that in Ti-6Al-4V containing slow diffusive V although the two alloys have the same strength level and the similar phase constitution. In the specimens cooled at cooling rate of 300°C/s or higher, the Fe-enriched β phase retained between the lath α plates, showing that diffusional phase transformation occurred even at relatively high cooling rate of 300°C/s. In the specimens cooled at cooling rates ranging from 1 to 300°C/s, cooling rate dependence on Vickers hardness at room temperature was considerably small compared to Ti-6Al-4V. It is similar to that in Ti-5Al-2.5Sn containing around 0.2% Fe, indicating that the diffusional component of phase transformation dominated the phase transformation during cooling and shear component was suppressed at cooling rates lower than 300°C/s. In the specimen cooled at 0.1°C/s, the ω phase formed in the retained β phase probably as a result of an aging effect due to the low cooling rate, while equilibrium FeTi was not detected. Based on the microstructure observations, the CCT diagram of Ti-5Al-1Fe was formulated.
Dual Phase (DP) steel is used in automotive body parts for weight saving and crashworthiness, however there is an issue of DP steel in low stretch flange ability evaluated by hole expanding tests. In order to improve stretch flange ability of DP steel, it is important to estimate the damage of punching quantitatively and to clarify the change of microstructure before and after punching because the hole expansion ratio is decided in the ductility remained after pre-strain equivalent to punching. Therefore we tried to measure the damage of punching by unique techniques of Electron Backscatter Diffraction (EBSD), nano-indentation and micro-tensile testing and to observe fracture surface by Scanning Transmission Electron Microscope (STEM). Average EBSD-Kernel Average Misorientation (KAM) value and pre-strain damage have strong correlation, thus average KAM value can become the index of the damage. The nanohardness and tensile strength using micrometer-sized specimens increased with increasing average KAM value in the ferritic phase as approaching the punching edge. A shear type fracture occurred without necking in the specimen cut out in the area of the edge. The ultrafine-grained ferritic microstructure was observed in the sample cut out in the same area with STEM. It seems that the ductility loss of the punched DP steel was probably attributed to localized strain into the ultrafine-grained ferritic microstructure.
Relationship between Ave.KAM value and Nano hardness, Micro-tensile strength.
To evaluate heterogeneous strain distribution developed by pre-deformations in dual phase (DP) steel accurately, a combinational technique of Electron Backscatter Diffraction (EBSD) and Digital Image Correlation (DIC) methods was newly introduced in this study. A good correlation is established between kernel average misorientation calculated by EBSD and local equivalent strain measured by DIC in ferrite matrix of DP steels regardless of the difference in deformation process, which means that an EBSD orientation map can be easily converted into an applicative strain map by employing the individual correlation formula. This new technique reveals that very high strain region is locally formed within dozens of micrometer from the punched edge in a punched DP steel. On the other hand, hard martensite grains dispersed in DP steel remarkably promote the heterogeneity of strain distribution in ferrite matrix. As a result, the high strain region is also developed in the form of bands in a cold-rolled DP steel by only 60% thickness reduction at least, as if it is affected by the distribution and morphology of martensite grains. In addition, the local strain mapping demonstrates that the equivalent strain of the high strain band in cold-rolled material is comparable to that of the heavily deformed edge in punched one. The very high strain band in ferrite matrix is characterized by ultrafine grained structure, which leads to the possibility for the losing ductility in ferrite matrix and the martensite cracking.
The deformation behavior of inhomogeneous microstructures developed by pre-straining was studied by micro-tensile testing to elucidate the cause of low hole expandability of ferrite-martensite dual-phase (DP) steels. Slip bands developed in the ferritic phase, when the DP steel was cold-rolled (CR) at a reduction of 60% in thickness; in the 88% CR microstructure, ultrafine ferrite grains with a strong texture were locally observed. While the nanohardness increased with increasing pre-strain in the ferritic phase, it was invariable in the martensitic phase. Tensile tests using micrometer-sized specimens with ferritic and martensitic phases revealed that the ultrafine-grained ferritic microstructure exhibited high yield strength but low ductility when compared to the slip band ferritic microstructure. While a shear type fracture occurred without necking in the former, the latter exhibited a chisel-edge type failure. Without ultra-grain refinement by pre-straining, the inhibition of slip transfer by the interphase boundary was a major contributor to the strengthening in the DP steel. The ductility loss of the severely deformed DP steel was presumably attributed to localized strain into the ultrafine-grained ferritic microstructure.
The fatigue crack initiation and propagation behavior of a water-quenched binary Fe-C fully ferritic steel was investigated though rotating-bending fatigue testing. Intergranular and transgranular crack initiation and propagation were observed. The intergranular crack propagation did not stop, while the transgranular crack propagation was retarded by crack closure and strain aging. As a result, intergranular cracking was the dominant cause of fatigue damage in the steel. A considerable number of cracks were initiated; these propagated through coalescence, which occurred mainly at the grain boundaries. Dominancy of the intergranular fatigue crack propagation increased with increasing stress amplitude. In addition, the steel showed coaxing effect significantly. The coaxing effect suppresses crack initiation as well as crack propagation.