Takahito Ohmura

Dislocation-grain boundary interactions in martensitic steel observed through in situ nanoindentation in a transmission electron microscope

Dislocation–interface interactions in Fe–0.4 wt% C tempered martensitic steel were studied through in situ nanoindentation in a transmission electron microscope (TEM). Two types of boundaries were imaged in the dislocated martensitic structure: a low-angle (probable) lath boundary and a coherent, high-angle (probable) block boundary. In the case of a low-angle grain boundary, the dislocations induced by the indenter piled up against the boundary. As the indenter penetrated further, a critical stress appeared to have been reached, and a high density of dislocations was suddenly emitted on the far side of the grain boundary into the adjacent grain. In the case of the high-angle grain boundary, the numerous dislocations that were produced by the indentation were simply absorbed into the boundary, with no indication of pileup or the transmission of strain. This surprising observation is interpreted on the basis of the crystallography of the block boundary.

The combined effect of molybdenum and nitrogen on the fatigued microstructure of 316 type austenitic stainless steel

The mechanical properties of austenitic stainless steels, including the low cyclic fatigue behavior have been a subject of numerous studies. Since the role of Mo was not considered in most previous studies, the present study aimed at understanding the combined effect of Mo and Ni on the fatigue properties of 316 type austenitic stainless steels. For this purpose, the authors have investigated the fatigued microstructure and the distribution of nitrogen by a transmission electron microscope (TEM), a conventional atom probe (1DAP) and a three dimensional atom probe (3DAP).

Relationship between nanohardness and microstructures in high-purity Fe–C as-quenched and quench-tempered martensite

The relationship between the nanohardness and the microstructures in the Fe–C martensite was studied to understand the contributions of the matrix and the grain boundary to the macroscopic strength. As-quenched martensite was examined for five kinds of Fe–C alloys with various carbon contents in the range of 0.1–0.8 mass%, while quench-tempered martensite was investigated for an Fe–0.4% C alloy. The ratio of the nanohardness to the macrohardness H n / H v was much smaller for the Fe–C martensite than those for the single crystals, indicating that there is a significant grain-boundary effect for the martensite. The ratio H n / H v of the as-quenched martensite decreased with an increase in the carbon content since the size of the block structure decreased with increasing carbon content. For the quench-tempered specimens, a significant reduction of the grain-boundary effect occured at the tempering temperature of 723 K. It is mainly due to the depression of the locking parameter caused by the disappearance of the film-like carbides on the boundaries.

Evaluation of the matrix strength of Fe-0.4 wt% C tempered martensite using nanoindentation techniques

Abstract Nanoindentation techniques were applied to evaluate the matrix strength of the as-quenched and the tempered martensite of an Fe-0.4 wt% C binary alloy. The matrix strength was compared with the micro Vickers hardness H V. The nanohardness Hn of the matrix decreases with increasing tempering temperature, which is similar to the temper softening observed in the micro Vickers hardness. The ratio H n/H V, which corresponds to the contribution of the matrix strength to the macroscopic strength, increases markedly when the specimen is tempered at 450°C. This is due to a reduction in the grain size effect caused by the grain growth of the blocks that are analogous to the effective grain. The pop-in behaviour that appears in the specimens tempered at or above 550°C is due to the reduction in the dislocation density caused by recrystallization.

Evaluation of mechanical properties of ceramic coatings on a metal substrate

Abstract Nanoindentation and tensile tests were performed for TiN coatings on a stainless steel substrate to investigate the indentation induced deformation behavior of the coatings and the adhesion of the coating–substrate interface. Load–displacement curves show the transition of plastic deformation state from coating only to coating–substrate composite in a penetration depth of 1/10 of the coating thickness, which is approximately two times smaller than that of metal coatings. We employed two specimens with different interface characteristics for the evaluation of the adhesion. The tensile test can clearly detect the difference by the analysis of statistics of extremes for the maximum crack spacing, while the indentation results show no difference, suggesting that the tensile test is superior in detecting the adhesion of the coating–substrate interface.

Indentation-Induced Deformation Behavior in Martensitic Steel Observed through In Situ Nanoindentation in a Transmission Electron Microscopy

Deformation behavior in the vicinity of grain boundary in Fe-0.4wt%C tempered martensitic steel were studied through in-situ nanoindentation in a TEM. Two types of boundaries were imaged in the dislocated martensitic structure: a low-angle lath boundary and a high-angle block boundary. In the case of a low-angle grain boundary, the dislocations induced by the indenter piled up against the boundary. As the indenter penetrated further, a critical stress appears to have been reached and a high density of dislocations was suddenly emitted on the far side of the grain boundary into the adjacent grain. In the case of the high-angle grain boundary, the numerous dislocations that were produced by the indentation were simply absorbed into the boundary, with no indication of pile-up or the transmission of strain.

Electron diffraction analysis of quenched Fe–C martensite

Martensite has a body-centered tetragonal (bct) structure in high carbon steels. However, body-centered cubic (bcc) {112} 〈111〉-type twins instead of bct twins always be observed as the substructure of martensite in high carbon steels. In this paper, martensitic substructure in a quenched high carbon Fe-1.4C (wt%) alloy has been investigated in detail using selected area electron diffraction (SAED) technique in a conventional transmission electron microscopy. The reciprocal lattice of martensite has been built based on the experimental SAED patterns. Two sets of diffraction spots (one face-centered cubic lattice and one hexagonal lattice) in the built reciprocal lattice suggest that two crystalline phases with bcc (or α-Fe) and hexagonal (ω-Fe) structure actually coexist in the twinned martensite. The two-phase diffraction spot patterns from the reciprocal lattice can match perfectly with the experimental results. The fact that the {0001}ω diffraction spot at the 1/3{222}α position and the {0002}ω at 2/3{222}α can support the ω-Fe existence in the twinned martensite.

A Novel Design Approach for Self-Crack-Healing Structural Ceramics with 3D Networks of Healing Activator

Self-crack-healing by oxidation of a pre-incorporated healing agent is an essential property of high-temperature structural ceramics for components with stringent safety requirements, such as turbine blades in aircraft engines. Here, we report a new approach for a self-healing design containing a 3D network of a healing activator, based on insight gained by clarifying the healing mechanism. We demonstrate that addition of a small amount of an activator, typically doped MnO localised on the fracture path, selected by appropriate thermodynamic calculation significantly accelerates healing by >6,000 times and significantly lowers the required reaction temperature. The activator on the fracture path exhibits rapid fracture-gap filling by generation of mobile supercooled melts, thus enabling efficient oxygen delivery to the healing agent. Furthermore, the activator promotes crystallisation of the melts and forms a mechanically strong healing oxide. We also clarified that the healing mechanism could be divided to the initial oxidation and additional two stages. Based on bone healing, we here named these stages as inflammation, repair, and remodelling stages, respectively. Our design strategy can be applied to develop new lightweight, self-healing ceramics suitable for use in high- or low-pressure turbine blades in aircraft engines.

Strength evaluation of α and β phases by nanoindentation in Ti–15Mo alloys with Fe and Al addition

Abstract The strengths of the α precipitate and the β matrix were evaluated by nanohardness in the Ti−15Mo−1Fe and Ti−15Mo−3Al alloys and compared to those of the Ti−15Mo alloy. The α phases with similar size (a long axis of a few micrometres and a short axis of a few hundred nanometres), distribution and volume fraction were obtained in three alloys by adjusting the aging temperature. Analyses by SEM-EDS confirmed that Fe and Mo were enriched in the β phase and depleted in the α phase, while Al was enriched in the α phase and depleted in the β phase. Tensile tests were carried out, and the tensile strength was shown to be higher in the Ti−15Mo−1Fe and Ti−15Mo−3Al alloys than in the Ti−15Mo alloy. The nanohardness measurements indicated that the α phase was softer than the β phase in both Ti−15Mo−1Fe and Ti−15Mo alloys, while it was harder in the Ti−15Mo−3Al alloy. The increased tensile strength was mainly caused by the strength of the Fe enriched β phase in the Ti−15Mo−1Fe alloy and by the strength of the Al enriched α phase in the Ti−15Mo−3Al alloy.

Effect of Inhomogeneity of Carbide Precipitation on Nanohardness Distribution for Martensitic Steels

Nanoindentation technique was applied to evaluate nanohardness distribution in a submicron scale for two kinds of martensitic steels: Fe-0.4C binary steel and Fe-0.05C-0.22Ti steel with a stoichiometric composition of TiC. AFM images showed that Fe-C steel includes relatively coarse cementite particles with about 100~200 nm in diameter and a couple of hundreds nanometer in average spacing, while high-resolution TEM observation showed that the Fe-C-Ti steel has fine TiC precipitates with 5 nm in diameter and 15 nm for average spacing. Nanoindentation results revealed that the standard deviation was much higher for the Fe-C than that for the Fe-C-Ti. Since the typical indent size was a couple of hundreds nanometer, which was about two orders larger than the size of the TiC and comparable to the cementite size, the small distribution of nanohardness of the Fe-C-Ti was attributed to the homogeneous microstructure in sub-micron scale, while the inhomogeneity of cementite particles in the Fe-C steel leaded to large nanohardness.