Dislocation structures developed in hinge-type plastic zones associated with cracks in silicon crystals have been studied using a high voltage electron microscope (HVEM). Fine slip bands due to those dislocations have been also examined by an atomic force microscope (AFM). {100} and {110} cracks were introduced into {110} silicon wafers at room temperature by Vickers indentation method. The temperature of the wafer chips indented was raised to higher than 823 K to activate dislocations around crack tips under a residual stress due to the indentation. In specimens with the heat-treatment, prominent dislocation arrays corresponding to the hinge-type plastic zone were observed not only near the crack tip but also in the crack wake. AFM observations showed that very fine slip bands with the step height of a few nano-meters were formed with the regular spacing of a few microns. Based on the analyses of those dislocations and slip bands, it has been revealed that those dislocations were shielding-type increasing the fracture toughness.
Thermal sprayed coatings are applied to lubricated sliding surfaces of engine components recently.Small size peelings were observed on the ceramic coated test pieces tested with a pin on disk wear tester. The mechanism of peelings was investigated.A test in which a ball was rolled on the coated surface was carried out. The test was considered to be appropriate for evaluating the peeling property of coatings for the correlation to wear test results. The rolling test results show that the peeling was not affected by the change of temperature but was strongly affected by the existence of lubricating oil. The same tendency was also observed in a cyclic loading test. Cross sectional view of tested surface shows a Hertz crack connected to a horizontal crack. Peelings can be caused by the formation of these cracks. Crack propagation is accelerated by the existence of lubricating oil. It is considered that the oil enclosed in the crack is pressurized by load and it expands the crack.
Impact tests and tensile tests were conducted between 77 K and 450 K in order to elucidate the temperature dependence of absorbed-impact energy, yield stress, effective shear stress, activation volume, and activation enthalpy. The impact-absorbed energy decreased with decreasing test temperature, however, this alloy did not undergo low-temperature embrittlement although it has a bcc structure. Tensile tests showed changes in both the work-hardening rate and the temperature dependence of yield stress at approximately 150 K. This suggests a change in the mechanism behind the plastic deformation at the temperature. The temperature dependence of the activation enthalpy for dislocation glide suggests that the process of climbing over the Peierls potential (kink-pair nucleation) is the dominant mechanism for the dislocation glide from 150 K to 200 K, while the interaction between a dislocation and solute atoms dominantly controls the dislocation glide above 200 K. Superelasticity appears in stress-strain curves tested below 120 K, suggesting that the yielding is governed by transformation-induced plasticity below 120 K. The enhanced toughness at low temperatures in these alloys is discussed from the viewpoint of dislocation shielding theory.
Czochralski silicon single crystals were deformed in tensile tests along the direction at between 1173 K and 1373. Yield point phenomenon were observed in the specimens deformed at below 1273 K while continues yield was observed in the specimens deformed at above 1323 K. It is due to the effect of dislocation starvation in the used crystals. Work-hardening rates in stage II were consistent with those reported in fcc crystals such as copper. The onset of stage II was found to be active before the Schmid factor of the second slip system becomes larger than that of the primary slip system. Electron backscattered diffraction images indicated clear kink bands near grips and in the parallel portion. The kink bands were formed at the middle of stage I, which suggest that the formation of kink bands is a trigger of stage II.
High-pressure torsion (HPT) was conducted on commercial grade pure titanium (99.4%) by applying pressures in a wide range from 1.2 to 40 GPa. When the microhardness was plotted against equivalent strain, the hardness saturates to a constant level at each applied pressure. Such a level at the saturation depends on the applied pressure: for up to the pressure of 4 GPa, the saturation level is independent of the pressure but, for the pressures above 4 GPa, the hardness gradually increases with pressure because of the formation of an phase. Bending tests showed that an excellent ductility as well as high bending strength was achieved for the sample processed at 2 GPa. The bending ductility was reduced for the sample at 6 GPa because of the phase formation.
The strain distribution due to martensitic transformation in a Fe–Ni alloy was investigated using high-precision markers drawn via electron-beam lithography. This study focused on the strain distribution within the lenticular martensite plate, which developed immediately below the martensite start temperature.
Dwell sensitive fatigue is a problematic issue when titanium alloys are used as aero engine parts. It has been found that displacement/load holding accelerates local strain evolution, crack initiation, and crack growth, which causes deterioration of fatigue resistance. The present study focused on elucidating the crack growth acceleration mechanism during dwell fatigue. The dwell fatigue test was performed under load control with a triangular waveform, which included 10-min displacement holding at the peak load for only one cycle at ΔK = 25, 30, 40, 50 MPa√m in the Ti-6Al-4V alloy with a bimodal microstructure. Digital image correlation-based strain maps of the in-situ fatigue test showed the strain evolution near the crack tip during 1-cycle dwell. Moreover, locally extended striations corresponding to 1-cycle dwell were observed on the fracture surface. From the two results, it was proposed that the crack propagation during dwell fatigue occurs with crack tip blunting that is originated from dislocation emission assisted by thermally activated process during the stress holding. Another important finding was that the substructure beneath the fracture surface consisted of planar dislocations bundles and a locally thick bundle was observed in the region corresponding to 1-cycle dwell.
