Tetsuro Shiraishi

Strain induced martensite formation characteristics of austenite stainless steel during various loading conditions

Abstract The strain induced martensitic transformation (SMT) of the austenite stainless steel (SUS 304) under cyclic loading and static loading was investigated directly using electron backscattered diffraction. Two different SMT characteristics are observed, which are attributed to the differences of plastic and twinning deformation. The maximum cyclic stress has a strong influence on the SMT. The total area fraction of the Fe-α′ phase increases significantly when the maximum cyclic load is >80% σUTS. In other words, the SMT is apparently absent when the samples are loaded with less than σmax = 70% σUTS, although such samples are fractured completely. Moreover, there is a clear R ratio effect on the SMT. For example, the loading condition R = −1 gives rise to a strong SMT compared to R = 0·1 due to the more severe strain caused by the compressive stress. In contrast, no clear frequency effect (1 versus 30 Hz) on the SMT is detected, which may be attributed to the same maximum cyclic stress. Like the SMT characteristics under cyclic loading, the proportion of Fe-α′ phase shows no clear increase until the sample is loaded statically to a tensile stress <70% σUTS.

Crystallisation characteristics of primary silicon particles in cast hypereutectic Al-Si alloy

Abstract The crystallisation characteristics of primary silicon particles in a cast hypereutectic Al–Si alloy were examined using various casting processes. The size of the primary silicon particles depends on the cooling rate. However, the primary silicon particles cannot be crystallised completely when the cooling rate is >20 K s−1, since uncrystallised Si atoms migrate to the regions at low cooling rates, producing large primary Si particles. Moreover, turbulent metal flow under conditions of high cooling rate leads to the macrosegregation of the primary Si particles, resulting in poor mechanical properties. From the crystallisation characteristics obtained, a suitable solidification criterion can be proposed.

Mechanical properties of AC4CH alloys produced by heated mould continuous casting process

Abstract This paper reports an investigation of the mechanical properties of cast aluminium AC4CH alloys produced by a unique casting method known as the heated mould continuous casting (OCC) process. The tensile and fatigue strengths and material ductility of the OCC samples are much higher than that for the same alloy produced by conventional gravity casting. The high tensile and high fatigue strengths of the OCC samples are reflected by the presence of tiny round α-Al grains, and the high ductility of OCC is a result of the regularly organized crystal orientation.

Corrosion and mechanical properties of cast aluminium alloys

Abstract The corrosion and mechanical properties of cast aluminium alloys [ADC6(Al–Mg) and ADC12(Al–Si–Cu)] have been investigated. Specifically, cast samples with different microstructural characteristics were produced using two different casting techniques, namely, gravity casting (GC) and heated mould continuous casting (HMC). For ADC6, intergranular corrosion occurs mainly through the eutectic Mg2Si and Al6(Mn,Fe) phases and the precipitate free zone around the eutectic structures. On the other hand, pitting corrosion in the α-Al phase is the dominant feature for ADC12. Such corrosion characteristics can be interpreted from the material properties (polarisation resistance and electrode potential) of the individual structures, such as the α-Al phase, eutectic α-Al phase and precipitate phase. It also appeared that the extent of corrosion is affected by the size of the microstructure; the ADC6 sample with small second phases has a high corrosion resistance due to the limited area for corrosion. In addition, the higher proportion of solid solution in the α-Al phase of ADC6, arising from the high cooling rate, makes a high corrosion resistance because of the change of electrode potential. A different trend is obtained for ADC12 due to the pitting corrosion, where low corrosion resistance is obtained for the sample with tiny grain. The tensile strength and ductility for both HMC samples (ADC6 and ADC12) are higher than those for the GC equivalents, which is caused by the presence of tiny microstructures and uniform crystal orientation. However, the mechanical properties of HMC samples decrease dramatically during corrosion, although these are still high compared to those of the GC samples without corrosion.

Material properties of cast aluminium alloys produced by various casting processes

The material and mechanical properties of cast aluminium alloys, produced using various casting technologies, have been investigated experimentally. In this study, several casting processes were selected, including gravity casting (GC), cold-chamber diecasting (CD), hot-chamber diecasting (HD), squeeze diecasting (SQ), twin rolled continuous casting (TRC) and heated mould continuous casting (HMC). Although all cast samples were made of the same aluminium alloy of Al-Si-Cu (ADC12), different material properties were obtained, in particular microstructural characteristics, crystal orientation and residual stress. In this instance, the microstructures of the GC and CD samples were formed mainly of coarse α-Al phase and needle-shaped eutectic structures; and a microstructure of fine round grain and tiny eutectic structures were observed for the HC, TRC and HMC samples. On the other hand, spherical shaped α-Al phase with a relatively large size was detected in SQ. The different microstructural characteristics led to different mechanical properties. A uniformly orientated lattice structure was detected in the HMC and SQ samples, which gave high material ductility. High internal compressive stress high dislocation density, arising from the rolling process, led to the high tensile properties for the TRC procedure. Various cast defects, such as blow holes and shrinkage porosity, were detected in GC and CD, which gave a reduction in their tensile properties.

Effect of domain wall characteristics on material properties of lead zirconate titanate piezoelectric ceramics

Abstract To better understand the material properties of lead zirconate titanate (PZT) ceramics, the influence of domain wall characteristics on the electrical and mechanical properties of PZT ceramics has been investigated. To obtain various domain wall characteristics, the poling process was carried out with different patterns with respect to the PZT ceramic. The domain walls were aligned in the PZT ceramic in the direction perpendicular to the poling direction. Such domain wall characteristic produced different crack growth behaviours. The crack growth occurred mainly along the domain walls of {110} for a31 (crack growth direction perpendicular to the poling direction) because of the high stress concentration between the domain walls. In contrast, the crack propagated along both domain walls and grain boundaries for a33 (crack growth direction parallel to the poling direction), leading to high crack growth resistance and good mechanical properties. The fracture toughness KIC values for the PZT ceramics in the a31 and a33 directions were about 0·5 and 2·6 MPa m1/2 respectively. It also appeared that the domain walls collapsed when a number of poling processes are conducted along different directions. These PZT ceramics had high mechanical properties due to the low stress concentrations.

Fatigue properties (S–N and da/dN–ΔK) of cast aluminium alloys

Abstract In this paper, the fatigue strengths of cast aluminium alloys have been examined using the S–N and da/dN–ΔK approaches on various cast samples having different microstructural characteristics. These were made by several different casting techniques, namely gravity casting, die casting and twin rolled continuous casting. Owing to the different microstructures, the fatigue properties varied. The S–N relations, e.g. the endurance limit, of the twin-rolled casting were apparently high compared to the other cast samples. The high fatigue strength was related to the high tensile strength and high ductility caused by the tiny spherical α-Al phase and fine eutectic structure. On the other hand, the low fatigue strength for the gravity and die cast samples was caused by the high stress concentration caused by the α-Al phase and needle-like eutectic silicon. Unlike the fatigue strength in the S–N approach, the da/dN–ΔK relationships for the twin rolled casting sample, i.e. the resistance to fatigue crack propagation, were low compared to the gravity casting and the die cast samples. Such a difference in the crack growth resistance was influenced by the crack growth characteristics. For example, for the gravity cast and the die cast samples, the crack growth occurs in the grain boundaries of the coarse α-Al phase and along the long eutectic silicon grains (zigzag crack path), which promotes a strong crack closure tendency, leading to high crack growth resistance. The difference in the fatigue strength determined from the S–N relation and da/dN–ΔK relation was interpreted by experimental and numerical examination of the stress–strain distribution.

Study of resonance fatigue properties of piezoelectric ceramics using new fatigue testing system

Abstract To better understand the material damage of lead zirconate titanate (PZT) ceramics, fatigue tests have been carried out using a newly proposed testing system. The material damage was evaluated with the change in sonar intensity from the ceramics, where a sensing control was operated using a condenser microphone system. This testing system produces cyclic loading with a wide range of fatigue frequencies: 1 Hz–72 kHz. The influence of fatigue frequency on the fatigue properties of a PZT ceramic has been investigated, and it appeared that the fatigue strength is not linearly correlated with the fatigue frequency. The reason for this is due to the resonance caused by the oscillation. Thus, the fatigue frequency can change the severity of sample vibration. The extent of sample vibration is associated with the sonar intensity from the ceramic, which is linearly related to the crack growth rate. Details of the resonance fatigue properties have been systematically examined.

Acceleration and retardation of fatigue crack propagation and fracture surface appearance in polycarbonate.

Fatigue crack propagation behavior under single and multiple overloading conditions in polycarbonate was studied fractographically. The results obtained are summarized as follows:(1) The fracture surface of the crack extension accelerated by a single peak overload consists of two different regions, one contains many fine tearing lines oriented parallel to the crack propagation direction, and the other contains many voids. The former is similar to that found in the striation, and the latter is similar to that found in the discontinuous growth band.(2) The amount of crack extension accelerated by the overload is almost constant inside the specimen, but decreases markedly near the boundary between the interior fracture surface and the shear lips formed at the specimen surface layers.(3) The acceleration factor of crack propagation due to the overload is dominantly dependent on the ratio of peak to baseline stress, and is not affected appreciably by the crack propagation history.(4) The degree of retardation of crack propagation after a single peak overload shows the dependence on baseline stress, which can be explained from the viewpoint of crack propagation mechanisms.(5) The width of characteristic retardation region which is produced after the multiple overloads is about equal to the size of plastic zone at the crack tip produced by the overloads.