Katsushi Kubo

Very high strain-rate superplasticity in a particulate Si3N4/6061 aluminum composite

The study deals with constant true strain tests carried out over a wide strain-rate range from 0.0001 to 100/s at an optimum superplasticity temperature of 833 K in air. The tensile axis is selected to be parallel to an extrusion direction for all tests, and the flow stress for each strain rate is determined at a fixed small true strain. The microstructural morphology of composites is discussed as well as the flow stress behavior and elongation. A high-strain-rate superplasticity is found in an as-extruded 20-vol-pct Si3N4/6061 aluminum composite; the maximum elongation of 620 pct is recorded at a strain rate of 2/s. 13 refs.

Superplasticity in as-extruded Si3N4w6061 aluminum composites processed at a ratio of 100: 1

Abstract 6061 aluminum matrix composites reinforced with 20 vol% Si 3 N 4 whiskers have been extruded at 773 K with a reduction ratio of 100: 1, and their properties have been investigated at an elevated temperature of 818 K. A large elongation of 600% was obtained at an initial strain rate of 2 × 10 −1 s −1 .

Superplastic-like behavior in as-extruded AlZnMg alloy matrix composites reinforced with Si3N4 whiskers

Abstract For as-extruded AlZnMg alloy without reinforcement of Si 3 N 4ω , a maximum elongation is 140%, which was measured near a relatively high strain rate 2 × 10 −2 s −1 at 818 K. On the other hand, a maximum elongation of 160% for α-Si 3 N 4ω /AlZnMg composites, whose grain size was less than 4 μm, was obtained at 798 K and at the highest end in strain rate range of 8 × 10 −1 s −1 evaluated in this work, and for β-Si 3 N 4ω /AlZnMg composites with grain size of about 4 μm, a maximum elongation of 230% was obtained at 818 K and a high strain rate of 2 × 10 −1 s −1 . In these AlZnMg system alloys the composites reinforced with α-Si 3 N 4ω exhibited a high superplastic elongation at higher strain rates than the composites reinforced with β-Si 3 N 4ω .

Joining of silicon nitride ceramics by hot pressing

The joining of hot-pressed silicon nitride ceramics, containing Al2O3 and Y2O3 as sintering aids, has been carried out in a nitrogen atmosphere. Uniaxial pressure was applied at high temperature during the joining process. Polyethylene was used as a joining agent. Joining strength was measured by four-point bending tests. The effects of joining conditions such as temperature (from 1400 to 1600°C), joining pressure (from 0.1 to 40 MPa), holding time (from 0.5 to 8 h) and surface roughness (Rmax) of the joining couple (about 0.12, 0.22 and 1.2μm) on the joining strength were examined. The joining strength was increased with increases in joining temperature, joining pressure and holding time. Larger surface roughness caused lower joining strength. The higher joining strength was attributed to a larger true contact area. The area was increased through plastic deformation of the joined couple at elevated temperatures. The highest joining strength attained was 567 MPa at room temperature, which was about half the value of the average flexural strength of the original body. The high temperature strength measured at 1200° C did not differ very much from the room-temperature value.

Influence of temperature and strain-rate on superplastic elongation in a powder metallurgy AlZnMg composite reinforced with Si3N4 whiskers

Abstract The superplastic behavior of a powder metallurgy Si 3 N 4w /AlZnMg composite was investigated under temperature conditions between 798 and 843 K and strain rates from 3 × 10 −5 to 10 s −1 in a constant strain-rate test. A maximum elongation of 380% was recorded at a strain rate of 10 −1 s −1 at 833 K.

Superplastic-like behavior in AlMgSi composites reinforced with α-Si3N4 or β-Si3N4 whiskers

Abstract Superplastic-like behavior has been investigated for AlMgSi alloy matrix composites reinforced with either α-Si3N4 or β-Si3N4 whiskers. Both composites exhibited large elongations in the high strain rate range near 10−1 s−1. The superplastic potential of the composite reinforced with β-Si3N4 whiskers was superior to that of the composite reinforced with α-Si3N4 whiskers.

Superplastic properties in a Si3N4w/Al-Zn-Mg composite extruded at a reduction ratio of 100:1

Abstract Al-Zn-Mg system alloy matrix composite reinforced with 20 vol% Si3N4 whiskers has been extruded at 773 K with a reduction ratio of 100:1, and its superplastic properties have been investigated at an elevated temperature by a constant velocity tension test. A maximum elongation of 240% was obtained at 5 × 10−1 s−1.

Fatigue Behaviour of Structural Ceramics

4-point bending static and cyclic fatigue tests are carried out on several structural ceramics both at room and elevated temperatures. Measured fatigue behaviours are compared with the lifetime predicted from the dynamic fatigue properties and cyclic loading effects on delayed failure are discussed. Experimental results indicate that a delayed failure of ceramics under a cyclic loading is caused mainly by extension of a pre-existed flaw and the lifetime is substantially predictable from a subcritical crack growth behaviour estimated under a static or quasi-static loading condition. It is also pointed out that at elevated temperatures the crystallization of glassy phase at grain boundaries extends the lifetime and that a crack growth is accelerated by compressive stress if grain (cluster) bridgings and/or crack interlockings are formed on a crack surface. Humidity controlled stress corrosion of a glassy phase at grain boundaries and mechanical fracture through the grain boundaries due to softening of the glassy phase are considered as main crack growth mechanisms at room and elevated temperatures, respectively. From these results it is concluded that the fatigue behaviour of ceramics is strongly affected by the properties of grain boundaries.

Lubricational characteristics of hypereutectic Al-Si P/M alloy in hot forging

Recently, some kinds of hypereutectic A1-Si P/M alloys have been developed as advanced materials possessing excellent wear resistance and relative strength. Although these alloys are brittle and hard at room temperature, their ductility increases and flow stress decreases at elevated temperatures. We have investigated the effects of temperature and strain rate on the deformability of a rapidly solidified hypereutectic A1-Si P/M alloy and suggested that this alloy showed superplastic behaviour [1]. Static hot forging in the temperature range 400 to 500°C was recommended for this alloy, because the improvement in ductility and the decrease in flow stress were accelerated with increasing temperature and with slower working speed. For successfull hot forging it is necessary to clarify the influence of friction on the formability and also to chose good lubricants with a low coefficient of friction. In this study, the influence of temperature, strain rate and kind of lubricant on the coefficient of friction were investigated, using the ring compression test. A rod of the P/M alloy for the tests was prepared by hot extrusion of rapidly solidified powders. Its chemical composition is shown in Table I. The crystal grain size is extremely fine, the mean grain size being about 2.5 pm. The volume content and grain size of silicon are 15 to 20% and 1 to 1.5pm, respectively. Ringshaped specimens were cut out parallel to the direction of extrusion. The dimension of the specimen was 5 mm height, 7.5mm inner diameter and 15mm outer diameter. The end surfaces of the specimen were

Surface damage in ground silicon carbide ceramics

Hot pressed (HP) and reaction bonded (RB) silicon carbide (SiC) ceramics were ground with a # 400 diamond wheel, and the induced damage was evaluated by the specimen strength measured by four-point bending test. The surface layers of the ground specimens were removed by polishing with a # 3000 diamond disc. The relationships between the strength and the thickness of removed layers were examined. In HP-SiC and RB-SiC, when the thickness was smaller than about 10μm, the strength was lower than that of the ground specimen. The maximum strength reductions were more than 30% for HP-SiC and more than 10% for RB-SiC of those of the original ground specimens, respectively. In addition, when the removed surface layers were more than about 10μm, the strength of each SiC ceramics recovered, and each showed a constant value regardless of the thickness. The strength seemed to have a similar value to the inherent strength of each SiC ceramics, and it was considered to be independent of the surface damage which was induced during grinding. In order to explain these reduction in strength, a simple model for the surface damage caused by grinding was proposed by considering internal cracks near the surface. The relationships between the thickness of removed layer and the theoretical specimen strength based on the model showed the same tendency as the experimental results.