Influence of Preaging Temperature on the Indentation Strength of 3Y-TZP Aged in Ambient Atmosphere

Yttria-stabilized zirconia (3Y-TZP) containing 0.25% Al2O3, which is resistant to low temperature degradation (LTD), was aged for 10 h at 130–220 °C in air. The aged specimens were subsequently indented at loads ranging from 9.8 to 490 N using a Vickers indenter. The influence of preaging temperature on the biaxial strength of the specimens was investigated to elucidate the relationship between the extent of LTD and the strength of zirconia restorations that underwent LTD. The indented strength of the specimens increased as the preaging temperature was increased higher than 160 °C, which was accompanied by extensive t-ZrO2 (t) to m-ZrO2 (m) and c-ZrO2 (c) to r-ZrO2 (r) phase transformations. The influence of preaging temperature on the indented strength was rationalized by the residual stresses raised by the t→m transformation and the reversal of tensile residual stress on the aged specimen surface due to the indentation. The results suggested that the longevity of restorations would not be deteriorated if the aged restorations retain compressive residual stress on the surface, which corresponds to the extent of t→m phase transformation less than 52% in ambient environment.

Performance of Precipitates in the Stress Aging Treatment of 7075 Al Alloy

The effect of elastic stress on the precipitation process of 7075 Al alloy aged at 438 K for 1 hour has been investigated in this paper. The mechanical properties have been measured by hardness testing and scanning electron microscopy (SEM) and the microstructures have been characterized by optical microscopy (OM), together with transmission electron microscopy (TEM). Results indicate that the microhardness can reach 180 HV after a 25 MPa elastic tensile stress aging treatment, much higher than the stress-free ones'. The fracture is more brittle and the formation of the precipitates is promoted in the case of the stress-aged specimen. Our study provides a new process to bring down the energy consumption.

The Ageing Characteristics and Strengthening Mechanism of a Cu-2.1Ni-0.5Si-0.2Zr-0.05Cr Alloy

The ageing process of a Cu-2.1Ni-0.5Si-0.2Zr-0.05Cr alloy in the temperature range from 673 K to 773 K is investigated in this research. The phase transformation process, electric conductivities and mechanical properties during the ageing process are also studied by experimental methods. The precipitation sequence during the aging process of Cu-Ni-Si alloy is GP zone→ (Cu,Ni)3Si→ δNi2Si phase. The conductivity of the aged specimen increases from 20% IACS to 40-45% with prolonging time during aging process in the temperature range from 673 K to 773 K. The dynamic curve of precipitation is obtained by Avrami-equations, based on which the increase of yield strength can be estimated according to Orowan mechanism. Experimental data have been compared with the calculated results in this paper, indicating this method is reasonable, suggesting that strengthening is mainly attributed to Orowan by passing mechanism.

Fatigue Crack Growth of Ti-15V-3Cr-3Sn-3Al Alloy

In this study, the fatigue crack growth rates (FCGRs) of Ti-15V-3Cr-3Al-3Sn alloy aged in the temperature ranges from 426 to 593 oC for 8 hrs was determined at stress ration of 0.1 or 0.5. The results of fatigue crack growth tests revealed the FCGRs decreased with increasing aging temperature. Generally, the 426oC aged specimen had the highest FCGRs among the specimens being tested, regardless of stress ratio. It was noted obvious increase in FCGRs occurred for the 482oC aged specimen at stress ratio of 0.5. Fatigue fracture appearance of various specimens tested in air exhibited predominantly transgranular fatigue fracture with coarse stripes for the specimens aged at/above 538oC. Intergranular fracture was more likely to be found in the 426oC aged specimen, and for the A482 specimen tested at high stress ratio.

HRTEM Observation of Early Stage of Precipitation in Mg-Gd-Y Alloys

HRTEM observation was performed to clarify the early stage of precipitation in Mg-Gd-Y alloy aged at 473 K. The hardness increased with increasing aging time after quenching. The hardness increased slowly at early stage of aging, increased rapidly, and then reached to peak hardness. At as-quenching and the early stage of aging, the weak spots appeared at a half 100Mgin SAED pattern. The intensity of these weak spots changed to higher with increasing aging time. Monolayers on {100}Mgplanes and β” phase existed at this aging stage. Monolayers, β” phase, the precipitates with five layers corresponding the feature of (020)β’plane and β’ phase co-existed after the rapidly increasing of hardness. In the peak aged specimen at 473 K, both of β” and β’ phases co-existed and β’ phase were formed dominantly.

Annealing Response of Al-0.22Sc-0.13Zr Alloy Processed by Accumulative Roll Bonding

Ultra-fine grained (UFG) materials can be produced by several techniques involving severe plastic deformation (SPD). Accumulative Roll Bonding (ARB) is one of the SPD methods that enable the production of large amounts of UFG sheets. UFG sheets were prepared by up to six cycles of ARB at ambient temperature from an Al-0.22Sc-0.13Zr alloy in two states: a non-agehardened and a peak-aged. The effect of Al3(Sc1-xZrx) precipitates on the thermal stability of the UFG structures produced by ARB was investigated by isochronal annealing at temperatures between 200 and 550 °C. Additionally, the non-age-hardened ARB material was peak-aged prior to annealing and annealed together with both as-ARB-processed materials. The changes of microstructure and hardness due to annealing were studied. Annealing at 300 °C induces an additional strengthening in both non-pre-aged ARB materials that may be ascribed to precipitation and growth of coherent Al3(Sc1-xZrx) particles. This result suggests that the hardness decrease introduced by ARB in the peak-aged specimen is due to dissolution of precipitates during deformation. The annealing response of the materials above 300 °C does not depend on their thermal pre-treatment. However, the finely dispersed Al3(Sc1-xZrx) precipitates stabilise the refined deformed microstructure suitable for superplastic forming up to relatively high temperatures.

HRTEM Observation of the Age Hardening Precipitates in Mg-Zn Alloy

The age hardening precipitates of Mg-4.7mass%Zn alloy aged at 423K,473K were studied by using high-resolution transmission electron microscope (HRTEM). Contrasts of mono layers were confirmed to exist on the (0001) and (1100) matrix planes. It was considered that the contrast of mono layer was plate-like shape, and identified as pre-precipitates from as-quenched stage to early stage of aging at 473K for 32h . In the peak aged specimen of aged at 473K, the β1’ phase was observed. The β1’ phase has a rod-like shape and parallel to c-axis of Mg matrix. It can be observed orientation relationship between Mg matrix and β1’phase has not only same parts to previous reports but also different parts in one β1’ phase .

HRTEM Observation of Precipitates at Early Stage of Aging in Mg-Gd-Zr Alloy

High resolution transmission electron microscopy（HRTEM）observation was performed to clarify morphology of precipitates at early stage of aging in Mg-Gd-Zr alloy. In the specimen aged at 423K, monolayer was observed on {1100}Mg planes. With increasing aging time, the monolayer and β” phase grew along the <1120>Mg directions. Arrangement of bright dots corresponding to {020}β’ except for the monolayer and β” phase was observed before peak aging stage. β’ phases formed at the same aging stage. Thus, these precipitates co-existed before peak aging stage. The monolayer, β” phase and β’ phase still co-existed in the peak aged specimen. The β’ phase was dominated in the peak hardness. It is revealed that the monolayer lying on {1100}Mg act as precursor of β” phase and β’ phase.

Superplastic Deformation Mechanism of a Zn-Al Eutectoid Alloy

The deformation mechanism in the nanometer grain size range has been basically investigated from the results of microstructural observation after superplastic deformation in a Zn-Al eutectoid alloy in which a reverse grain size dependence of superplasticity was previously reported: flow stress increases and elongation decreases with decreasing grain size when grain size is markedly reduced to nanometer range. By controlling the aging condition after solution treatment and subsequent quenching, two specimens are prepared: the as-quenched specimen with ultrafine grains of 83nm in diameter and aged specimen with normally fine grains of 2.6μm. The elongation is confirmed to be smaller in the as-quenched specimen than in the aged specimen, although the flow stress is lower. As a result of TEM observation on the interior of the grains, dislocations are rare in the as-quenched specimen, while a significant density of dislocations are observed in the aged specimen. This result strongly supports the mechanism previously proposed by Mishra et al. that the accommodation process, i.e., the dislocation glide inside the grains, becomes more difficult with decreasing grain size in the nanometer grain size range, even though the grain boundary sliding as the major process becomes facilitated. Roughly assessed m-value was in accord with this mechanism.

Effect of Aging and Microstructure on Mechanical Rolling CuAlNi Shape Memory Alloy

CuAlNi shape memory alloy with Cu-13.5Al-4.5Ni(wt%) composition was prepared by cross-rolling method and the aging effect, phase transformation characteristics, microstructural variation were investigated. Transformation temperature was greatly increased in aged specimens at 250°C. Transformation temperature was not changed after the second reversed transformation, and Ms point was same in most specimens with third reversed transformation cycle. The variation of Ms point was not seen with aging at 100°C, but it was decreased with aging at 250°C. Transformation temperatures appear to be constant, to an measurable extent, in specimens prepared by both hotrolling and cross-rolling at 900°C. Plate shaped-specimen with the thickness of about 1mm was prepared by cross-rolling treatment at 350°C. The transformation temperatures did not change after the second transformation-reverse transformation cycling, and specially transformation start temperature was the same in most specimens experiencing third thermal cycle and thereafter. Undesirable tweed-like structure was observed in 250°C-aged specimen after a cross-rolling at 350°C. On aging the specimen prepared by cross-rolling, G.P zone was formed easier than that of γ2 phase. This phase was transformed to plate-like θ phase during aging.