Qianqian Hu

The transformation behavior and the shape memory effect due to cyclic stress/strain for Ti-49.6Ni alloy

Abstract The transformation behavior for Ti–49.6 at.%Ni shape memory alloy was investigated under cyclic stress/strain by electrical resistance method and differential scanning calorimetry (DSC) method. After solution no R transformation occurred during the martensite transformation in the R – T curve, and the reverse martensite transformation temperature points A s as well as A f could be discerned clearly in the heating process. With the same kind of heat treatment, R transformation occurred after the specimen underwent cyclic stress or strain; in the meantime, the reverse transformation temperature points A s and A f were illegible during the reverse martensite transformation. The characteristics of resistance–temperature curve ( R – T curve) of Ti–49.6Ni shape memory alloy changed apparently and regularly. The results also showed that after cyclic strain, the transformation temperatures M s and A s were stable while the cyclic stress is not. That implied that the low-strain cycling itself could be used as a better method for mechanical training. For both stress and train cycling, the martensite transformation temperature M s was lower than that of the raw material. It indicated that stress/strain cycling would inhibit the martensite transformation and directly result in the R transformation occurring. The shape memory effect after cyclic stress/strain were also studied, respectively.

Optical enhancement brought by doping Gd(3+) ions into Ce: YAG ceramics for indoor white light-emitting diodes.

We dope Gd(3+) ions into Ce: YAG ceramics to induce red-shift in the photoluminescence, of which the degree is proportional to the Gd(3+) doping concentration. This kind of ceramic, when collaborating with InGaN blue chips, proves to be a promising fluorescent material of white light-emitting diodes, for not only its high in-line transmittance and decent quantum yield, but also the improvement in color rendering ability brought by the red-shift, which enhances the ratio of red portion in spectra. We demonstrate that 10% is the optimized value of Gd(3+) doping concentration, as it provides the maximum color rendering index of ~78 with luminous efficiency as high as 128 lm/W.

Influence of fully reversed stress/strain-controlled cycling on the transformation behavior of Ti-49.6Ni shape memory alloy

The transformation behavior of Ti-49.6Ni shape memory alloy was investigated under fully reversed stress/strain cycling using electrical resistance measurement. The results showed, that there was no R-transformation in the original alloy and that R-transformation occurred after stress/strain cycling. The R-phase was induced by a rhombohedral distortion of the austenite in the transition process from the high-temperature phase, also called parent phase, to the low-temperature phase, the martensite, and was characterized well by the sharp increase of the electrical resistivity. The austenite start- and finish-transformation temperatures A(s) and A(f) could be observed clearly in the original alloy, whereas they appeared less clearly after stress/strain cycling. Under plastic strain-amplitude controlled cycling with Deltaepsilon(p) = 0.05%, the transformation temperature M-s remained stable between 10(3) to 10(4) cycles. Finally we assumed that low plastic strain-amplitude controlled cycling itself, which could not do any damage to the material, might be used as a better kind of mechanical training method in comparison with the thermo-mechanical training for a stabilization of the transformation temperatures for Ti-49.6Ni shape memory alloy.