Cooling rate effects on the structure and transformation behavior of Cu-Zn-Al shape memory alloys

Different fragments of a hot-rolled and homogenized Cu–Zn–Al shape memory alloy（SMA） were subjected to thermal cycling by means of a differential scanning calorimetric（DSC） device. During thermal cycling, heating was performed at the same constant rate of increasing temperature while cooling was carried out at different rates of decreasing temperature. For each cooling rate, the temperature decreased in the same thermal interval. During each cooling stage, an exothermic peak（maximum） was observed on the DSC thermogram. This peak was associated with forward martensitic transformation. The DSC thermograms were analyzed with PROTEUS software： the critical martensitic transformation start（Ms） and finish（Mf） temperatures were determined by means of integral and tangent methods, and the dissipated heat was evaluated by the area between the corresponding maximum plot and a sigmoid baseline. The effects of the increase in cooling rate, assessed from a calorimetric viewpoint, consisted in the augmentation of the exothermic peak and the delay of direct martensitic transformation. The latter had the tendency to move to lower critical transformation temperatures. The martensite plates changed in morphology by becoming more oriented and by an augmenting in surface relief, which corresponded with the increase in cooling rate as observed by scanning electron microscopy（SEM） and atomic force microscopy（AFM）.

Phase Morphology Evolution in AISI301 Austenite Stainless Steel under Different Cooling Rates

Quenching experiments were performed at different cooling rates under non-directional solidification by differential thermal analysis, and the morphologic variation of primary phase, phase transition temperature and hardness change at the same quenching temperature were investigated. The experimental results show that, with the gradual decrease of the cooling rate from 25 K/min, the morphology of ferrite starts to transform experiencing the dendrite, radial pattern, Widmanstatten-like and wire-net. Sample starts to present the Widmanstatten-like microstructure at 10 K/min which does not exist at higher or lower cooling rates, and this microstructure is detrimental to the mechanical property. Except 10 K/min, the hardness decreases with decreasing cooling rate.

Thermal Analyses of Power Electronics Integrated with Vapour Chamber Cooling

Insulated gate bipolar transistor(IGBT)power module is used for power switching transistor devices in the power supply and motor control circuits in both hybrid electric vehicles and electric vehicles.The target of heat flux of IGBT is continuously increasing due to the demand for power rating improvements and miniaturisation.Without suitable efficient cooling technolo-gies,excessively high temperature and uneven temperature distribution can cause high thermal stress,eventually leading to severe module failures.Therefore,highly efficient cooling solutions are highly required.Vapour chamber with phase change can provide quick heat transfer and low temperature gradient.This study proposes a new IGBT structure integrated with vapour chamber.The tests and simulation results indicate that the thermal and thermo-mechanical performances of IGBT integrated with vapour chamber are better than those of the IGBT with copper baseplate module.The thermal resistance between the junction and heat sink is reduced from 0.25 to 0.14°C/W,and the temperature uniformity is greatly improved due to the phase change in the vapour chamber.The simulation also investigates the thermal stress distribution,deformation and thermal fatigue lifespan of IGBT power electronics module.A reduction of 21.8%in thermal stress and an increase of 9%in lifespan of Sn-3.5Ag solder are achieved.