Influence Of Diameter And Grain Size On The Superelasticity And Shape Memory Effect Of Ni-Mn-Ga Fiber

With the development of miniaturized micro-devices, the small size ofintelligent materials has become increasingly demanding. Smaller size of intelligentmaterials brings some changes of their characteristics, so the formation and themechanism of this size effect will be of significance in application. This paperstudied the influence of diameter on the superelasticity and shape memory effect ofNi-Mn-Ga fiber, and analyzed the causes of the size dependence.The high vacuum arc melting was used to prepare the ingot ofNi_50.3Mn_26.9Ga_22.8. DSC, OM, XRD were used to analyze the ignot of the phasetransition temperature, microstructure, phase composition of the as-cast andheat-treatment state. Fibers of different diameters were fabricated by melt-extractionmethod with different process parameters and then SEM analysis was followed. Theresault is the molybdenum wheel is suitable for different diameter fibers fabrication.With the speed of the wheel1000rad/min or1200rad/min, a large amount of fiberswith high quality surface were obtained. By XRD analyzing, the phase transitiontemperature near room temperature NMG4F1fiber was selected finally and threegroups of different diameter fibers were obtained.Exploration of the heat treatment process, SEM and EDS were used to analyzethe growth of grain and the variations of composition with different processparameters. Founding shows that grain size changes with time as the law ofB=Ctⁿ, varies with temperature followingD₁²|-D₀²|=K₁·e^-Qm/RT-1；founding that thetrend of composition change orderly with heat treatment to homogenize thecomposition. Meanwhile, with grain grew up, Mn trends to volatile under heattreatment. When temperatures above1050°C, the Mn element volatiles severely.Based on heat treatment process, NMG4F1fiber were studied at1000°C withdifferent times to get oligocrystal, bamboo-shaped crystal fibers. SEM, EDSanalysis were verified to select a group of fibers of different grain size.The shape memory effect and superelasticity of the fibers with differentdiameters were investigated. The results showed that the size effect of differentdiameters on two-way shape memory effect （TSME） was: the smaller the diameter,the phase transition temperature on the stress sensitivity （dT/dσ）, phase transitiontemperature hysteresis （ΔT） were also smaller; for superelasticity: the smaller thediameter, the sensitivity of phase transformation stress on temperature （d σ/dT）,phase transformation stress hysteresis （Δσ）, and the sensitivity of energydissipation unit strain on temperature (d Δ E_1%dT) become greater. After analyzing the process of martensitic transformation, found that heat transfer and the frictionwork are the main causes of size-dependent and the mechanism is, when TSME, thefiner fiber has the faster rate of heat transfer in transformation process, is easier toheat exchange with the environment timely, so dT/dσ and Δ Tare smaller;superelasticity training, the finer fiber has greater specific surface area, produce thestronger pinning efect of surfaces, degrading more useful mechanical work intofrictional work which is then eventually dissipated in the form of heat, so thatd σ/d T, Δσ andd Δ E_1%/dTbecome greater.Heat treatment fibers for different grain sizes of the TSME and superelasticityin training were found that M_s, Δ T, dT/dσ varied with the grain size, the largergrain size of oligocrystal fiber, the greater Δ Tand dT/dσ, but when tobamboo-shaped crystal fiber, Δ Tand dT/dσ reduced simultaneously.