Precipitate hardening is the most easiest and effective way to enhance strain recovery properties in NiTiHf high-temperature shape memory alloys.This paper discusses the precipitation,coarsening and age hardening of H...Precipitate hardening is the most easiest and effective way to enhance strain recovery properties in NiTiHf high-temperature shape memory alloys.This paper discusses the precipitation,coarsening and age hardening of H-phase precipitates in Ni_(50)Ti_(30)Hf_(20)alloy during isothermal aging at temperatures between 450℃and 650℃for time to 75 h.The H-phase mean size and volume fraction were determined using transmission electron microscopy.Precipitation kinetics was analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law.From these analyses,a Time-Temperature-Transformation diagram was constructed.The evolution of H-phase size suggests classical matrix diffusion limited Lifshitz-Slyozov-Wagner coarsening for all considered temperatures.The coarsening rate constants of H-phase precipitation have been determined using a modified coarsening rate equation for nondilute solutions.Critical size of H-phase precipitates for breaking down the precipitate/matrix interface coherency was estimated through a combination of age hardening and precipitate size evolution data.Moreover,time-temperature-hardness diagram was constructed from the precipitation and coarsening kinetics and age hardening of H-phase precipitates in Ni_(50)Ti_(30)Hf_(20)alloy.展开更多
The martensitic phase transformation in Ti_(40.4)Ni_(48)Hf_(11.6) shape memory alloys is leveraged for bi-directional actuation with TiNiHf/SiO_(2)/Si com-posites.The shape memory properties of magnetron sputtered Ti_...The martensitic phase transformation in Ti_(40.4)Ni_(48)Hf_(11.6) shape memory alloys is leveraged for bi-directional actuation with TiNiHf/SiO_(2)/Si com-posites.The shape memory properties of magnetron sputtered Ti_(40.4)Ni_(48)Hf_(11.6) films annealed at 635℃-5 min are influenced by film thickness and the underlying substrate.Decreasing TiNiHf film thick-ness from 21μm to 110 nm results in the reduction of all characteristic transformation temperatures until a critical thickness is reached.Particularly,Ti_(40.4)Ni_(48)Hf_(11.6) thin films as low as 220 nm show transfor-mations above room temperature when deposited on SiO_(2) buffer layer,which is of great interest in nano-actuation.In comparison,220 nm films on Si substrates are austenitic at room temperature,and thus not suitable for actuation.Thermal fatigue tests on TiNiHf/SiO_(2)/Si bimorphs demonstrate better functional fatigue characteristics than freestanding films,with an average reduction of 15℃ after 125 cycles,with tempera-ture stabilization subsequently.Experimental bi-directional actuation results are promising in the development of bistable actuators within a PMMA/TiNiHf/Si trimorph composite,whereby the additional PMMA layer undergoes a glass transition at 105℃.With the aid of constitutive modeling,a route is elaborated on how bistable actuation can be achieved at micro-to nanoscales by showing favorable thickness combinations of PMMA/TiNiHf/Si composite.展开更多
基金supported by the National Natural Sci-ence Foundation of China(Nos.52050410340 and 51971072)the Fundamental Research Funds for the Central University(No.3072021CFJ1002).
文摘Precipitate hardening is the most easiest and effective way to enhance strain recovery properties in NiTiHf high-temperature shape memory alloys.This paper discusses the precipitation,coarsening and age hardening of H-phase precipitates in Ni_(50)Ti_(30)Hf_(20)alloy during isothermal aging at temperatures between 450℃and 650℃for time to 75 h.The H-phase mean size and volume fraction were determined using transmission electron microscopy.Precipitation kinetics was analyzed using the Johnson-Mehl-Avrami-Kolmogorov equation and an Arrhenius type law.From these analyses,a Time-Temperature-Transformation diagram was constructed.The evolution of H-phase size suggests classical matrix diffusion limited Lifshitz-Slyozov-Wagner coarsening for all considered temperatures.The coarsening rate constants of H-phase precipitation have been determined using a modified coarsening rate equation for nondilute solutions.Critical size of H-phase precipitates for breaking down the precipitate/matrix interface coherency was estimated through a combination of age hardening and precipitate size evolution data.Moreover,time-temperature-hardness diagram was constructed from the precipitation and coarsening kinetics and age hardening of H-phase precipitates in Ni_(50)Ti_(30)Hf_(20)alloy.
文摘The martensitic phase transformation in Ti_(40.4)Ni_(48)Hf_(11.6) shape memory alloys is leveraged for bi-directional actuation with TiNiHf/SiO_(2)/Si com-posites.The shape memory properties of magnetron sputtered Ti_(40.4)Ni_(48)Hf_(11.6) films annealed at 635℃-5 min are influenced by film thickness and the underlying substrate.Decreasing TiNiHf film thick-ness from 21μm to 110 nm results in the reduction of all characteristic transformation temperatures until a critical thickness is reached.Particularly,Ti_(40.4)Ni_(48)Hf_(11.6) thin films as low as 220 nm show transfor-mations above room temperature when deposited on SiO_(2) buffer layer,which is of great interest in nano-actuation.In comparison,220 nm films on Si substrates are austenitic at room temperature,and thus not suitable for actuation.Thermal fatigue tests on TiNiHf/SiO_(2)/Si bimorphs demonstrate better functional fatigue characteristics than freestanding films,with an average reduction of 15℃ after 125 cycles,with tempera-ture stabilization subsequently.Experimental bi-directional actuation results are promising in the development of bistable actuators within a PMMA/TiNiHf/Si trimorph composite,whereby the additional PMMA layer undergoes a glass transition at 105℃.With the aid of constitutive modeling,a route is elaborated on how bistable actuation can be achieved at micro-to nanoscales by showing favorable thickness combinations of PMMA/TiNiHf/Si composite.
