This paper investigates the effect of hydrogen on the transformation ratcheting of NiTi shape memory alloy(SMA)wires in the experimental and theoretical aspects.In the aspect of experiments,the NiTi SMA orthodontic wi...This paper investigates the effect of hydrogen on the transformation ratcheting of NiTi shape memory alloy(SMA)wires in the experimental and theoretical aspects.In the aspect of experiments,the NiTi SMA orthodontic wires are hydrogen charged by the electrochemical charging method at room temperature with varying charging durations and charging lengths.After that,the ex-situ cyclic tension-unloading experiments are performed for the charged and non-charged wires.Experimental results reveal that the two transformation platforms(two-step MT)occur during the forward MT at the beginning and end of cyclic deformation for hydrogen-charged wires,which can be regarded as a global response of the non-charged and charged regions.Furthermore,this two-step MT and transformation ratcheting aggravate with the increase of the charging duration.In the aspect of the theoretical model,a diffusional-mechanically coupled constitutive model is developed.In this constitutive model,the strain is considered as four components:elasticity,transformation(MT),hydrogen expansion and transformation-induced plasticity(TRIP).Combining Helmholtz free energy and Clausius–Duhem inequality,the thermodynamic driving forces of MT and TRIP are obtained.Fick’s law and the mass conservation equation are incorporated to derive the evolution of hydrogen concentration.A transition from material points to the whole wire is employed to extend the model from a material point to the entire wire,and the overall response with a heterogeneous hydrogen concentration field is obtained.The proposed model's ability to predict the transformation ratcheting of the non-charged and charged NiTi SMA wires is verified by contrasting predictions and experimental results.展开更多
使用模具并采用磁控溅射法在铁电陶瓷PZT基体上沉积具有条形分布结构的Ni Ti SMA薄膜。显微组织结构观察发现,以条形分布结构方式沉积的Ni Ti SMA薄膜晶化处理后具有等轴晶结构。比较所制备PZT/Ni-Ti SMA薄膜复合材料与纯PZT的介电常数...使用模具并采用磁控溅射法在铁电陶瓷PZT基体上沉积具有条形分布结构的Ni Ti SMA薄膜。显微组织结构观察发现,以条形分布结构方式沉积的Ni Ti SMA薄膜晶化处理后具有等轴晶结构。比较所制备PZT/Ni-Ti SMA薄膜复合材料与纯PZT的介电常数及介电损耗发现,两者的介电损耗水平接近;复合材料的介电常数比纯PZT的提高约18%。Ni Ti SMA的沉积使基体中靠近薄膜区域的Zr/Ti物质的量比恰好落在准同型相界区内,致使所制备复合材料的介电性能优于纯PZT。展开更多
A multiscale nonlocal continuum model is proposed to describe the superelastic deformation of gradient nano-grained NiTi shape memory alloys(SMAs).At the mesoscopic scale,the polycrystalline aggregate is regarded as a...A multiscale nonlocal continuum model is proposed to describe the superelastic deformation of gradient nano-grained NiTi shape memory alloys(SMAs).At the mesoscopic scale,the polycrystalline aggregate is regarded as a composite,i.e.,the graininterior(GI)phase is assumed to be a cuboidal inclusion embedded in a matrix of grain-boundary(GB)phase.An intrinsic energetic length and a gradient energy are introduced into the Helmholtz free energy of the GI phase.The criterion of martensite transformation(MT)is derived based on the principle of virtual power and second law of thermodynamics.The hindering effect of GB on MT in GI phase is addressed.By deriving the analytical solution of the proposed model and introducing a scale transition rule,the overall and local stress-strain responses of the specimen at the macroscopic scale are obtained.The prediction capability of the proposed model is verified by comparing the analytical solution with the experiment.The influences of the distribution form for the grain size(GS)on the superelastic deformation of gradient nano-grained NiTi SMAs are further predicted and discussed.The analytical form and low computational cost of the proposed model make it an appropriate theoretical tool to design the gradient nano-grained SMAs with desired mechanical property.展开更多
A macroscopic based multi-mechanism constitutive model is constructed in the framework of irreversible thermodynamics to describe the degeneration of shape memory effect occurring in the thermo-mechanical cyclic defor...A macroscopic based multi-mechanism constitutive model is constructed in the framework of irreversible thermodynamics to describe the degeneration of shape memory effect occurring in the thermo-mechanical cyclic deformation of NiTi shape memory alloys (SMAs). Three phases, austenite A, twinned martensite and detwinned martensite , as well as the phase transitions occurring between each pair of phases (, , , , and are considered in the proposed model. Meanwhile, two kinds of inelastic deformation mechanisms, martensite transformation-induced plasticity and reorientation-induced plasticity, are used to explain the degeneration of shape memory effects of NiTi SMAs. The evolution equations of internal variables are proposed by attributing the degeneration of shape memory effect to the interaction between the three phases (A, , and and plastic deformation. Finally, the capability of the proposed model is verified by comparing the predictions with the experimental results of NiTi SMAs. It is shown that the degeneration of shape memory effect and its dependence on the loading level can be reasonably described by the proposed model.展开更多
Thermal explosion method was used to prepare porous NiTi shape memory alloy. The process of thermal explosion was investigated. The effect of process parameters on thermal explosion reaction and properties of products...Thermal explosion method was used to prepare porous NiTi shape memory alloy. The process of thermal explosion was investigated. The effect of process parameters on thermal explosion reaction and properties of products was analyzed. The results showed heating rate, green density, particle size of initial powder strongly affected combustion temperature, porosity and compressive strength of final products. The mechanism of thermal explosion and the microstructure of reacted products were studied by XRD and SEM photographs. The results showed the final products mainly comprised of NiTi, Ti2Ni and TiNi3 phases and their strength decreased with the increase of porosity.展开更多
基金Financial supports from the National Natural Science Foundation of China NSFC(No.12322203,12072296)are greatly appreciated.
文摘This paper investigates the effect of hydrogen on the transformation ratcheting of NiTi shape memory alloy(SMA)wires in the experimental and theoretical aspects.In the aspect of experiments,the NiTi SMA orthodontic wires are hydrogen charged by the electrochemical charging method at room temperature with varying charging durations and charging lengths.After that,the ex-situ cyclic tension-unloading experiments are performed for the charged and non-charged wires.Experimental results reveal that the two transformation platforms(two-step MT)occur during the forward MT at the beginning and end of cyclic deformation for hydrogen-charged wires,which can be regarded as a global response of the non-charged and charged regions.Furthermore,this two-step MT and transformation ratcheting aggravate with the increase of the charging duration.In the aspect of the theoretical model,a diffusional-mechanically coupled constitutive model is developed.In this constitutive model,the strain is considered as four components:elasticity,transformation(MT),hydrogen expansion and transformation-induced plasticity(TRIP).Combining Helmholtz free energy and Clausius–Duhem inequality,the thermodynamic driving forces of MT and TRIP are obtained.Fick’s law and the mass conservation equation are incorporated to derive the evolution of hydrogen concentration.A transition from material points to the whole wire is employed to extend the model from a material point to the entire wire,and the overall response with a heterogeneous hydrogen concentration field is obtained.The proposed model's ability to predict the transformation ratcheting of the non-charged and charged NiTi SMA wires is verified by contrasting predictions and experimental results.
文摘使用模具并采用磁控溅射法在铁电陶瓷PZT基体上沉积具有条形分布结构的Ni Ti SMA薄膜。显微组织结构观察发现,以条形分布结构方式沉积的Ni Ti SMA薄膜晶化处理后具有等轴晶结构。比较所制备PZT/Ni-Ti SMA薄膜复合材料与纯PZT的介电常数及介电损耗发现,两者的介电损耗水平接近;复合材料的介电常数比纯PZT的提高约18%。Ni Ti SMA的沉积使基体中靠近薄膜区域的Zr/Ti物质的量比恰好落在准同型相界区内,致使所制备复合材料的介电性能优于纯PZT。
基金supported by the National Natural Science Foundation of China(NSFC)(Grant Nos.12072296,12322203 and 12202294)Sichuan Science and Technology Program(Grant No.2024NSFSC1346)are greatly appreciated。
文摘A multiscale nonlocal continuum model is proposed to describe the superelastic deformation of gradient nano-grained NiTi shape memory alloys(SMAs).At the mesoscopic scale,the polycrystalline aggregate is regarded as a composite,i.e.,the graininterior(GI)phase is assumed to be a cuboidal inclusion embedded in a matrix of grain-boundary(GB)phase.An intrinsic energetic length and a gradient energy are introduced into the Helmholtz free energy of the GI phase.The criterion of martensite transformation(MT)is derived based on the principle of virtual power and second law of thermodynamics.The hindering effect of GB on MT in GI phase is addressed.By deriving the analytical solution of the proposed model and introducing a scale transition rule,the overall and local stress-strain responses of the specimen at the macroscopic scale are obtained.The prediction capability of the proposed model is verified by comparing the analytical solution with the experiment.The influences of the distribution form for the grain size(GS)on the superelastic deformation of gradient nano-grained NiTi SMAs are further predicted and discussed.The analytical form and low computational cost of the proposed model make it an appropriate theoretical tool to design the gradient nano-grained SMAs with desired mechanical property.
基金Financial supports by the National Natural Science Foundation of China (Grant 11532010)the project for Sichuan Provincial Youth Science and Technology Innovation Team, China (Grant 2013TD0004)
文摘A macroscopic based multi-mechanism constitutive model is constructed in the framework of irreversible thermodynamics to describe the degeneration of shape memory effect occurring in the thermo-mechanical cyclic deformation of NiTi shape memory alloys (SMAs). Three phases, austenite A, twinned martensite and detwinned martensite , as well as the phase transitions occurring between each pair of phases (, , , , and are considered in the proposed model. Meanwhile, two kinds of inelastic deformation mechanisms, martensite transformation-induced plasticity and reorientation-induced plasticity, are used to explain the degeneration of shape memory effects of NiTi SMAs. The evolution equations of internal variables are proposed by attributing the degeneration of shape memory effect to the interaction between the three phases (A, , and and plastic deformation. Finally, the capability of the proposed model is verified by comparing the predictions with the experimental results of NiTi SMAs. It is shown that the degeneration of shape memory effect and its dependence on the loading level can be reasonably described by the proposed model.
文摘Thermal explosion method was used to prepare porous NiTi shape memory alloy. The process of thermal explosion was investigated. The effect of process parameters on thermal explosion reaction and properties of products was analyzed. The results showed heating rate, green density, particle size of initial powder strongly affected combustion temperature, porosity and compressive strength of final products. The mechanism of thermal explosion and the microstructure of reacted products were studied by XRD and SEM photographs. The results showed the final products mainly comprised of NiTi, Ti2Ni and TiNi3 phases and their strength decreased with the increase of porosity.
