Variations in Wear Resistance of a Novel Triboalloy Pseudoelastic TiNi Alloy-with Respect to its Pseudoelasticity and Hardness

It has recently been found that TiNt shape memory alloy has another attractive property; high resistance to wear. The wear resistance of this alloy benefits from its pseudoelasticity (PE). It has, however, been noticed that other mechanical properties also affect the wear resistance, especially the hardness. Research was conducted to investigate the correlation between the wear resistance and both the PE and hardness. It has been demonstrated that when the PE is high, lower hardness leads to higher wear resistance.

Influence of Pseudoelasticity on Mechanical Behavior of TiNi Alloy under Dynamic Impact Loading

The mechanical behavior of TiNi alloy and Cr12MoV alloy under dynamic impact loading was investigated with a self-made impact testing system. The real-time contact force was measured with a piezoelectric force sensor and digital signal processing system during impact. Equations for predicting instantaneous velocity anti displacement were presented. The results showed that the TiNi alloy exhibited a plateau of maximum contact force with increasing impact height. At the plateau stage, TiNi alloy in the parent phase can absorb impact energy and keep the maximum contact force nearly identical due to its pseudoelasticity.

INTERNAL VARIABLES AND THERMODYNAMIC MODELLING OF PSEUDOELASTICITY

Solid-solid phase transitions Often exhibt hysteresis and to describe the hysteretic behavior we need infernal variables Fos. shape memory single crystal under uni-axial tensile loading the internal variable has been identified experimentally as the number of interfaces between the austenitic and the martensitic phase regions . Experimental evidences are discussed here. Thermodynamical modelling based ont he internal variable is presented.

Effect of Ultrasound on the Pseudoelasticity of Shape Memory Alloys

The paper aims to develop a model describing the ultrasound-assisted pseudoelastic deformation of shape memory alloys. Experimental results record that acoustic energy reduces the value of stresses needed to induce pseudoelastic deformation (martensitic transformation). At the same time, the ultrasound-assisted deforming develops with a more significant strain hardening. The model presented here is based on the synthetic theory of inelastic deformation. To catch the phenomena caused by acoustic energy, we enter into the basic equation of the synthetic theory terms reflecting the effect of ultrasound on the processes governing the peculiarities of pseudoelastic deformation in the acoustic field. The analytical results fit good experimental data.

In situ observation of the pseudoelasticity of twin boundary

Twin boundary(TB)is a special and fundamental internal interface that plays a key role in altering the mechanical and physical properties of materials.However,the atomistic deformation mechanism of TB re-mains under debate,of which the most concerned aspect is how TB would affect the mechanical strength and plasticity of a material.Herein,we introduce our new discovery that the pseudoelastic strain of a TB can recover with decomposition and escape of pile-up dislocations,demonstrated by imposing a sponta-neous pseudoelastic deformation with recoverable plastic bending strain up to 5.1%on a TB.We found that the steps on the curved TB gradually annihilated during the migration of the TB,which was in-duced by the slip of decomposition dislocations on the TB.The TB not only provides local strain harden-ing through interaction with dislocations during the loading stage but also acts as a channel for the fast movement of decomposition dislocations during the recovery stage.Beside,the TB can maintain excellent pseudoelasticity under a multicycle bending test,which may play an important role in improving the fa-tigue resistance of materials.These findings could open up a new avenue for optimizing the mechanical properties of materials by manipulating their twin boundaries at the nanoscale.

Pseudoelasticity and elastocaloric effect of Fe_(75.5)Ga_(24.5) single crystal

<100>-,<112>-and <149>-oriented single crystals of Fe_(75.5)Ga_(24.5) alloy were prepared by optical float zone melting method.The pseudoelasticity behavior and elastocaloric effect of the single crystals were investigated,as well as the associate microstructures.D0_(3) phase structure was realized by solution treatment at 800 0C and annealed at 600℃ for 10 h.The compressive deformation behavior exhibits significant dependence on the crystalline directions.Excellent pseudoelasticity with recoverable strain up to 5% is obtained by compression along <149>direction.The pseudoelasticity disappears after five deformation cycles.Adiabatic temperature change is simultaneously detected during the pseudoelasticity,especially during loading process.

STRESS-STRAIN HYSTERESIS OF Cu-Zn-Al POLYCRYSTALLINE SMAs─Ⅱ: EXPERIMENTS AND NUMERICAL SIMULATIONS

In this paper, the stress-strain bys torests at various temperatures was investigated experimentally for Cu-Zn-Al polycrystalline shape memory alloys (SMAs). Numerical simulations of the pseudoelastic hysteresis were performed based on a model, which had been proposed by the authors. As observed in the experiments, the shapes of the outer loops of the hysteresis varied strongly from specimen to specimen, which have the same chemical composition but different heat-treatment. Rather complicated inner hysteretic curves were obtained at testing temperatures higher than Af. The numerical simulations of the stress-strain hysteresis and the inner hysteretic curves agreed quite well with the experimental results.

DYNAMIC RESPONSE OF SHAPE MEMORY ALLOY SYSTEM AND ITS VIBRATION CONTROL

A hysteric model is represented to describe the dependence of restoring force on deformation of pseudoelastic SMA.The dynamic response of the system is investigated by means of mathematical models.The result shows that this kind of vibration absorbing system can suppress vibration with large amplitude effectively.Furthermore,the vibration absorbing system can work in optimum state by adjusting temperature and using piezoelectric sensors and actuators.

The Deformation Behavior of TiNi Shape Memory Alloy under Axial Dynamic Compression

The deformation behavior of Ti-50.9at%Ni shape memory alloy under axial compression dynamic loads was investigated by an MTS 858Mini Bionix test machine. The alloy were aged at 500℃ for an hour before being machined into specimens. The compression experiments were conducted at 20℃ and the variety of dynamic loads were controlled by the strain rate, which was 3mm/min, 15mm/min, 30mm/min and 50mm/min, respectively. The experimental results indicate that in the case of 3mm/min, stress-induced martensitic transformation occurs at about 350MPa when loading and reverse transformation at about 200MPa when unloading, during which the aged Ti-50.9at%Ni alloy shows the recoverable nonlinear pseudoelastic strain of 4.3% with the residual strain of 1.2% reserved. With the strain rate increasing, the area encloses by loading-curve and unloading-curve, i.e stress (strain) hysteresis becomes smaller and smaller and the residual strain also decreases, while critical stress for inducing martensitic transformation rises. At a higher strain rate the alloy exhibits linear-like pseudoelasticity, which is up to 4.5%.

A NEW MODEL OF SHAPE MEMORY ALLOYS

A new constitutive model of shape memory alloys ( SMAs) based on Tanaka' s martensite fraction exponential expression is produced. this new model can present recoverable shape memory strain during different phase transformation, and reflect the action of martensite reorientation. Also it can overcome the defect of Tanaka' s Model when the SMAs' microstructure is fully martensite. The model is very simple and suitable for using, and the correct behavior of the model is proved by test.

A MICROMECHANICS ANALYSIS FOR THE MICROSTRUCTURE DESIGN OF A TWO-PHASE PSEUDOELASTIC COMPOSITE

A micromechanics analysis on the possibility of designing a two-phase pseudoelastic composite is made for the case where ductile transformable shape mem- ory alloy plastic particles are imbedded coherently in an elastic matrix. It is demon- strated that a pseudoelastic stress-strain loop in a macroscopic loading-unloading cy- cle can be obtained by microscopically stress induced forward and reverse martensitic transformations in the SMA particles. The relation between the macroscopic stress- strain response and the material parameters of the constituents of this composite is quantified through the micromechanics calculations, which reveals that the best duc- tility and thus the greatest energy absorption capacity of this novel microstructure can be obtained by the optimum material design.

Quick Estimation for Macroscopic Anisotropic Properties of Polycrystalline Cu and CuZnAl Sheets

The macroscopic anisotropy in polycrystalline materials may be predicted through anisotropic prolierty insingle crystal and texture analysis also. As practical examples, the Young' s modulus of copper sheet and thepseudoelastic elongation of CuZnAl sheet are evaluated. The results derived from this method are consistentwith the preceding conclusions obtained by others. It is noted that the method of drawing the elastic anisotropydiagram in different direction [hkl] in copper single crystal and quantitatively evaluating the pseudoelastic elongation of CuZnAl sheet have not been seen anywhere.

Indentation Size Effect inβCuAlBe and Cu-2Be Alloys

The indentation size effect(ISE)was studied by instrumented nanoindentation and Vickers hardness measurements on polycrystalline copper,βCuAlBe pseudoelastic and age-hardenable CuBe alloys.Variations in the load-depth curves at different loads would suggest a change in the behavior of the materials as the load increases.Nanohardness was estimated at different loads taking into account the pile-up in each case,which was estimated from the images of the topography obtained by atomic force microscopy(AFM).The effect of the microstructure of Cu-2Be(wt%)samples on the ISE was also studied.For each copper alloy,the Nix and Gao model was applied to the hardness-depth curves,and significant differences were found for the parameters obtained.Cu is in the constant nanohardness regime for depths below 500 nm,while microhardness exhibits the ISE effect.The behavior of the nanohardness and the plastic strain with the depth is almost inverse.CuAlBe presents the lowest values of plastic strains due to the pseudoelastic effect,while Cu presents an almost constant value around 8%,which corresponds to the highest plastic strain obtained.