Gradient Ageing Of Ni-Rich Niti Shape Memory Alloy

To date, the shape memory behaviour are limited only for discrete action application leaving no option for any progressive or intermediate state of control. The creation of gradient plateau is one of the best solutions and it is possible by many ways, and one of it is by microstructure modification via heat treatment that includes annealing and ageing. The purpose of this study is to create a functionally graded NiTi wire with an optimum pseudoelastic behaviour through a gradient ageing treatment. This approach is based on the sensitivity of the alloys’ thermomechanical properties with respect to temperature and duration of ageing treatment. The experimental works are divided into two stages. The first stage involved the study of the effect of temperature and duration of isothermal ageing on thermal and deformation behaviour of NiTi shape memory alloys. Due to the heterogeneous precipitate distribution, prolonged ageing led to the gradient deformation on the isothermal aged specimen with respect to gradual variation of Ni composition within the alloy matrix. The result shows that the stress plateau of the phase transformation exhibits positive stress gradient with a value of 3.4 GPa

Pseudoelasticity And Cyclic Behaviours Of Nickel-Rich NiTi Shape Memory Alloy

Near-equiatomic NiTi is one of the famous shape memory alloys used in vibration isolation and dampening application. This is due to its pseudoelastic behaviour. Pesudoelasticity refers to the ability of shape memory alloy to instantly recover from very large deformation after the deformation load is released. However, the yield strength of this alloy is low thus susceptible to localized dislocation. In this regard, preserving the pseudoelasticity of the alloy becomes a challenge. The alloy degrades, involving progressive accumulation of residual strain and reduction of the transformation stress when subjected to cyclic loading. Aging treatment is widely used to improve the yield strength of the alloy through precipitation hardening. Only coherent precipitates can profoundly increase the yield strength. Also, the precipitation is more effective in high Ni content of NiTi alloy. The effect of ageing on the yield strength and functional fatigue resistance are less explored especially for composition above 51at%Ni. This study aims to quantify the ageing effect of Ni-rich NiTi alloy towards the yield strength and the pseudoelastic behavior. The response of the yield strength towards the cyclic performance of Ni-rich NiTi alloys is also investigated. Commercial Ni-rich NiTi alloys with composition Ti-51at%Ni and Ti 50.8at%Ni were used in this study. The experimental works involved the isothermal ageing at a temperature range of 400°C to 550°C for 15, 30 and 60 minutes after solution treated at 900°C. Thermal analysis was done using Differential Scanning Calorimetry (DSC) to specify the phase transformation temperature. The tensile tests at room temperature were carried out to characterise the pseudoelasticity, yield strength and the cyclic performance using universal tensile machine (UTM). The results show that the yield strength increased as the ageing temperature increased, and the specimen of 51at%Ni alloy aged at 450°C for 30 minutes recorded the highest martensite yield stress value. Ageing at above 500°C caused the yield strength to decrease significantly. By comparison, 50.8at%Ni specimens showed lower yield strength than 51at%Ni. The pseudoelasticity of the aged specimens was found to decrease with the increase of ageing times. NiTi alloy with 51at%Ni showed less than 1% unrecovered strain at room temperature even though the austenite finish temperature, Af is higher than the testing temperature. Unlike 51at%Ni, 50.8at%Ni specimens aged at 425°C to 500°C showed poor recoverability with more than 2% residual strain. The cyclic deformation behaviour of the 51at%Ni alloy was found to be more stable for specimen aged at lower ageing temperature. Ageing at very high temperature (525°C and 550°C) caused the residual strain to increase and the transformation stress to decrease significantly over the first few cycles indicating functional behaviour degradation. Ageing of Ti-51at%Ni at 450°C for 30 minutes is recommended as the best ageing condition for cyclic deformation purpose. It produced the highest martensite yield stress, σyM of 1335 MPa, performing considerably good pseudoelasticity with residual strain, εr of 0.41% and high functional fatigue resistance with residual strain difference, ∆εr of 0.08%

Nanoscale Strain Fields Research of Boundaries between B2 Matrix and G.P. Zone in Ni-Ti Alloy Thin Films

Ti-47at.%Ni alloy films were prepared by magnetron sputtering followed by 460°C for 40 minutes heat-treatment. The strain fields between B2 phase matrix and G.P. zone were mapped by a combination of high-resolution transmission electron microscopy and geometric phase analysis method. It was found that there is a compressive strain region parallel to the longitudinal axis of G.P. zone with 2 nm in width, −2.2% in average strain at the boundaries between B2 phase and G.P. zone

Influence of Aging and Severe Plastic Deformation on the Shape Memory Behavior of Ti-50.6at%Ni

Martensitic phase transformations were investigated for Ti -50.6 at % Ni processed via severe plastic deformation (SPD) and subsequent aging for temperatures between 400 and 550C and times ranging from 1hr to 100hrs. Phase transformations of the SPD material were compared to material processed via solution treated at 800C and 850C for 1hr. Transformations were investigated with differential scanning calorimetry (DSC) and confirmed by X-ray diffraction (XRD) with optical microscopy being used to determine grain size. Solution treated samples displayed a single (1-1) B2↔B19\u27 transformation prior to and upon initial aging, i.e. 400C/1h, 450C/1/10h. Intermediate aging conditions, 450C/100h; 500C/1/10/100h; and 550C/1h, resulted in multiple B2↔B19\u27 transformations (2-2). Aging at 400C/10/100h, induced the R phase and 3-2 transformation behavior. Intensive aging caused the solution treated material and the SPD material to revert to a single (1-1) B19\u27↔B2 transformation. A 2-1 transformation was exhibited by the bulk of the SPD samples, i.e. unaged SPD, 400C and 450C at 1/10/100h, 500C 1/10h, 550C 100h. XRD of SPD material indicated the presence of metastable Ni4Ti3 precipitates prior to aging, and equilibrium Ni3Ti precipitates after aging for 500C/100h. Multiple peaks in the solution treated samples result from composition heterogeneities. 3-2 behavior is the result critical microstrains inducing the R phase. 2-1 behavior of the SPD material results from a homogenous microstructure and macrostrains which induce the R phase. Aging the SPD material causes recovery, suppressing the R phase transformation and increasing the Ms temperature, eventually resulting in a 1-1 transformation

Effect of Laser Welding Parameters on Formation of NiTi Shape Memory Alloy Welds

In this work experimental trials of welding of NiTi flat plates with 2.0 mm thickness were conducted using a 4.5 kW continuous wave (CW) Nd:YAG laser. The influences of laser output power, welding speed, defocus amount and side-blow shielding gas flow rate on the morphology, welding depth and width, and quality of the welded seam were investigated. Meanwhile, the effects of heat input on the mechanical and functional properties of welded joints were studied. The results show that laser welding can take better formation in NiTi alloys. The matching curves with laser power and welding speed affecting different formation of welds were experimentally acquired, which can provide references for laser welding and engineering application of NiTi alloy. The heat input has obvious effects on the ultimate tensile strength (UTS) and shape memory behavior of the welded joints

Room Temperature Ageing Effect in Ni-rich Ni-Ti Shape Memory Alloys

[eng] Ni-Ti system of alloys has the widest range of applications among all shape memory alloys. This system has traditionally been considered as an example of alloys stable close to ambient temperatures due to the strongly suppressed atomic diffusion. It is generally accepted that the lower temperature limit of ageing effects in Ni-Ti is close to 100-150ºC. In the first part of this work it is shown that the cubic–monoclinic martensitic transformation (B2-B19´) in the Ni50.8Ti49.2 alloy alloy is strongly affected by room temperature exposure after quenching from 1270 K and temperature cycling through the temperature range of martensitic transformation. Main manifestations of the effect of ageing are: - strong decrease of the transformation temperatures; - increase of the transformation temperature hysteresis; - eventually, the suppression of the transformation. In the second part we show that the necessary condition of the room temperature ageing is cycling through the martensitic transformation range under sufficiently low temperature scanning rate. Sweeping-up of quenched-in defects (presumably, excess Ni atoms) by interfaces moving during martensitic transformation which provokes local agglomerations of defects is proposed as a key ingredient of the intense room temperature ageing. It is suggested that the nuclei of precipitates are formed from the agglomerations of defects during consequent room temperature ageing.[spa] Entre todas las aleaciones con memoria de forma, las que presentan un sistema Ni-Ti son las que tienen una aplicación más extendida. Tradicionalmente, este sistema se ha considerado como un ejemplo de estabilidad en el rango de la temperatura ambiente debido a una fuerte supresión de la difusión atómica. En general, la bibliografía de referencia acepta que el límite de temperatura inferior de los efectos del envejecimiento en Ni-Ti está cerca de 100-150ºC. En la primera parte de este trabajo observamos como la transformación martensítica cúbicomonoclínica (B2-B19’), en la aleación de aleación Ni50.8Ti49.2, se ve fuertemente afectada por la exposición a temperatura ambiente después del enfriamiento desde 1270 K y el ciclado térmico a través de la temperatura de transformación martensítica. Las principales muestras del efecto del envejecimiento son: - Un importante descenso de las temperaturas de transformación; - Se detecta un aumento de la histéresis de la temperatura de transformación; - Además, eventualmente, se observa la supresión completa de la transformación; En la segunda parte del trabajo, observamos que, fundamentalmente, la condición necesaria para el envejecimiento a temperatura ambiente se debe al efecto del ciclado térmico cercano a la transformación martensítica, principalmente a una velocidad de ciclado térmico suficientemente baja. Se observa un efecto de barrido de defectos (presumiblemente, exceso de átomos de Ni), mediante el movimiento de las interfaces durante la transformación martensítica. Este barrido puede ser el origen de aglomeraciones locales de defectos, factor fundamental del envejecimiento intenso a temperatura ambiente. Se sugiere que los núcleos de los precipitados se forman a partir de las aglomeraciones de defectos durante el envejecimiento a temperatura ambiente

The influence of aging on shape memory effect in Ti‑50.7at.%Ni and Ti‑51.7at.%Ni single crystals

The shape memory effect (SME) during stressassisted thermal cycles under compressive load in [001]-oriented Ti-50.7at.%Ni and Ti-51.7at.%Ni single crystals aged at 823 K for 1 h has been studied. Ti3Ni4 particles with a diameter of 300–400 nm were precipitated with volume fractions of 11 and 22% and interparticle distances of 300–500 nm and 50–150 nm, respectively. In quenched single crystals, the SME parameters were determined by the transformation type (thermal-induced martensitic transformation (MT) or strain glass transition). In contrast, the SME parameters of aged single crystals were determined by the volume fraction of particles and interparticle distances. Differing volume fractions of particles and interparticle distances led to different temperatures ( M0s ) for the formation of B19′-martensite, different strain (εrev), different dependences of the interval of forward MT ( Δσ1) and thermal hysteresis ( ΔT1= Aσf−Mσs and ΔT2= Aσs−Mσf) on applied stresses, and changes in the morphology of martensite crystals. Practically, these differences do not affect the stresses (σmin and σmax) required to achieve the minimum strain and maximum reversible strain (εrev) and strain growth coefficient (dεrev/dσ). The influence of aging on the dependence of the SME parameters on the chemical composition was analysed in comparison with quenched crystals

Optimization Possibilities of Properties of NiTi alloy used as traumatology implants

Cílem této diplomové práce je studium vlivu tepelného zpracování na změnu fyzikálně-metalurgických vlastností slitiny NiTi. Pro experimenty byla použita slitina NiTi se složením 50,6 at. % Ni a 49,4 at. % Ti ve formě plochých vzorků o dvou různých tloušťkách. Jako tepelné zpracování bylo zvoleno rozpouštěcí žíhání při 850 °C a následné stárnutí při 450 °C. Studium mikrostruktury po jednotlivých tepelných úpravách bylo zkoumáno na optickém metalografickém mikroskopu a skenovacím elektronovém mikroskopu a doplněno měřením mikrotvrdosti. Pro zjištění transformačních teplot bylo využito diferenční skenovací kalorimetrie. Dále bylo využito energiově disperzní spektroskopie pro zjištění chemického složení jednotlivých fází ve struktuře.The aim of this Master thesis is the study of an influence of thermal treatment on physical and metallurgical properties of NiTi alloy. The alloy of NiTi composed of 50,6 at. % Ni and 49,4 at. % Ti in a rendition of flat samples of two different thicknesses were used for the experiments. Moreover, as a thermal processing, a solution treatment at 850 °C was employed, with subsequent aging at 450 °C. The study of the microstructures was examined after each thermal processing on an optical metallographic microscope, as well as on a scanning electron microscope and was accomplished by microhardness measurement. Furthermore, differential scanning calorimetry was used to finding transformation temperatures. Additionally, energy dispersive spectroscopy was utilized to determine chemical composition of individual phases in the structure.637 - Katedra neželezných kovů, rafinace a recyklacevýborn

Multiple Memory Material Processing for Augmentation of Local Pseudoelasticity and Corrosion Resistance of NiTi-based Shape Memory Alloys

Possessing unique thermomechanical properties, the discovery of nickel-titanium shape memory alloys (SMAs) has sprouted a plethora of applications in various fields, including aerospace, automotive, microelectronics, and medical devices. Due to its excellent biocompatibility and its ability to mimic biological forces, the medical implant industry has shown strong interest in expanding the application of NiTi SMAs. However, traditional SMA functional properties are limited by a single set of thermomechanical characteristics in a monolithic component. Past efforts in overcoming this limitation have had little success until recently with the invention of the multiple memory material (MMM) processing technology. This novel processing technology enables multiple functional responses through the augmentation of local microstructure and composition using a high power density source such as a laser. This thesis presents an investigation of the effect of laser processing on pseudoelastic behaviour and corrosion response of medical grade SMAs.1 yea