Transformation Finish Temperature Research Articles

Mechanical behavior of shape memory alloys considering the effects of body fluids corrosion for biomedical applications

Abstract Shape memory alloys (SMAs) are widely used in biomedical engineering, including cardiovascular stents, artificial skeletons, and orthodontic implants. For the above applications, the body fluids corrosion processes will inevitably cause deterioration in the mechanical properties of the SMAs actuator during its service life, which will threaten the safety of human health. To analyze such problems, experimental measurements have been carried out to investigate the influence of body fluid corrosion on the mechanical properties of SMAs. Changes in the mechanical properties, such as Young’s modulus and phase transformation temperatures of SMAs under body fluids corrosion were tested firstly in the simulated body environment with the 0.9 wt. % NaCl solution at 37. With an increase of the immersion time, the results show that the Ti (titanium) percentage, austenitic (reverse) transformation start temperature, austenitic (reverse) transformation finish temperature, and maximum residual strain all increase, the Ni (nickel) percentage, martensitic transformation finish temperature, tensile strength, and yield strength decrease, and the martensitic transformation start temperature first decreases and then increases. The research in this work can provide an experimental basis for further study of the SMAs materials in biomedicine applications.

Selective laser melting fabrication of body-temperature shape memory NiTi alloy for 4D-printed orthopedic implants

ABSTRACT SLM fabrication of NiTi alloy is receiving increasing attention. However, the relatively high austenite transformation finish (Af) temperature is the main problem for biomedical applications. In this study, it was found that phase transformation temperature (PTT) increased with increasing laser power, decreasing scanning speed, and decreasing hatch spacing. The combination of PPs with an energy density of less than 75 J/mm³ is expected to yield the Af temperature below 37 °C. P = 80 W, v = 600 mm/s, and h = 70 μm were identified as the optimal combination because of a good tensile strength of 612 MPa and a peak shape recovery rate of 96.94%. Biocompatibility assessment showed that SLM-NiTi porous scaffolds supported pre-osteoblast survival and proliferation. A patient-specific mesh-like supporting prosthesis was designed to show the prospect of 4D-printed orthopedics implant. In clinical application, SLM-NiTi prosthesis could reduce bony window and facilitate easier insertion through compressive deformation.

Interpass temperature strategies for compressive residual stresses in cladding low-transformation-temperature material 16Cr8Ni via wire arc additive manufacturing

Low-transformation-temperature (LTT) materials induce compressive residual stresses within an effective depth in wire arc additive manufacturing (WAAM), which is well suited for cladding applications. However, this role was found to be sensitive to the phase transformation sequence. We developed an LTT material—16Cr8Ni (16Cr8Ni-LTT)—and to explore its potential, it was clad onto a KA36 substrate via WAAM. Three different interpass temperature strategies were designed as follows: (1) no interpass temperature control; (2) interpass temperature controlled at approximately 270 °C above the martensite transformation start temperature (Ms = 200 °C) for simultaneous phase transformation; and (3) interpass temperature controlled at approximately 50 °C below the martensite transformation finish temperature (Mf = 60 °C) for sequential phase transformation. A thermal elastic-plastic numerical model considering the phase transformation-induced plasticity (TRIP) was developed using in-house software, JWRAIN-Hybrid, to reproduce the temperature field and residual stresses of 16Cr8Ni-LTT cladding. X-ray diffraction (XRD) and contour methods were employed for residual stress measurements. The high accuracy of the model was validated in terms of the temperature, deformation, and residual stress. The reproduced maximum historical temperature distributions were combined with hardness tests to identify the depths and temperatures of the heat-affected zone (HAZ). The measured maximum compressive residual stresses induced by the three interpass temperature strategies for cladding 16Cr8Ni-LTT were −503, −420, and −720 MPa. Moreover, irrespective of the adopted interpass temperature strategy, both longitudinal and transverse residual stresses were compressive in the 16Cr8Ni-LTT cladding. This study provides scientific insights into LLT-induced compressive residual stresses and highlights the superiority and flexibility of cladding LTT materials via WAAM for improving the resistance of large metal components to fatigue and corrosion.

Transformation Behaviors and Microstructure Modifications in Hydrogen-Charged Ti–Ni Shape Memory Alloy

AbstractThe effect of hydrogen charging duration on the transformation behavior, microstructural evolution, and dynamic microstructural changes associated with thermoelastic martensitic transformation in Ti–Ni shape memory alloy was investigated. Compared with the uncharged specimen, the martensitic transformation start (Ms) and reverse transformation finish (Af) temperatures increased with charging time, whereas the martensitic transformation finish and reverse transformation start temperatures remained almost unchanged. In situ SEM results were consistent with these behaviors. Upon cooling, the transformation progressed from the center to the surface in charged specimens, indicating a higher transformation temperature in the center than the surface. The latent heat of transformation decreased with increasing charging time, quantitatively attributed to an untransformed region consisting of hydrogen-induced martensite and a hydrogen-affected layer. The hydrostatic effect from those layers on the interior B2 phase was proposed as the origin of the increased Ms and Af temperatures.

Uncovering the isothermal transformation dynamics and elemental diffusion behavior of reversed austenite in PH13-8Mo steel

The isothermal transformation kinetics and microstructural evolution of the reversed austenite significantly influence the properties of PH13-8Mo steel. However, the microscopic phase transformation behavior related to elemental diffusion during isothermal processes for PH13-8Mo steel needs further examination. Herein, we systematically investigate the isothermal transformation and elemental diffusion behavior of reversed austenite in PH13-8Mo steel. Phase transformation kinetic measurements unravel that the complete elimination of residual austenite can be achieved. Only hierarchical martensitic microstructure without austenite is detected after the solution-cooling treatment at 927 °C with a cooling rate of 10 °C/s, determining the critical martensite transformation start and finish temperature to be 176.6 °C, 11.6 °C. The isothermal kinetics of reverted austenite satisfy the Johnson-Mehl-Avrami relationship. The fraction of reversed austenite increases from 0.14 % at 510 °C to 8.73 % at 593 °C. Lath martensite boundaries with large kernel average misorientation values promote elemental diffusion at the interface. Further, the underlying behavior of elemental diffusion is elucidated during the isothermal aging process. The interfacial model indicates the growing width of the reversed austenite up to 100 nm and the compositional gradient formed by the elemental diffusion during aging for 0–5 h at 593 °C. The interdiffusion coefficient calculations interpret the diffusion direction of alloying element Ni from martensite to reversed austenite; Cr and Mo may form alloying carbides at the austenite-martensite interface; Al possesses the fastest diffusivity in martensite and austenite phases. The present work unveils the phase transformation kinetics of PH13-8Mo steel, achieving quantitative regulation for maraging stainless steel.

Effects of Building Directions on Microstructure, Impurity Elements and Mechanical Properties of NiTi Alloys Fabricated by Laser Powder Bed Fusion.

For NiTi alloys prepared by the Laser Powder Bed Fusion (LPBF), changes in the building directions will directly change the preferred orientation and thus directly affect the smart properties, such as superelasticity, as well as change the distribution state of defects and impurity elements to affect the phase transformation behaviour, which in turn affects the smart properties at different temperatures. In this study, the relationship between impurity elements, the building directions, and functional properties; the effects of building directions on the crystallographic anisotropy; phase composition; superelastic properties; microhardness; geometrically necessary dislocation (GND) density; and impurity element content of NiTi SMAs fabricated by LPBF were systematically studied. Three building directions measured from the substrate, namely, 0°, 45° and 90°, were selected, and three sets of cylindrical samples were fabricated with the same process parameters. Along the building direction, a strong <100>//vertical direction (VD) texture was formed for all the samples. Because of the difference in transformation temperature, when tested at 15 °C, the sample with the 45° orientation possessed the highest strain recovery of 3.2%. When tested at the austenite phase transformation finish temperature (Af)+10 °C, the 90° sample had the highest strain recovery of 5.83% and a strain recovery rate of 83.3%. The sample with the 90° orientation presented the highest microhardness, which was attributed to its high dislocation density. Meanwhile, different building directions had an effect on the contents of O, C, and N impurity elements, which affected the transformation temperature by changing the Ni/Ti ratio. This study innovatively studied the impurity element content and GND densities of compressive samples with three building directions, providing theoretical guidance for LPBFed NiTi SMA structural parts.

Formation mechanism of inherent spatial heterogeneity of microstructure and mechanical properties of NiTi SMA prepared by laser directed energy deposition

Ni51Ti49 at.% bulk was additively manufactured by laser-directed energy deposition (DED) to reveal the microstructure evolution, phase distribution, and mechanical properties. It is found that the localized remelting, reheating, and heat accumulation during DED leads to the spatial heterogeneous distribution of columnar crystal and equiaxed crystal, a gradient distribution of Ni4Ti3 precipitates along the building direction, and preferential formation of Ni4Ti3 precipitates in the columnar zone. The austenite transformation finish temperature (A f) varies from −12.65 °C (Z = 33 mm) to 60.35 °C (Z = 10 mm), corresponding to tensile yield strength (σ 0.2) changed from 120 ± 30 MPa to 570 ± 20 MPa, and functional properties changed from shape memory effect to superelasticity at room temperature. The sample in the Z = 20.4 mm height has the best plasticity of 9.6% and the best recoverable strain of 4.2%. This work provided insights and guidelines for the spatial characterization of DEDed NiTi.

Degradation of recovery properties after heat treatment in hot-forged NiTiFe alloy

NiTiFe is a kind of low-temperature shape memory alloy (SMA) with great application potential in aerospace, such as pipe joints. Heat treatment can effectively regulate the microstructure and recovery properties (recovery strain and recovery stress) of SMAs. In this study, the microstructure, recovery properties, and transformation temperatures of hot-forged Ni47Ti50Fe3 shape memory alloy (SMA) before and after heat treatment (HT) with different HT temperatures (550–950 ℃) were analyzed. The results show that when the HT temperature is above 650 ℃, the density of high- and low-angle grain boundaries was reduced. The recovery properties of the hot-forged NiTiFe alloy deteriorate with increasing HT temperature. The initial hot-forged samples showed the best recovery strain of 8.32% and recovery stress of 449 MPa, which were reduced to 5.99% and 299 MPa, respectively, after 950 ℃ HT. The R↔B2 phase transformation temperature changes little with the increasing HT temperature. But the reverse martensitic phase transformation finish temperature increased from − 75 ℃ to − 50 ℃ after tensile deformation at − 180 ℃. The EBSD and HRTEM analyses of the microstructure after the free and constrained recovery tests were carried out. The {114}B2 and {112}B2 twins remained after the free recovery process, and more of those twins remained after the constrained recovery process. Although the recrystallization and grain growth can reduce the defects density which are the obstacles to the form of detwinning martensite, it also makes the deformation twins prone to form during the tensile and constrained recovery process, thus leading to poor recovery properties.

Effects of ambient temperature on cyclic response and functional fatigue of shape memory alloy cables

In this paper, the effects of ambient temperature on the tensile response and superelastic fatigue behavior of the shape memory alloy (SMA) cable are investigated. The tested SMA cable is made of Nickel–Titanium and has an outer diameter of 8 mm with a 7 × 7 configuration. The SMA cables are subjected to an incrementally increasing loading protocol up to 14% strain amplitude under ambient temperatures varying from −20 °C to 60 °C. The effects of deformation amplitude and test temperature on stress-induced martensitic phase transformations are assessed. SMA cable specimens are also subjected to cyclic loading at a constant strain amplitude of 6% up to 1000 loading cycles at different test temperatures. The evolution of superelastic response with loading cycles is revealed by analyzing the maximum stress, residual deformation, hysteretic energy, and equivalent viscous damping ratio extracted from hysteretic curves. The results indicate that the ambient temperature considerably alters the superelastic behavior of SMA cables by shifting the stress-strain loops upward and narrowing them. When the ambient temperature is close to the reverse phase transformation temperature, the superelastic response degrades more progressively with both increasing loading amplitude and number of loading cycles. The least degradation in functional properties of SMAs takes place when the cable is tested at a temperature close to its austenitic transformation finish temperature, while increasing temperature decreases low cycle fatigue life of SMA cables when it is deformed up to the end of phase transformation. Overall, it is important to consider this temperature dependent response of SMA cables in applications where the cables will be subjected to varying outdoor temperatures.

Superelastic behavior and elastocaloric effect in a Ni51.5Fe21.5Ga27.0 ferromagnetic shape memory single crystal under compression

Ni51.5Fe21.5Ga27.0 single crystals have been subjected to different heat treatments resulting in a different degree of L21 ordering. Superelastic response has been measured at different temperatures in compression mode. The mechanical behavior strongly depends on axis orientation. In the [001] direction, perfect superelasticity over a wide range of temperatures is found. For the [110] orientation, the material fails by brittle fracture short above austenite transformation finish temperature, Af. A linear dependence of the critical stress with temperature has been found in agreement with Clausius-Clapeyron equation. The slope does not significantly change with the degree of order, but it is notably affected by the crystal orientation. The microstructure of the samples after mechanical tests has been studied by transmission electron microscopy. The superelastic cycling produces dislocations with a Burgers vector that suggests local microplastic deformation of the martensitic phase. Finally, the adiabatic temperature change has been used to chacterize the elastocaloric effect in this alloy. The adiabatic cooling is found to be larger in the [110] than in the [001] orientation at 240 K. However, the brittleness of [110] samples avoid testing the adiabatic temperature change at room temperature. The adiabatic cooling in [001] orientation decreases systematically with temperature, which is related to decrease of the strain and entropy change of transformation.

Effect of Sn content on microstructure, texture evolution, transformation behavior and superelastic properties of Ti–20Zr–9Nb‒(2–5)Sn (at.%) shape memory alloys

In the present study, the effects of Sn content on microstructure, texture evolution, transformation behavior and superelastic properties of Ti–20Zr–9Nb‒(2–5)Sn (at.%) shape memory alloys were systematically investigated. After severe cold-rolling at room temperature, the sheet was annealed at 900 °C. A dominant α″ martensite and minor β phase were formed in the 2Sn alloy specimen. (Ti,Zr)5(Nb,Sn)3 s phase started to be detected in the 3Sn alloy specimen, and its area fraction increased from 0.02% to 1.75% with increasing Sn content from 3 at.% and 5 at.%. Increasing Sn content prevented recrystallization, affecting the texture evolution. A dominant (112)β[1‾3‾2]β and weak γ-fiber deformation textures were observed in the 5Sn alloy specimen and the 001β<110>β recrystallization texture was developed in the 3Sn alloy specimen. Also, increasing Sn content was found to effectively stabilize the β phase. 1 at.%‒Sn addition decreased the α″ → β reverse transformation start temperature (As) and finish temperature (Af) by 75 °C and 106 °C, respectively. The superelasticity was distinctly observed in the 900 °C annealed 5Sn alloy specimen at room temperature because its Af was below the room temperature.

Atomic scale modeling of the coherent strain field surrounding Ni4Ti3 precipitate and its effects on thermally-induced martensitic transformation in a NiTi alloy

Precipitation of Ni4Ti3 in NiTi alloy profoundly affects material properties, while the coherency strain fields and their effects on the thermally-induced martensitic transformation are not known in detail, especially for the B19’ variants and morphologies. Therefore, molecular dynamics simulations, as well as an Eshelby solution and phase-field microelasticity theory were applied to investigate the strain fields caused by the Ni4Ti3 precipitates with different aspect ratios. The maximum strain (along the central axis of the precipitate) in the matrix and its relative position are formulated as function relationships. Ms (martensitic transformation start temperature) and Af (austenitic transformation finish temperature) during martensitic transformation were determined and analyzed. Two previously published self-accommodation B19’ structures, the triangular and “herring-bone’’ morphologies are investigated in detail. An intermediate state between the triangular and “herring-bone’’ morphologies, termed mixed self-accommodation, is observed for the first time and proved to be the most unstable structure, providing a potential design route for low hysteresis microactuators. The formation processes of these self-accommodation morphologies are analyzed and discussed. Our simulations are the first time to reveal a variety of twinned B19’ morphologies selected by precipitates and their effects on thermally-induced martensitic transformation at the atomic scale.

Lastnosti simulirane grobozrnate mikrostrukture poboljšanega visokotrdnega jekla

Quenched and tempered high-strength steels have a favorable strength-to-weight ratio. These steels are usually welded with the use of standard procedures. Due to the heat input during welding, steels undergo changes in the heat-affected zone and as a result an unfavorable microstructure and mechanical properties can occur. The coarse-grained heat-affected-zone microstructure was studied with the use of simulated thermal cycles. They were applied to the experimental quenched and tempered steel. The steel samples were heated to a peak temperature of 1350 °C and then cooled with different rates. A microstructure examination, dilatometry analysis and hardness measurements of the steel samples were performed. The transformation start and finish temperatures were determined using the dilatation vs. temperature data analysis. The investigation was performed to determine the appropriate cooling time t8/5 interval for further investigation of the experimental steel.

Effect of Mo on the Microstructure and Superelasticity of Ti-Ni-Cu Shape Memory Alloys

In this paper, the microstructure and superelasticity of Ti(50−x)Ni44Cu6Mox (x = 0-2.5) alloys were studied. The main phase in Ti(50−x)Ni44Cu6Mox alloys at room temperature is B2 austenite. The martensite transformation finish temperatures of the 0Mo and 0.2Mo samples are approximately − 34.2 and − 45.4 °C, respectively, while the transformation temperature cannot be detected above − 50 °C for the samples with Mo contents greater than 0.2%. The content of Mo has little effect on the compressive strength of the Ti(50−x)Ni44Cu6Mox alloy, but the fracture strain decreases with increasing Mo content. The 0.6Mo sample shows elastic deformation characteristics and has the lowest fracture strain due to the precipitation of Ni-rich compounds. Cyclic compression tests with an increased and constant prestrain were adopted to study the superelasticity properties of the alloys. The residual stress can increase approximately 5-6% after 5 cycles when the prestrain increases from 2 to 10%, while the residual strain drops approximately 2-3% after 20 cycles when the prestrain is constant at 6%. In both experiments, the results show that the recoverable strain can be generally improved by substituting Mo for Ti in Ti(50−x)Ni44Cu6Mox alloys.

The Effect of Cooling Conditions on Martensite Transformation Temperature and Hardness of 15% Cr Chromium Cast Iron.

The research reported in the paper concerned the conditions of cooling high-chromium cast iron with about 15% Cr content capable to ensure completeness of transformation of supercooled austenite into martensite in order to obtain high hardness value of the material and thus its high resistance to abrasive wear. For testing, castings were prepared with dimensions 120 mm × 100 mm × 15 mm cast in sand molds in which one of cavity surfaces was reproduced with chills. From the castings, specimens for dilatometric tests were taken with dimensions 4 mm × 4 mm × 16 mm and plates with dimensions 50 mm × 50 mm × 15 mm for heat treatment tests. The dilatometric specimens were cut out from areas subject to interaction with the chill. The austenitizing temperature and time were 1000 °C and 30 min, respectively. Dilatograms of specimens quenched in liquid nitrogen were used to determine martensite transformation start and finish temperatures TMs and TMf, whereas from dilatograms of specimens quenched in air and in water, only TMs was red out. To secure completeness of the course of transformation of supercooled austenite into martensite and reveal the transformation finish temperature, it was necessary to continue cooling of specimens in liquid nitrogen. It has been found that TMs depended strongly on the quenching method whereas TMf values were similar for each of the adopted cooling conditions. The examined cooling variants were used to develop a heat treatment process allowing to obtain hardness of 68 HRC.

A feasibility study on warm forming of an as-quenched 22MnB5 boron steel

In this paper, the feasibility of a newly proposed forming method, being the warm forming of as-quenched 22MnB5 boron steels, was studied through a series of proof of concept experiments. To assess the material thermo-mechanical behaviours under the proposed forming conditions, first, the as-received 22MnB5 boron steel was austenized and quenched to below the martensite transformation finish temperature to obtain a martensitic microstructure; second, uniaxial tensile tests of the as-quenched steel were conducted under proposed warm forming conditions on a Gleeble 3800 materials simulator. To evaluate the material post-form properties, first, tempering treatments on the as-quenched steel samples were performed to simulate the heat-treating conditions in the proposed warm forming process; second, the mechanical properties (hardness, strength, and ductility) of as-tempered samples were measured and a microstructure analysis was conducted. From the experimental results, it was found that, under the proposed warm-forming process conditions (420 °C–620 °C), the material showed significant strain softening, which would increase the tendency of necking during stamping and adversely affect its drawability. In addition, it was found that the heating of martensite in a 22MnB5 boron steel to higher temperatures (>400 °C) adversely affected its post-form strength and ductility due to the tempering effect. Therefore, according to the results obtained in this study, the warm forming of as-quenched 22MnB5 boron steel may reduce the strength of formed parts by more than 50% in comparison to the possible strength the material could achieve under the investigated process.

Influence of Ni content on impact-abrasion wear resistance of medium-carbon low alloy cast steel

Medium-carbon low alloy wear-resistant cast steel with different Ni contents (0.08 mass%, 0.77 mass%, 1.74 mass%, 2.43 mass%) were heat treated at 1050 °C, followed by water quenching and tempering at 250 °C. The influence of Ni content on the microstructure, hardness, impact toughness and abrasive resistance of the cast steel were investigated. The results showed that the Ni contents increasing hindered the transformation of martensite phase by reducing martensite transformation start temperature (Ms) and martensite transformation finish temperature (Mf). The fine martensite phase after quenching improved the hardness, impact toughness and wear-resistance of the cast steel. The cast steel with low Ni content (0.08% or 0.77%) consists of lath tempered martensite, while the microstructure turns to mixed lath martensite and lamellar twin martensite with higher Ni content. The cast steel with 1.74% Ni showed a hardness of HRC49.5, an impact absorbing energy with V-notch impact absorbing energy of 16.7 J, and the best wear resistance among the samples.

Characterizing Transformation Phenomena and Elastic Moduli of Austenite and Oriented Martensite of Superelastic Thin NiTi Wire through Isothermal Dynamic Mechanical Analysis

In this paper, superelastic behavior of Nickel Titanium thin wires is characterized using the method of dynamic mechanical analysis. Nominal dynamic storage modulus ′ is measured as function of nominal strain and stress during isothermal superelastic tensile tests at three testing temperatures above the reverse martensitic transformation finish temperature. The method brings new information on deformation mechanisms compared to the consideration of only tensile stress-strain curves. It is shown that determination of the elastic moduli E, especially at high strain, requires to calculate the true storage modulus ′. Using ′ and not ′, elastic modulus of oriented martensite is determined equal to 73 GPa of the same order than the elastic modulus of austenite equal to 70 GPa. Two models are then proposed to simulate experimental storage moduli evolution during the tests. A first model explains the ′ evolution during stress plateau by the localization phenomenon ; it leads to express ′ as function of the nominal strain. A second model describes the evolution of ′ after the stress plateau as function of true stress and test temperature. This model permits to determine the Clausius-Clapeyron coefficient of the forward transformation.

The heating–cooling rate effect on thermal properties of high nickel-rich NiTi shape memory alloy

In this study, the effects of heating and cooling rate on the thermal properties of high nickel-rich Ni55Ti45 shape memory alloy were investigated. Transformation temperatures and enthalpies were determined by differential scanning calorimetry. R-phase was observed with two-stage phase transformation during cooling. Transformation finish temperatures were affected, while the transformation start temperatures were not changed by heating–cooling rate. Kissinger, Takhor, and Ozawa methods were applied to find activation energy that is needed for phase transformation. The average activation energy value of 200.5 kJ mol−1 was calculated for Ni55Ti45 alloy. The calculated activation energy was found lower than the ones reported in the literature.

Microstructure responses and deformation mechanisms of solutionized Ti-51.5 at.%Ni alloy during reciprocating sliding

Abstract To explore the microstructure response to repeated sliding of materials showing stress-induced martensitic (SIM) transformation, we have investigated the microstructure evolution of solutionized Ti-51.5 at.% Ni introduced during the dry sliding against the GCr15 sphere at room temperature. It was found that B19′ was the dominant phase in the worn subsurface of solutionized case exhibiting extremely low martensitic transformation start ( M s ) temperature (lower than −120 °C) and reverse transformation finish temperature ( A f = − 3.4 °C), albeit amorphous structure observed in the topmost layer. Remarkably, nanotwins and faulting were experimentally revealed to contribute to the special microstructure. Moreover, particles of R phase were also discovered. The elementary mechanisms illustrated will be essential for the future modelling of the tribologically deformed layer of the alloys.