Cold-rolled Alloy Research Articles

Formation of Structural-Phase State and Elastic and Durometric Properties of Biocompatible Cold-Rolled Titanium Ti–Nb–Zr-Based Alloys during Aging

The methods of scanning electron microscopy, X-ray diffraction analysis, and microindentation are used to study the effect of alloying with zirconium (within 3 to 6 at %) and complex Zr + Sn and Zr + Sn + Ta additions on the evolution of the structure, phase composition, and properties (effective modulus of elasticity, hardness, and wear-resistance parameters) of quenched biocompatible β-titanium (at %) Ti–6% Nb–% Zr, Ti–6% Nb–% Zr, Ti–6% Nb–% Zr, Ti–6% Nb–% Zr–% Sn, and Ti–6% Nb–% Zr–% Sn–0.7Ta alloys during aging (at 400°C for 4, 16, and 64 h) after multipass cold rolling with a total degree of strain of 85%. As compared to the quenching, the cold rolling of the studied Ti–b–r alloys is shown to suppress the occurrence of the β → ω transformation in the course of aging and to favor the acceleration of the decomposition of β solid solution with the formation of nonequilibrium αl phase in the course of aging. The increase in the zirconium content from 3 to 6 at % in the cold-rolled ternary Ti–6% Nb –х% Zr alloys and introduction of complex Zr + Sn and Zr + Sn + Ta additions to the Ti–6% Nb alloy instead of only zirconium addition hinder the decomposition processes of the β phase during aging; this impacts the intensity of variations of the effective modulus of elasticity and microhardness. The aging of the cold-rolled alloys under study was found to allows us to obtain the higher values of the parameters H/Er and (Н is the hardness and Er is the resolved modulus of elasticity) associated with the wear resistance as compared to those for the widely used medical Ti–Al–V alloy. The compositions of the alloys and conditions of their treatment are determined, which allow us to obtain the combination of the highest-level properties.

Effect of thermo-mechanical treatment on cryogenic mechanical properties of a medium-entropy Fe65Co12.5Ni12.5Cr9.5C0.5 alloy produced by the laser-based powder bed fusion

Microstructure and mechanical properties of the Fe65Co12.5Ni12.5Cr9.5C0.5 (in at.%) medium-entropy alloy obtained by the laser-based powder bed fusion (PBF-LB) was investigated in the as-produced condition and after (i) annealing in the interval 800–1100 °C or (ii) cold rolling followed by annealing at 800 °C. The as-produced microstructure was found to be surprisingly stable; noticeable recrystallization occurred only after annealing at 1100 °C. Cold rolling resulted in the formation of typical deformed microstructure, while further annealing led to the formation of a fine-grained recrystallized microstructure. A considerable improvement in cryogenic mechanical properties was obtained after some thermo-mechanical treatments due to the transformation-induced plasticity (TRIP) effect. The cold-rolled alloy had the best cryogenic strength characteristics; yield strength and ultimate tensile strength were found to be 1740 MPa and 2235 MPa, respectively. High strength was combined with a pronounced value of elongation to fracture of 24 %. Detailed analysis of microstructure and corresponding mechanical behavior changes caused by various processing of the as-produced alloy are presented.

Ultrafast recrystallization and grain refinement of 2A97 Al-Cu-Li alloy by electropulsing assisted annealing at lower temperature

In the current work, a pulsed current was applied on recrystallization annealing to promote the recrystallization of cold-rolled 2A97 Al-Cu-Li alloys. Besides, the effect of stored deformation energy on electropulsing assisted recrystallization was also investigated. The results indicated that, with the assistance of electropulsing, the complete recrystallization temperature of the 55 % cold-rolled alloys was decreased from 480 °C to 440 °C, and the holding time was significantly decreased from 30 min to 15 s. Moreover, the recrystallization promotion effect of electropulsing was enhanced with the increment of stored deformation energy. As the rolled reduction increased from 23 % to 55 %, the drop in complete recrystallization temperature induced by electropulsing rose from 20 °C to 40 °C. Additionally, the average grain size decreased from 46.8 μm to 22.6 μm, leading to an enhancement in the mechanical properties of the 2A97 alloy. The microstructure observation showed that the electropulsing assisted recrystallization was a process of nucleation and nuclei growth dominated by dislocation rearrangement, and the ultrafast recrystallization at lower temperature induced by electropulsing may be attributed to the reduction of the nucleation energy barrier and the promotion of dislocation rearrangement caused by the ‘local hot spot effect’ and ‘electromigration effect’.

Comparative analysis of the effect of magnesium and zinc on the structure and mechanical properties of hot-rolled and cold-rolled alloy Al–2%Cu–1.5%Mn sheets

Comparative analysis of the effect of magnesium and zinc on the structure and mechanical properties of hot-rolled and cold-rolled alloy Al–2%Cu–1.5%Mn sheets

Superelastic Properties of Aged FeNiCoAlTaB Cold-Rolled Shape Memory Alloys

In the present study, microstructure and cyclic tensile tests were used to measure the superelastic responses of Fe40.95Ni28Co17Al11.5Ta2.5B0.05 (at.%) shape memory alloys after 97% cold rolling. Cold-rolled samples underwent annealing heat treatment (1250 °C/1 h) followed by quenching in water or aging heat treatment (700 °C/6 h and 700 °C/12 h) followed by quenching in water. The microstructure results showed that the average grain size increased from 210 μm to 1570 μm as annealing times increased from 0.5 h to 1 h. X-ray diffraction (XRD) spectra for FeNiCoAlTaB (NCATB) showed that in cold-rolled alloys after solution, the strong peak was in the face-centered cubic (γ, FCC) <111> structure. In aged samples, a new peak (γ’, FCC) emerged, the intensity of which increased as aging times rose from 6 to 12 h. Transmission electron microscope (TEM) images showed that the average precipitate size was around 10 nm in 700 °C/6 h specimens and 18 nm in 700 °C/12 h specimens. The precipitate was enriched in Ni, Al, and Ta elements and exhibited an L12 crystal structure. Tensile samples aged at 700 °C for 6 and 12 h exhibited recoverable strains of 1% and 2.6%, respectively, at room temperature. Digital image correlation (DIC) results for the sample aged at 700 °C for 12 h showed that two martensite variants were activated during the superelastic test. Such variants can form corresponding variant pairs (CVPs), which promote tensile deformation. The tensile sample exhibited a gradual cyclic degradation, and a large irrecoverable strain was observed after the test. This irrecoverable strain was the result of residual martensite, which was pinned by dislocations.

Understanding the strength increment of Al–Zn–Mg–Cu aluminum alloys by enhanced solution-treatment (EST)

Enhanced solution-treatment (EST) tends to increase the strength of aluminum alloys by increasing the solute concentrations due to the sufficient dissolution of soluble constituents. However, results of spray-formed 7055 Al–Zn–Mg–Cu alloy indicate that the strength increment occurs only in hot-extruded alloy not in cold-rolled one and is related to increasing the average Taylor factor (MA) instead of the solute concentrations. The increased MA of hot-extruded alloy is attributed to the transformation of some S texture grains into the Copper texture ones with increasing the solution-treating temperature, whereas recrystallization and grain growth of cold-rolled alloy exerts little effect on the MA.

A 3D Non-Linear FE Model and Optimization of Cavity Die Sheet Hydroforming Process

Cryo-rolled aluminum alloys have a much higher strength-to-weight ratio than cold-rolled alloys, which makes them invaluable in the aerospace and automotive industries. However, this strength gain is frequently accompanied by a formability loss. When uniformly applied to the blank surface, hydroforming provides a solution by generating geometries with constant thickness, making it possible to produce complex structures with “near-net dimensions”, which are difficult to achieve with conventional approaches. This study delves into the cavity die sheet hydroforming (CDSHF) process for high-strength cryo-rolled AA5083 aluminum alloy, focusing on two primary research questions. Firstly, we explored the utilization of a nonlinear 3D finite-element (FE) model to understand its impact on the dimensional accuracy of hydroformed components within the CDSHF process. Specifically, we investigated how decreasing fluid pressure and increasing the holding time of peak fluid pressure can be quantitatively assessed. Secondly, we delved into the optimization of process parameters—fluid pressure (FP), blank holding force (BHF), coefficient of friction (CoF), and flange radius (FR)—to achieve dimensional accuracy in hydroformed square cups through the CDSHF process. Our findings reveal that our efforts, such as reducing peak fluid pressure to 22 MPa, implementing a 30 s holding period, and utilizing an unloading path, enhanced component quality. We demonstrated this with a 35 mm deep square cup exhibiting a 16.1 mm corner radius and reduced material thinning to 5.5%. Leveraging a sophisticated nonlinear 3D FE model coupled with response surface methodology (RSM) and multi-objective optimization techniques, we systematically identified the optimal process configurations, accounting for parameter interactions. Our results underscore the quantitative efficacy of these optimization strategies, as the optimized RSM model closely aligns with finite-element (FE) simulation results, predicting a thinning percentage of 5.27 and a corner radius of 18.64 mm. Overall, our study provides valuable insights into enhancing dimensional accuracy and process optimization in CDSHF, with far-reaching implications for advancing metal-forming technologies.

A study on the quantitative recrystallization of cold deformed copper affected by tin-lead solder

It has been observed that the fractional recrystallization characteristics of commercially pure copper is affected by the presence of individual or both the constituent elements of the Sn-Pb solder alloy. In order to design the experiment, commercially pure Cu, binary copper alloys (Cu-Sn and Cu-Pb) and ternary copper alloy, Cu-Sn-Pb are investigated. Cast alloys are homogenized, solution treated, and then quenched to complete the thermal treatment. In order to recrystallize, alloys are cold rolled to a 75% thickness and then annealed at 700°K isothermally for varying durations, up to 3600 seconds. In this experiment, the fractional recrystallization of annealed samples is evaluated as the normalized difference in microhardness recorded at various time steps. Additionally, in an attempt to verify the experimental results, the well-known Johnson-Mehl-Avrami-Kolmogorov equation is also used to predict the associated recrystallization behavior. From the study, it can be inferred that the presence of Sn-Pb solder-alloy elements have a positive impact on the recrystallization behavior of pure copper due to the solid solution strengthening, in which the effect of tin is greater than that of lead. Quantitative analysis indicates that recrystallization of pure Cu, Cu-Sn, Cu-Pb, and Cu-Sn-Pb alloys attains 99.4%, 95.4%, 98.4%, and 89.5%, respectively. Sn forms intermetallic with Cu but Pb does not. Additionally, Sn forms different intermetallic with impurities and has a BCC crystal structure dissimilar to the FCC of Cu and Pb. As a result, the formation of GP zones and the intermetallic phases during annealing show greater differences in the recrystallization behavior between the two approaches. By combination, microstructural studies of the cold-rolled alloys reveal the elongated grains of the second phases, and the alloys almost completely re-crystallized after 1800 seconds of annealing at 700°K.

Microstructural origin of the superior strength–ductility synergy of γ′-strengthened high-entropy alloy with heterogeneous grain structure and discontinuous precipitation configuration

The pursuit of a superior combination of strength and ductility in metallic materials for structural applications has long been a challenge. Recently, L12 (γ′) precipitation-strengthened high-entropy alloys (HEAs) that undergo direct aging from cold-rolled states have exhibited exceptional strength-ductility synergy. However, the origins of this synergetic effect in direct aging are still a subject of controversy. Herein, we aim to unravel the fundamental factors contributing to the improved mechanical properties of directly aged HEAs. Two distinct aging processes, namely cold-rolling and direct aging (CA), and recrystallization and aging (RA), are employed on the Cr18Fe18Co28Ni28Al4Ti4 alloy. The RA process induces continuous precipitation (CP) behavior, while the discontinuous precipitation (DP) is confined near the grain boundaries. On the contrary, the CA process leads to DP-dominant reaction accompanied by simultaneous recrystallization. Additionally, the inherent heterogeneity of the cold-rolled alloy results in a heterogeneous structure in the CA alloys. The CA alloy aged at 600 °C exhibits significantly increased strength and ductility compared to the cold-rolled state, with a remarkable yield strength of 1649 MPa, tensile strength of 1741 MPa, and total elongation of 19.5%. This ultra-high strength is attributed to the precipitation strengthening and heterogeneous deformation-induced (HDI) strengthening resulting from the heterogeneous grain structure. Furthermore, the fine grain and ultra-fine grain regions promote additional HDI strengthening during plastic deformation, leading to sustained strain hardening. The findings of this study establish a correlation between microstructural features and mechanical behavior in directly aged γ′ precipitation-strengthened HEAs, providing valuable insights for the design of such alloys.

Hydrogen permeability of 48Cu52Pd cold-rolled alloy foil and different methods of its surface pretreatment

The process of atomic hydrogen penetration into the metal phase is complicated by the phase-boundary transition from the liquid and/or gas phase. That is why the cleanliness of metal and alloy surfaces is of particular importance. The purpose of this work was to determine the effect of surface pretreatment using photon pulses, ultrasound, and potential cycling on the parameters of hydrogen permeability for 48Cu52Pd metal cold-rolled membranes. The study was focused on a foil of copper-palladium homogeneous alloy with 48 at. % Cu and 52 at. % Pd composition. The studied samples were obtained by cold rolling and their thickness were 10 and 16 μm. Surface pretreatment included rinsing in acetone, using ultrasound, pulsed photon treatment, and quadruple potential cycling over a wide range of potentials. Electrochemical studies included cyclic voltammetry and cathode-anodic chronoamperometry in a deaerated 0.1 M H2SO4 solution. Hydrogen permeability was calculated using mathematical models for samples of finite and semi-infinite thickness. It was found that the surface treatment of a 48Cu52Pd foil with photon pulses leads to both an increase in the ionisation rate of atomic hydrogen and an increase in the roughness of the foil surface. The diffusion coefficient of atomic hydrogen does not depend on the method of surface pretreatment with ultrasound and photon pulses. The extraction rate constant for the extraction of the atomic hydrogen after photon treatment increases, which facilitates the processes of both H introduction and ionisation due to the release of active centres of the surface. Electrochemical cleaning of the surface during the quadruple potential cycling contributes to the growth of the extraction rate constant for the extraction of atomic hydrogen

Improvement of mechanical properties and thermal stability of Al–Cu–Mg–Ag alloy by warm rolling

In this paper, the microstructure, mechanical properties and thermal stability of Al–Cu–Mg–Ag alloys processed by warm rolling were compared with cold rolling with same strains using scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the strengthening mechanism has been quantitatively investigated. The mechanical properties of the Al–Cu–Mg–Ag alloy are significantly improved by warm rolling at 200 °C and subsequent artificial aging at 180 °C, both before and after heat exposure at 200 °C for 100 h. Microstructure investigations revealed that compared to cold-rolled samples, warm-rolled samples have a higher number density of Ω plates after artificial aging, and the increase in yield strength caused by precipitation strengthening is greater. After heat exposure at 200 °C for 100 h, although the Ω plates in the warm-rolled samples are coarser compared to the cold-rolled alloys, the number densities are higher. Therefore, warm-rolled samples exhibit better mechanical properties than cold-rolled samples.

An investigation on annealing process and strengthening mechanism of cold rolled Mg-10Li-3Al-2.8Zn alloy

In this study, we investigated the annealing process of a cold-rolled dual-phase Mg-10Li-3Al-2.8Zn alloy and used the crystal defect strengthening model to explain the strengthening mechanism of the annealed Mg-Li alloy. To maximize the strength of the alloy, it is annealed at 553 K for 1 h and then water quenched. The maximum yield strength is 321 MPa and the ultimate tensile strength is 346 MPa. Compared with the cold-rolled alloy, the yield strength is increased by 145 MPa and the ultimate tensile strength is increased by 139 MPa. The high strength of annealed alloy is the result of solution strengthening, fine grain strengthening, second phase strengthening and dislocation strengthening, in which solution strengthening contributes the most. The size, content and distribution of the second phase AlLi and MgLi2Al have significant effects on the strength of the alloy. Higher annealing temperature and rapid cooling are all conducive to the dissolution of AlLi in the matrix, lower temperature and rapid cooling are conducive to the solid solution of MgLi2Al, and higher on the contrary, MgLi2Al will be precipitated quickly at a high temperature. Hard AlLi and MgLi2Al both cause the strengthening of the second phase, but their strengthening effect are far less than that of the second phase solid solution in β-Li.

Shape Memory Properties and Microstructure of FeNiCoAlTaB Shape Memory Alloys

The three-point-bending shape memory properties, microstructure, and magnetic properties of Fe40.95Ni28Co17Al11.5Ta2.5B0.05 (at.%) alloys were investigated. The magnetic results showed a martensitic transformation in the samples that were aged at 700 °C for 6 and 12 h under the applied magnetic fields of 0.05 and 7 Tesla. The martensitic start temperature increased from −113 °C to −97 °C as aging times increased from 6 to 12 h. Increasing the magnetic fields from 0.05 to 7 Tesla, the transformation temperatures increased to a higher temperature. Both samples reach saturation magnetization (140 emu/g) under 7 Tesla. The 98.5% cold-rolled alloys that were annealed at 1250 °C for 0.5 h presented a strong <100> texture in the rolling direction with an average grain size of 360 μm. Increasing the annealing time to 1 h, the intensity of texture reduced from 31.61 to 23.19. The fraction of low angle grain boundaries (LABs) for the 98.5% CR samples after annealing at 1250 °C for 0.5 h and 1 h was about 24.6% and 16.1%, respectively. Three-point-bending results show that the sample aged at 700 °C for 6 h displayed 0.2% recoverable strain at a stress level of 800 MPa. Failure occurred before the 900 MPa cycle could be completed. The sample aged at 700 °C for 12 h shows no transformation before the applied stress level of 300 MPa. As the stress levels increase to 400 MPa, the sample shows the shape memory effect and displayed 0.8% recoverable strain at a stress level of 400 MPa. The samples are failures during the 500 MPa cycle. The observed recoverable strain values were lower than those that were theoretically predicted, which was possibly due to the larger volume fraction of high-angle grain boundary and the slightly lower than expected average grain size.

Microstructure, residual stress, and mechanical properties evolution of a Cu–Fe–P alloy under different conditions

The evolution of the mechanical characteristics, residual stresses, and microstructure of a Cu–Fe–P alloy during machining was investigated, and the influence mechanism of the microstructure on the residual stress and strength of the Cu–Fe–P alloy was confirmed. The findings suggest that the tensile strength, transverse direction (TD), and rolling direction (RD) residual stresses in the cold-rolled state are 516 MPa, −80 MPa and −36 MPa, respectively, which are 76.1%, 93.8%, and 72.2% higher than those in the hot-rolled state, respectively. In addition, after annealing at high temperatures, the tensile strength, TD, and RD residual stresses of the Cu–Fe–P alloy decreased to 344 MPa, −9 MPa and −24 MPa, respectively, which were 33.3%, 88.8%, and 33.3% smaller than those of the cold-rolled alloy, respectively. The strength and residual stress of the Cu–Fe–P alloy are intriguing. The primary influencing aspects of the strength and residual stress are dislocations, grain boundaries, and textures. The residual stress of the Cu–Fe–P alloy may be controlled while maintaining strength by appropriately increasing the proportion of recrystallized grains and decreasing the dislocation density and volume fraction of the Brass texture, respectively. These outcomes serve as a guide for improving the service performance of Cu–Fe–P alloys.

Microstructures and Mechanical Properties of Annealed Ti50Ni47Fe3 Shape Memory Alloy

The effect of annealing temperature on the microstructures and mechanical properties of Ti50Ni47Fe3 (at. %) shape memory alloy was investigated by using a cold-rolled alloy sheet. For this purpose, a scanning electron microscope, electron backscatter diffraction, a transmission electron microscope, X-ray diffraction, tensile tests and Vickers hardness tests were used. The evolution of the microstructures, mechanical properties and fracture morphology of Ti50Ni47Fe3 alloy was studied. The results show that the recovery occurs at an annealing temperature of 500 °C, and the recrystallization occurs at 600 °C. Because of the recrystallization at 600 °C, the <110>//RD texture disappears, and the intensity of the <111>//RD texture decreases; the alloy reaches its maximum elongation while maintaining a high strength, and at this annealing temperature, the alloy has excellent comprehensive mechanical properties. After the temperature exceeds 600 °C, the mechanical properties of the alloy decrease sharply. With the increase of the annealing temperature, the quantity and distribution of elliptical Ti2Ni-phase particles show almost no specific changes. Additionally, with the increase of annealing temperatures to 600 °C, the fracture surface of Ti50Ni47Fe3 alloys becomes flatter.

Microstructure, properties and deformation mechanism of HCCM horizontal continuous casting Cu–Ni–Co–Si alloy strip during cold rolling

Cu-1.3 wt%Ni-1.0 wt%Co-0.9 wt%Si-0.1 wt%Mg alloy strip with 10 mm in thickness produced by heating-cooling combined mold (HCCM) horizontal continuous casting was cold rolled. Microstructure evolution and mechanical properties of the cold-rolled alloy strips with different reduction were studied, and the deformation mechanism was clarified. The results showed that the continuous casting strips had axial columnar grain with main <001> orientations and straight small angle grain boundaries, the elongation after fracture was up to 25.0%, which can be directly processed by large deformation cold rolling, and the maximum cumulative cold deformation was up to 99%.When the reduction was 40% and 60%, the dislocation bands crossed each other and shear bands formed, and the alternating distribution of soft/hard oriented grains were conducive to improve the deformation coordination ability of micro-region under high strain. When the deformation exceeded 80%, deformation twins formed and the twins had strong interaction with the shear band and grain boundaries, resulting in strong shear and rotation of the local grains. The tensile strength and hardness increased from 464 MPa to 137HV of the as-cast strip to 629 MPa and 208HV with reduction 90%, respectively, while the elongation decreased from 25.0% to 4.1%. These results can lay a foundation for the development of compact method for producing high performance Cu–Ni–Co–Si alloy strips.

Hot and cold rolling of a novel near-α titanium alloy: Mechanical properties and underlying deformation mechanism

In this work, mechanical properties, microstructure evolution mechanism, texture variation process and deformation mechanism of a near-α CT20 titanium alloy with different hot and cold rolling reductions are investigated and uncovered comprehensively. The tensile strength of the alloy gradually increases, and the ductility decreases as the rolling reduction grows along both rolling direction (RD) and transverse direction (TD) due to the increase in grain boundary and dislocation density, while the mechanical properties along RD are better than those along TD. The high strain carried in α phases reduces the proportion of high-angle grain boundaries and increases the content of low-angle grain boundaries, with growing dislocation density in α phase. During cold rolling process, basal <a> slip shifts {0001} pole toward ND, and prismatic <a> slip inclines {11 2‾0} pole and {10 1‾ 0} pole toward TD. The intragranular orientation is enhanced with increasing rolling reduction, and the texture intensity of α and β phase is weakened as rolling reduction gradually grows. It is also noteworthy that prismatic <a> slip dominates the slip mode along RD, while basal <a> slip and pyramidal slip are activated along TD during cold rolling. The work-hardening rates of cold-rolled alloys are higher than that of the hot-rolled alloy as a large number of dislocations and grain boundaries were introduced by severe plastic deformation. The different work-hardening rates and mechanical properties of the rolled alloy along RD and TD are verified to be associated with the preferred orientation of α phase and the slip distances of dislocation during deformation.

Precipitation behaviors and mechanical properties of a cold-rolled equiatomic FeCoCrNiMn high entropy alloy after long-time annealing treatment at 500 °C

The thermodynamic stability of extensively investigated equiatomic FeCoCrNiMn high entropy alloy has been doubted in recent investigations. In this study, we conducted a long-time annealing treatment on a cold-rolled FeCoCrNiMn alloy at 500 ℃ with the time from 240 to 960 h. The results show that three phases precipitated after a long-time annealing treatment at 500 ℃. The precipitation sequence of these phases starts with two phases (a Cr-rich phase and a NiMn-rich phase) at annealing time ranging from 240 to 720 h, followed by a third FeCo-rich phase at a longer annealing period of 960 h. Besides, the long-time annealing treatment at 500 ℃ results in partial recrystallization of the cold-rolled alloy. The formation of the precipitation affects significantly the mechanical properties of the alloy, including hardness, yield strength and also the ductility.

Effect of cold rolling and solution treatment on β stability and mechanical properties of a metastable β-Ti alloy

This work investigated the influence of cold rolling reduction and solution treatment on β stability and mechanical properties of a metastable β-Ti alloy (Ti–5Mo–2Cr–Fe–3Zr). The microstructure, phase composition and mechanical properties were analyzed by SEM, XRD, EBSD, TEM and tensile test. Deformation mechanism of cold-rolled alloy includes the reversion of athermal α" to β phase, deformation-induced {332}<113>β twinning and ω phase, dislocation slip, and microbands. The phase compositions of samples with different cold rolling reductions after solution treatment are different. Compared with 12.8% cold rolling and solution treatment (CR12.8%ST), the 27.7% and 51.3% cold rolling and solution treatment (CR27.7%ST and CR51.3%ST) have less α" phase and more β and ω phases, indicating that the β stability of CR27.7%ST and CR51.3%ST is higher than that of CR12.8%ST. Furthermore, the tensile strength and elongation of the samples increase simultaneously with the increase of cold rolling reduction. The CR51.3%ST alloy has good mechanical properties including a high ultimate tensile strength of 948 MPa, a uniform elongation of 15.7%, and an ultra-high strain hardening rate of 2000 MPa.

Synchrotron-based multiscale study on phase transformation in a cold-rolled NiTi shape memory alloy: Effects of preexisting defects

Deformation and phase transformation of a cold-rolled, textured, shape memory alloy sheet Ni50.6Ti49.4 are investigated via real-time, in situ synchrotron X-ray diffraction and digital imaging correlation. The specimens are subjected to tensile loading along two orthogonal directions. Bulk stress–strain curves (macroscale), strain fields (mesoscale) and X-ray diffraction patterns (microscale) are obtained simultaneously. Martensite nucleation is highly correlated with the preexisting defects. Different from the widely observed Lüders bands within defect-free NiTi, uniform martensite phase transformation occurs in the cold-rolled alloy as a result of the relatively widely distributed preexisting defects, giving rise to progressive growth of martensite, relatively uniform strain fields, the sigmoidal stress–strain curves and increased strain hardening rate. Different favorably oriented martensite variants are induced due to the initial texture, and the martensite phase transformation is more active for loading along the less favorable orientation, giving rise to the anisotropy in the phase transformation.