Decrease In Transformation Temperature Research Articles

Effect of Zr addition on the microstructure and mechanical properties of hot-rolled TiZrAlSn alloys

In this study, the microstructure and mechanical properties of hot-rolled χZr-Ti-5Al-2.5Sn (χ = 0, 10, 20, 30 and 35, wt%) alloys were systematically investigated. With the increase of Zr addition, phase constitutions of TiZrAlSn alloys change from single α phase to α + α´, α´ and α´ + αʺ + β phases in sequence while the β transformation temperature decreases. A lower content of α”+β phases appears in the 35Zr-Ti-5Al-2.5Sn alloy. The yield strength of TiZrAlSn alloys increased monotonically from 612 ± 15 MPa for basal Ti-5Al-2.5Sn alloy to an ultra-high strength of 1386 ± 11 MPa for 35Zr-Ti-5Al-2.5Sn alloy, which is ∼126% higher than that of basal alloy. The high content of Zr solute atoms, high density of dislocations and interfaces/boundaries in α´ martensite phase render the ultra-high yield strength and high strain hardening rate in 35Zr-Ti-5Al-2.5Sn alloy. With respect to ductility, the uniform and total elongations increase initially from Ti-5Al-2.5Sn alloy to 10Zr-Ti-5Al-2.5Sn alloy and decrease afterward until 35Zr-Ti-5Al-2.5Sn alloy. The bimodal microstructure of primary α grains and secondary α´ martensite (βtrans region) rendered 20Zr-Ti-5Al-2.5Sn alloy the best comprehensive mechanical properties which included the tensile strength of 992 ± 11 MPa, total elongation of 17.2 ± 1% and a high strain hardening rate. This high strain hardening ability of 20Zr-Ti-5Al-2.5Sn alloy comes from the compatible deformation of soft primary α and hard secondary α´ martensite phases with high density of dislocations and boundaries. In addition, the 20Zr-Ti-5Al-2.5Sn alloy shows a high elongation during tensile necking. The corresponding ductile fracture morphologies of TiZrAlSn alloys are also analyzed. These high-strength TiZrAlSn alloys could be helpful for developing novel TiZr alloys with a high strength and ductility.

Effect of Heat Treatment Time and Temperature on the Microstructure and Shape Memory Properties of Nitinol Wires.

In this study, the effect of heat treatment parameters on the optimized performance of Ni-rich nickel-titanium wires (NiTi/Nitinol) were investigated that were intended for application as actuators across various industries. In this instance, the maximum recovery strain and actuation angle achievable by a nitinol wire were employed as indicators of optimal performance. Nitinol wires were heat treated at different temperatures, 400-500 °C, and times, 30-120 min, to study the effects of these heat treatment parameters on the actuation performance and properties of the nitinol wires. Assessment covered changes in density, hardness, phase transition temperatures, microstructure, and alloy composition resulting from these heat treatments. DSC analysis revealed a decrease in the austenite transformation temperature, which transitioned from 42.8 °C to 24.39 °C with an increase in heat treatment temperature from 400 °C to 500 °C and was attributed to the formation of Ni4Ti3 precipitates. Increasing the heat treatment time led to an increase in the austenite transformation temperature. A negative correlation between the hardness of the heat-treated samples and the heat treatment temperature was found. This trend can be attributed to the formation and growth of Ni4Ti3 precipitates, which in turn affect the matrix properties. A novel approach involving image analysis was utilized as a simple yet robust analysis method for measurement of recovery strain for the wires as they underwent actuation. It was found that increasing heat treatment temperature from 400 °C to 500 °C above 30 min raised recovery strain from 0.001 to 0.01, thereby maximizing the shape memory effect.

Substituting Ce with La and Nd in Al11Ce3: Effect on microstructure and mechanical properties

Microstructure and mechanical properties are studied for five cast, coarse-grained intermetallic alloys: binary Al11Ce3 and Al11La3, ternary Al11(Ce0.75La0.25)3 and Al11(Ce0.5La0.5)3, and quaternary Al11(Ce0.54La0.27Nd0.19)3 with mischmetal composition. All compounds exhibit a single phase indicating a solid solution among the three rare-earth elements (RE=Ce, La, and Nd) on the Ce sublattice of Al11RE3, in disagreement with Thermo-Calc prediction of segregation of Ce and (La, Nd) into two insoluble compounds. The lattice parameters of this orthorhombic Immm-structured α-Al11RE3 phase increase as La concentration increases, while the α-β phase transformation temperature decreases. The five Al11RE3 compounds show very similar (i) propensity for twinning, (ii) high hardness (4.1–4.3 GPa), and (iii) low indentation fracture toughness (0.48–0.65 MPa m1/2). The Al-Al11RE3 interface lattice mismatches, as calculated for two stable interface orientations, have comparable values across all examined compounds. These similarities imply that these Al11RE3 compounds will exhibit comparable strengthening and coarsening resistance in Al-RE-based eutectic alloys, thus positioning them as economically- and environmentally-favorable alternatives to the well-developed binary Al11Ce3 compound formed in current eutectic Al-Ce alloys.

Theoretical analysis and design of Ti-based shape memory alloys correlating composition and electronic properties to transformation temperatures for high temperature applications

The influence of electronic parameters, such as the atomic number of the alloy (Z), valence electron concentration (cv), number of valence electrons per atom (ev/a ratio), and pseudopotential radius (P), on the phase transformation temperatures (Ms, As) of binary, ternary and quaternary shape memory alloys (SMAs) based on Ti is investigated. These temperatures (Ms, As) are correlated with the composition and ev/a of the alloys using multiple linear regression. The dependence of thermal hysteresis on the atomic radius of the alloys is also studied. The alloys investigated comprise of both low (ev/a < 5) and medium (5 < ev/a < 7.5) valence electrons groups. The phase transition temperatures are raised with an increase in ev/a ratio, Z and P for medium ev/a group alloys, while for low ev/a group alloys, the trend is quite the opposite. As cv increases, the transformation temperatures (TTs) decrease for the medium ev/a group, while they increase with cv for the low ev/a group. The two different trends observed for low and medium ev/a groups are explained in the light of elastic modulus of the alloys. The effect of Z, cv, electron orbital occupancy and alloying elements on the elastic modulus, and in turn on the temperatures of transition of the alloys, is also discussed.

Effect of Homogenization on the Transformation Temperatures and Mechanical Properties of Cu15Ni35Hf12.5Ti25Zr12.5 and Cu15Ni35Hf15Ti20Zr15 High-Entropy Shape Memory Alloys.

The major challenge of high-temperature shape memory alloys (SMAs) is the collocation of phase transition temperatures (TTs: Ms, Mf, As, Af) with the mechanical properties required for application. Previous research has shown that the addition of Hf and Zr into NiTi shape memory alloys (SMAs) increases TTs. Modulating the ratio of Hf and Zr can control the phase transformation temperature, and applying thermal treatments can also achieve the same goal. However, the influence of thermal treatments and precipitates on mechanical properties has not been widely discussed in previous studies. In this study, we prepared two different kinds of shape memory alloys and analyzed their phase transformation temperatures after homogenization. Homogenization successfully eliminated dendrites and inter-dendrites in the as-cast states, resulting in a reduction in the phase transformation temperatures. XRD patterns indicated the presence of B2 peaks in the as-homogenized states, demonstrating a decrease in phase transformation temperatures. Mechanical properties, such as elongation and hardness, were improved due to the uniform microstructures achieved after homogenization. Moreover, we discovered that different additions of Hf and Zr resulted in distinct properties. Alloys with lower Hf and Zr had lower phase transformation temperatures, followed by higher fracture stress and elongation.

Size effect on the microstructure, phase transformation behavior, and mechanical properties of NiTi shape memory alloys fabricated by laser powder bed fusion

In this work, NiTi samples with different thicknesses (0.15–1.00 mm) were fabricated by laser powder bed fusion (LPBF) under variable scanning speeds (500–1200 mm s–1). The densification behavior, phase transformation behavior, and mechanical properties of the sample with different thicknesses are studied. The results indicate a strong size effect in the LPBF-fabricated NiTi alloy. The decrease of the sample thickness results in (i) the increase of porosity, (ii) the decrease of the number of adhered NiTi powder particles at the surface, (iii) the monotonous decrease of the martensitic transformation temperatures (MTTs), and (iv) the decrease of the shape recovery temperature. The influence of sample thickness on the melt-pool behavior, and thus the microstructure and performance of NiTi alloys are discussed. It is suggested that the melt-pool is deeper and narrower in the thin samples than in the thick samples. We conclude that, apart from the LPBF process conditions, the sample dimensions have also to be considered to fabricate NiTi structures with predictable properties.

Atypical phase transformation behavior of Fe-33%Ni alloys induced by shot peening

The phase transformation behaviors of two Fe-33 mass%Ni alloy specimens subjected to shot peening (SP) were investigated. The first sample consisted solely of an austenite (γ) phase while the second also contained a high proportion of a martensite (α′) phase. When the specimen containing only the γ phase was subjected to SP, a stress-induced martensitic transformation occurred. In contrast, the second sample underwent the reverse transformation from the α′ to γ phase. Moreover, the volume fraction of the γ phase in the vicinity of the peened surface of each specimen was found to plateau as the SP treatment was prolonged. The final volume fraction of the γ phase was the same in both specimens regardless of their initial microstructure prior to the SP process. In addition, the volume fraction of the γ phase was found to increase with increasing peening pressure, meaning that the reverse transformation was promoted by SP at higher pressures. Calculations of the deformation heat generated during SP indicated that this heat was not primarily responsible for the reverse transformation induced by SP. Estimation of the change in the phase transformation temperature resulting from the residual compressive stress imparted by SP showed that this treatment changed the value of T0 (the arithmetic mean of the martensitic transformation start temperature and the reverse transformation start temperature) for the Fe-33 mass%Ni alloy from 328.5 K to 287.8 K. Because T0 after SP was lower than the temperature to which the specimens were raised during SP, this process could have caused stress-induced reverse transformation. Therefore, the decrease in the phase transformation temperature stemming from residual compressive stress is believed to explain the reverse phase transformation observed during SP of this alloy.

The effect of systematic substitution of Nb with Ti on a potential high-temperature shape memory alloy: B2 NbRu

NbRu has been considered as a potential high-temperature shape memory alloy due to its high martensitic transformation temperature, however, it has poor superelasticity and other shape memory properties, which limits its applications. In this paper, the effect of systematic substitution of Nb with Ti on the structural, thermodynamic, and elastic properties of supercell 2x2x2 B2 Ru50Nb50-xTix spanning the entire composition range (x = 0, 6.25, 12.50, 18.75, 25.00, 31.25, 37.50, 43.75, 50 at. %) is investigated using ab initio calculations. Elasticity results showed that the mechanical stability of the B2 phase improved with increasing Ti content, which could be associated with a decrease in transformation temperature. However, a suitable composition range within which Cʹ~0, resembling that of well-known TiNi, is identified. This predicts Ti addition to be effective in improving shape memory properties of B2 NbRu alloy.

Analysis of phase transformation in Mg2NiH4 via in situ synchrotron X-ray measurements

Mg-based alloys such as Mg2NiH4 are still attracting attention as high-capacity hydrogen storage materials. Mg2NiH4 undergoes the phase transformation between low-temperature (LT) and high temperature (HT) phases near the hydrogen desorption temperature. Herein, the crystallographic changes of Mg2NiH4 were investigated to clarify the behavior of the phase transformation by in situ synchrotron radiation X-ray measurements. The samples with three thermal histories (slow cooling, quenching from the HT phase, and hydrogenation below the transformation temperature) were prepared. These thermal histories affected the ratio of the LT phase without microtwinning (LT1) and with microtwinning (LT2). As the LT2/LT1 ratio increases, the phase transformation temperature decreases, and the phase transformation accelerates. X-ray diffraction peak analyses showed that the crystallite size and the lattice strain depend on the thermal history of the sample. X-ray absorption spectra indicated an increase in the short-range disorder of Mg2NiH4 when hydrogenated below the transformation temperature, which causes anomalous phase transformation and modulates the transformation temperature.

Where and when are dislocations induced by thermal cycling in Ti–Ni shape memory alloys?

Systematic experiments were performed to determine where and when dislocations are induced during the thermal cycling of Ti–Ni shape memory alloys. The decrease in transformation temperatures upon thermal cycling became less pronounced with increasing Ni concentration, which was supported by the dislocation densities estimated from scanning transmission electron microscopy observations. The arrangement of transformation-induced dislocations after the 1st thermal cycle reproduced the self-accommodation structure of B19ʹ martensite. Three-dimensional visualization revealed that the complex spiral appearance of the dislocations after the 1st thermal cycle was attributable to the duplicated projection of relatively simple elongated dislocation loops aligned in the depth direction parallel to 〈111〉B2 along {110}B2. No dislocations were observed along the twin boundary in the initially transformed martensite, whereas there were numerous dislocations parallel to the trace of the {110}B2 planes in the martensite formed upon the 2nd thermal cycle. The results suggest that the transformation-induced dislocation loops were generated during the reverse transformation rather than the forward transformation. The possible source site of the transformation-induced dislocations is also discussed.

Eutectoid lamellar structure strengthened ultra-strong Al13Fe29Co29Ni29 high-entropy alloy

Alloys with a lamellar structure exhibit excellent mechanical properties as a large number of interfaces hinder dislocation motion. Here, an ultra-strong Al13Fe29Co29Ni29 high-entropy alloy (HEA) reinforced by a pearlite-like lamellar structure is studied. The results show that the Al13Fe29Co29Ni29 alloy decomposes into submicron lamellar (face-centered cubic (FCC) + L12) and (body-centered cubic (BCC) + B2) phases during heat treatment between 560 °C and 600 °C. The layer thickness is determined to decrease with decreasing transformation temperature in the Al13Fe29Co29Ni29 alloy, and it is proportional to the reciprocal of the undercooling of the phase transition. Strengthened by the submicron lamellar structure, the yield strength of the alloy after annealing at 600 °C reaches 1301 MPa with a ductility of 6.1%. In addition, the yield strength increases with the refinement of the lamellar structure, and it follows the classical Hall–Petch relationship. Furthermore, the lamellar FCC and lamellar BCC phases are strengthened by the nanosized L12 and B2 phases, respectively. This lamellar-structure-strengthened alloy possesses a higher strength than other metallic nanolaminates with the same layer thickness, such as Cu–Ni, Ti–Al, and Cu–Ag alloys. Thus, it is envisaged that the proposed alloy will be useful for technological applications.

Grain size dependence of stress-assisted two-way memory effect in Ti-50.04 at.% Ni shape memory alloy

The grain size (GS) effects on stress-assisted two-way memory behavior of NiTi shape memory alloys (SMAs) were investigated by thermomechanical actuation tests over a wide GS range (from 10 nm to 230 nm). The actuation strain and transformation temperatures are observed to decrease monotonically as the GS is reduced, but the transition temperature window is enlarged. The irrecoverable strain decreases monotonically with GS reduction overall. The underlying mechanisms were investigated through microstructural and thermodynamic analysis. It is demonstrated that, as the GS is reduced, the increasing volume fraction of dislocations and grain boundaries (GB) occupying the space of austenite crystallite suppress the phase transition (PT) and lower the entropy change, leading to the decrease of transformation strain. GS reduction converts the PT from first-order type to a continuous one, and greatly expands the operating temperature range of actuation, tuning the discontinuous leap in strain changes into a continuous variation. When GS > 40 nm, transformation induced plasticity dominates the irrecoverable deformation and is monotonically affected by GS dependent hall-patch effect. Further modeling analysis reveals that, the increase in the volume of GB caused by GS reduction drastically modifies the free energy density convexity of the crystallite-GB composite system and leads to the monotonic decrease of transformation temperatures. The near-linear temperature dependency of actuation strain and wider transition temperature window of the nanostructured NiTi, compared to traditional coarse-grained NiTi, provide the opportunity of improved actuation control of SMA actuator and have the potential for some unique applications in the actuation area.

Multifunctional properties in both three and one-dimensional polycrystalline Cu-doped Co–Ni-Ga shape memory alloys

The effect of Cu doping on the microstructure and functional properties of polycrystalline Co–Ni-Ga-Cu shape memory alloy is investigated with the Cu composition ranging from 0.3 to 4.5 at.%. For the miniaturization applications in actuators and sensors, the alloy with 3 at.% Cu is selected to fabricate one-dimensional microwires. The results reveal that Cu promotes the growth of the γ phase precipitates, accompanied by the decrease of the martensitic transformation temperature in the bulk Co–Ni-Ga-Cu samples. At low Cu doping, a reversible strain of 4% is obtained. By increasing Cu to 4.5%, a compressive reversible strain can reach at least 2.5% at room temperature. During the compression process, the adiabatic elastocaloric effect of a 3.3 K temperature change is also achieved. Furthermore, the polycrystalline microwire displays a great tensile superelasticity with a larger reversible strain of 5% in a temperature range varying from 223 to 373 K, which is the best among Co–Ni-Ga polycrystalline alloys. The shape memory effect of the microwire with a 3% strain output ranging from 123 K to 353 K is achieved as well. The good multifunctional properties in the Co–Ni-Ga-Cu shape memory alloy bulks and microwires lay a foundation for their applications in sensing, actuating and solid-state refrigeration fields.

Transformation Mechanism and Precipitation Behavior of Nanoscale Cementite in Carbon Steels during Ultrafast Cooling

The precipitation behavior and transformation mechanism of cementite in low‐ and medium‐carbon steels (0.04–0.5% C) after hot rolling are studied using ultrafast cooling (UFC) technology. The results show that the carbon concentration and stop‐cooling temperature are the primary factors influencing the precipitation behavior of nanoscale cementite. It is found that 1) the pearlite grows discontinuously, and the nanoscale cementite is dispersed and precipitated in the 0.17% C and 0.33% C steels cooled by UFC from 953 to 873 K without the addition of microalloys, and 2) the yield strength increases gradually with decreasing UFC stop temperature. The discontinuous growth state of cementite is attained when ferrite and pearlite formation occurs alternately and competitively on the α/γ interface forefront. The growth length of ferrite when is equal to decreases significantly with the increase in carbon concentration. Moreover, the difference between the ferrite growth length and pearlite lamellar spacing decreases with the decreasing transformation temperature. The moderate carbon concentration and lower stop‐cooling temperature are more conducive to breaking up the continuity of the pearlite growth. This study of the transformation behavior of degenerate pearlite under continuous cooling conditions is more instructive for actual production.

Transformation temperature and corrosion behavior of porous NiTi and NiTiCu shape memory alloy

The present research work deals with the fabrication of porous NiTi and NiTiCu shape memory alloy (SMA) by powder metallurgy process. These SMAs have number of applications in medical and space industries. The transformation temperature of both SMAs is measured by differential scanning calorimetry (DSC). Corrosion behavior is measured by potentiodynamic polarization curve in the presence of artificial saliva (pH value 7.4). Result shows that transformation temperature decreases by 8 °C to 10 °C after copper addition. Also, the corrosion current density decreases for NiTiCu, which exhibit more corrosion resistant than equi-atomic NiTi alloy. The corrosion current density of NiTiCu is less than NiTi by a factor of 31.

An experimental evaluation of the microstructure, mechanical and functional fatigue properties of the boron-doped Cu-Al-Be SMA wires

An experimental evaluation of the microstructure, mechanical and functional fatigue properties of the Cu-11.70Al-0.45Be doped with Bx (x = 0.05, 0.10, 0.12, and 0.14 wt%) SMA wires has been carried out. The experiments were performed to investigate microstructure, phase/precipitates, and transformation temperatures for both as-cast and wire samples. Furthermore, tensile properties, shape recovery ratio, and functional fatigue evaluation have also been carried out for the wire samples. The investigation shows that the addition of the minor amount of boron and secondary processes involved during the specimen preparation induced excellent grain refinement. The addition of boron decreased transformation temperatures; however, there was not a considerable change observed due to the secondary process. It was observed that tensile properties increases with the boron addition, and complete shape recovery was observed for all the selected alloys. Finally, functional fatigue tests were conducted under constant stress condition and observed that the number of cycles until the failure has increased and more distance recovery was achieved with an increase in boron doping.

Transformation-induced strain of a low transformation temperature alloy with high hardness during laser metal deposition

Bulk crack-free Fe-Cr-Ni-Mo-Si-B alloy (with high hardness of 54.5 HRC) was fabricated via laser metal deposition (LMD). The solid-state phase transformation (PT) volume effect, transformation-induced plasticity (TRIP), and the influences of PT strain on the tensile properties of Fe-Cr-Ni-Mo-Si-B alloy were investigated, and the problem of how to release thermal contraction through PT strain in the LMD process was discussed. The results show that the volume strain of the martensite PT of Fe-Cr-Ni-Mo-Si-B alloy was equivalent to the thermal contraction strain caused by a temperature decrease of about 370 °C for the austenite of this alloy, and the PT volume expansion effect increased with a decrease in transformation temperature. In the LMD process, the Magee effect had an obvious contribution on the TRIP, but the decrease of tensile stress during the actual PT process must be considered. Under constant load conditions close to the yield strength, the strain of TRIP should be an order of magnitude higher than that of the transition volume effect. In situ neutron diffraction demonstrated that the martensite starting point (Ms) of PT can be significantly increased by stress and plastic. The highest martensite starting temperature of strain induced PT (Md) is about 360 °C higher than that of PT without external stress. PT strain can endow the materials, which are completely brittle at room temperature, with an elongation up to 34% in the tensile process. Thermal contraction strain the LMD process can be counteracted by PT strain, thus achieving the crack-free formation of high-hardness materials.

Effect of abc Pressing at 573 K on the Microstructure and Martensite Transformation Temperatures in Ti49.8Ni50.2 (at%)

This paper presents experimental data on the microstructure and martensite transformation temperatures of Ti49.8Ni50.2 (at%) after abc pressing (multi-axial forging) to different true strains e from 1.84 to 9.55 at 573 K. The data show that increasing the true strain results in grain–subgrain refinement on different scales at a time. With e = 9.55 at 573 K, the average grain–subgrain size measured approximately 130 nm. Decreasing the abc pressing temperature from 723 to 573 K caused a decrease in all martensite transformation temperatures, a change in the lattice parameters, R phase formation, and angular shifts of diffraction peaks and their broadening. The largest change in the microstructure of Ti49.8Ni50.2 was provided by abc pressing to e = 1.84. Increasing the true strain to e = 9.55 resulted in a much smaller effect, suggesting that the alloy obtained a high density of structural defects even at e = 1.84. Two possible mechanisms of grain–subgrain refinement are discussed.

Study on Kinetics of Transformation in Medium Carbon Steel Bainite at Different Isothermal Temperatures.

Ultra-fine carbide-free bainitic (UCFB) steel, also known as nano-bainite (NB) steel, is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths. The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in conventional austempering (CA) processes based on the shear mechanism combined with the dilatometer test. UCFB steels with medium and high carbon composition are designed in this work to systematically study the transformation kinetics of bainite, and the evolution of its microstructure and properties, and reveal the influence of heat treatment processes on the microstructure and properties the UCFB steels. The results show that the activation energy for BF nucleation decreases during the CA process and isothermal transformation temperature decreases. The bainite transformation is first nucleated at the grain boundaries, and then nucleated at the newly formed bainitic ferrite/austenite interface.

Effect of doping with rare-earth element La in MnCoGe system

<p indent=0mm>MnCoGe is one of the most representative materials that undergoes martensitic transformation in the MM′X system. Because the phase transition temperature of MnCoGe is higher than its magnetic ordering temperature, various methods have been used to decrease the phase transition temperature and realize magneto-structural coupling. However, there are rare reports about the effect of doping with rare-earth elements. In this work, the structures and physical properties of Mn_{1−<italic>x</italic>}La<italic>_x</italic>CoGe compounds with <italic>x</italic>=0, 0.01, 0.02, 0.03, and 0.05 were studied. The introduction of La causes a deficiency of both Mn and Ge atoms in the main phase, which leads to a decrease in the martensitic transformation temperature and the realization of magneto-structural coupling. However, due to its limited ability of deficiency formation, a further increase in the La content leads to large amount of secondary phase instead of a continuous decrease of the phase transition temperature. Additionally, the MnCoGe matrix is found to be divided into small “domains” by the needle-like secondary phase, which is the reason for the large amount of retained austenite phase in the sample with a high La content.