Transformation Temperature Range Research Articles

Effect of restraint stress on martensite transformation in low transformation temperature weld metal

In this study, martensite transformation behavior of low transformation temperature (LTT) weld metals under unrestrained and restrained conditions was analyzed by thermal simulation technology. It is found that martensite transformation start (Ms) temperature and martensite transformation temperature range of LTT weld metal under restrained condition increased by 85 K and 50 K, respectively, compared with that of under unrestrained condition, which is attributed to the significant increase Gibbs free energy produced by restraint stress for martensite transformation. The increasement of Ms due to the effect of restraint stress is quantitatively described by Fe–X–C ternary regular solution-elastic energy model. Based on the transformation kinetics curve, the effect of restraint stress on martensite transformation of LTT weld metal can be divided into three stages, including stress-induced nucleation, transition and without effect stages. In initial stage, the martensite nucleates at a relatively high temperature with a low phase transformation rate due to the directional effect of restraint stress; in second stage with martensite transformation fraction range of 0.2–0.6, the elastic energy caused by restraint stress decreases significantly, and the chemical free energy of martensite transformation under restrained condition is lower than that under unrestrained condition, which results in the martensitic transformation rate of LTT weld metal under restraint condition being lower than that under unrestrained condition; in final stage, the small value of restraint stress has little effect on martensite transformation of LTT weld metal, and the transformation rate of martensite mainly depends on the content of austenite. This study provides a theoretical base for better understanding phase transformation and microstructure of LTT weld metal under restrained condition.

Influence of the holding temperature on the kinetics of the isothermal B2 → B19′ transformation in TiNi-based shape memory alloy

The aim of the present paper was to study the kinetics of the isothermal B2 → B19′ transformation in the quaternary Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy in a wide temperature range to find the common features of this phenomenon in NiTi-based alloys. It was found that the isothermal B2 → B19′ martensitic transformation occurred during holding at a constant temperature both within and outside of the temperature range of the forward transformation. The holding temperature of T* influenced the maximum value of the martensite phase (ΦM) that appeared under isothermal condition and the rate of the martensite volume fraction variation. The maximum ΦM value of 80% was found when holding was at T* = Ms (the start temperature of the forward martensitic transformation on cooling), and it was two times larger than in quenched Ni51Ti49 alloy. An increase in holding temperature larger than Ms decreased the maximum ΦM value and the slope of the ΦM(t) curve. A decrease in the holding temperature less than Ms decreased the ΦM value and hardly influenced the dΦM/dt value. It was found that the variation in volume fraction of the martensite phase that appeared under isothermal holding at different temperatures in the Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy was close to the binary Ni51Ti49 alloy, and hence, it was concluded that the features of the kinetics of the isothermal B2 → B19′ transformation were common for this type of transformation in non-stoichiometric NiTi-based alloys.

Increasing efficiency of plasma hardening by local cooling of surface by air with negative temperature

The martensitic transformation interval of some hypoeutectoid, all eutectoid and all hypereutectoid steels covers to a large extent the region of negative temperatures. Due to the fact that the plasma hardening operation is carried out in workshops where the minimum temperature is +20 °С, the surface temperature of the part after plasma heating cannot reach negative values. Because of this, the temperature range of the martensitic transformation is not fully used and in the hardened structure there is a certain amount of austenite, which has not undergone martensitic transformation. This circumstance reduces the hardness of the hardened layer and often low tempering is required to convert residual austenite to tempered martensite, which lengthens and makes the heat treatment more expensive. Complete or almost complete martensitic transformation is possible if the surface heated by the plasma beam is immediately cooled to a negative temperature.It is shown that local cooling of the hardened surface to a temperature of –40 °C can be carried out by air using the Ranque-Hilsch tube, which significantly expands the possibilities of full hardening for eutectoid and hypereutectoid steels. The studies consisted in heating the surface with a plasma stream to a temperature of 750 °C and 900 °C. The temperature was changed by the plasma torch current and by changing the velocity of the plasma flow spot moving along the sample surface. The experiments were carried out on steels 45 (0.45 % C), U8 (0.8 % C) and U10 (1 % C). The study of the structures was carried out on a MIM-7 microscope with a video camera and with the image displayed on the screen. The approximate quantitative composition of austenite, martensite, and associated structures was determined by the areas on the screen.During plasma hardening of steel 45 from a temperature of 900 °C using the Ranque-Hilsch tube, there is practically no residual austenite in the structure. When hardening U8 steel, residual austenite is detected in a small amount. When hardening U10 steel, the amount of residual austenite is approximately 15 %. Local surface cooling allows high-quality hardening of steels of most grades, regardless of the carbon content.

Exploring the functional capabilities of NiTi shape memory alloy thin films deposited using electron beam evaporation technique

The viability of utilizing electron beam evaporation technique for the development of NiTi shape memory alloy (SMA) thin films on kapton polyimide (flexible substrate) has been investigated. The actuation characteristics of the SMA/polyimide structure were evaluated using shape recovery analysis and electrical actuation. The maximum shape recovery ratio of 1.375 was attained with butterfly configuration during shape recovery analysis. The SMA/polyimide stimulated via Joule heating has displayed a maximum displacement of 0.9 mm at 3 V. The surface morphology examined through scanning electron microscope shows the smooth surface without any pores. The thermal stability analyzed using thermogravimetric analysis shows the NiTi SMA thin film can withstand higher temperature above 550 °C without any damage to the SMA/polyimide structure. The stress/strain behavior of the SMA/polyimide structure was analyzed as a function of temperature. The differential scanning calorimetry has showed that the austenite phase transformation temperature lies in the range of 45°C-95 °C. The temperature sweep test performed using dynamic mechanical analyzer recorded a drastic drop in storage modulus in the same phase transformation temperature range as that of DSC. The UV–Vis spectroscopy reveals the NiTi/polyimide structure has high reflective surface in the visible region which aids in the application of reflectors. SMA/polyimide structure can be used as a smart optical mirror in interferometry technique owing to its optical reflectivity.

Effect of temperature-dependent phase transformation on UV detection properties of zinc sulfide nanocrystals

In this work, design and optimization of ultraviolet sensors based on ZnS nanoparticles with the focus on the effect of annealing temperature on optical properties of zinc sulfide were carried out. Synthesis of ZnS nanoparticles was performed using a chemical deposition method and annealing process was followed at 300 °C, 500 °C and, 700 °C under flow of argon. Scanning electron microscopy indicated that by increasing the annealing temperature, the particle size of ZnS nanostructures is increased. Differential thermal analysis showed that the temperature range of phase transformation in ZnS nanostructures is at 300 °C–450 °C. X-ray diffractometry demonstrated a phase change from zinc blende to hexagonal wurtzite at the temperature of 500 °C. The UV-visible spectra of the ZnS samples indicated an increase in light absorption of ZnS nanostructures regarding to the phase transformation at 500 °C. Photoluminescence (PL) study also demonstrated a slight blue-shift in PL peak position of the sample that was annealed at 500 °C which is attributed to the higher optical band gap of this sample. Moreover, the intensity ratio of UV to visible peak is the highest for this sample showing an enhancement in optical properties of ZnS nanostructures which is suitable for fabrication of visible-blind UV detectors. For studying the influence of annealing temperature on optoelectrical properties of UV sensors based on ZnS nanostructures, metal-semiconductor-metal devices were fabricated using deposition of interdigitated Schottky contacts on different ZnS samples. The suitable optical and structural properties of the optimum sample including increasing the electron-hole pairs under UV illumination was manifested in optoelectrical characteristics of ultraviolet detectors based on ZnS sample which was annealed at 500 °C. For this sample, the sensitivity under UV radiation was enhanced by 6.9 times compared to the other ZnS samples.

Solid-state thermal energy storage using reversible martensitic transformations

The identification and use of reversible Martensitic transformations, typically described as shape memory transformations, as a class of metallic solid-solid phase change materials are experimentally demonstrated here. Direct evidence of repeatable temperature leveling (9%–25% reduction in peak temperature rise) during transient heating and cooling using NiTi was obtained by cyclic Joule-heating in a simulated thermal energy storage application. Compared to standard solid-solid materials and solid-liquid paraffin, these experimental results show that shape memory alloys provide up to a two order of magnitude higher figure of merit (FOM). To calculate the material FOM and determine the crystal structure, direct measurements of latent heat, thermal conductivity, density, and diffraction were performed. Beyond these experimental results, a review of >75 binary NiTi and NiTi-based ternary and quaternary alloys in the literature shows that shape memory alloys can be tuned in a wide range of transformation temperatures (from −50 to 500 °C), latent heats (up to 35.1 J/g), and thermal conductivities (from 15.6 to 28 W/m K). This can be accomplished by changing the Ni and Ti balance, introducing trace elements, leveraging intermediate R-phase transitions, and/or by thermomechanical processing. Combining excellent corrosion resistance, formability, high strength and ductility, high thermal performance, cyclic stability, and tunability, shape memory alloys represent a class of exceptional phase change materials that circumvent many of the scientific and engineering challenges hindering progress in this field.

A Novel Approach for Analytical Description of the Isothermal Bainite Transformation in Alloyed Steels

Abstract A novel approach for modeling the bainite transformation in alloyed steels during isothermal quenching is presented. The approach is based on the application of the logistic function with the logarithmic argument of the isothermal holding time. The proposed model involves two time-independent fitting parameters, a and b, where are specified for the given isothermal quenching temperature and other conditions (e.g., steel grade, austenite grain size, etc.). The adequacy of the modeling results is estimated using the sum of squared differences between the experimental and calculated bainite fraction formed at every time step Δτ during isothermal quenching. The logistic function was found to provide a fit between the experimental and calculated bainite transformation kinetics that was up to ten times better than that of the conventionally applied Kolmogorov–Johnson–Mehl–Avrami equation. An excellent agreement is achieved between the modeling results and the experimental data for commercially produced high-strength alloyed steels 300M, HY-TUF, and D6AC isothermally quenched in the temperature range of the bainite transformation. The obtained temperature dependences of the model parameters a and b reveal the potential for further theoretical investigation and verification of their physical meaning.

Properties of a 49Ni–36Ti–15Hf High-Temperature Shape Memory Alloy in the As-Cast State

The properties of a 49Ni–36Ti–15Hf (at %) high-temperature shape memory alloy (HTSMA) are studied comprehensively. Data on the elemental and phase compositions, the phase transformation temperatures, and the mechanical characteristics are obtained. The studies are carried out on samples cut from alloy ingots. The temperature range of the phase transformation of the alloy determined by differential thermal analysis is from Mf = 186.8°C to Af = 275.2°C in the as-delivered state and from Mf = 188.1°C to Af = 276.5°C after vacuum annealing. These values are suitable for designing of the required safety device. However, as-cast 49Ni–36Ti–15Hf (at %) alloy specimens undergo brittle fracture at stresses from 461 MPa (as-delivered state) to 514 MPa (after vacuum annealing at 700°C). In this connection, it is impossible to deform such samples in the afterelastic state, and the shape memory effect and the degree of shape recovery are zero in them; i.e., the as-cast 49Ni–36Ti–15Hf (at %) alloy is not suitable for designing an efficient safety device. To improve the thermomechanical characteristics, it is planned to change the HTSMA elemental composition, the conditions of deformation treatment (forging, rolling) or heat treatment of the as-cast alloy, or the deformation conditions of articles. The authors will take into account these results of studying the as-cast 49Ni–36Ti–15Hf (at %) alloy when designing various devices for nuclear power engineering.

DEFORMATION AND FRACTURE OF STRENGTHENED HIGH-CARBON STEEL AFTER TREATMENT IN TEMPERATURE CONDITIONS OF PHASE PRE-TRANSFORMATION AND TRANSFORMATION

Traditional methods of heat treatment are energy-intensive and time-consuming, so the task of increasing their efficiency is very relevant. The process of repeated high-speed heating with short-term aging in the temperature range of polymorphic pre-transformation and transformation from the viewpoint of evolution of structure, properties and character of the fracture of quenched high-carbon steels was investigated. In particular, it was found that high-speed heating (600 – 700 °C/s) and short-term holding (0.5 sec) followed by cooling in salted water (6 °C) leads to the formation of a structure not differing from the structure of low-drawn (200 °C, 2 hours) martensite of traditionally hardened steel with 4 times increase in elongation and 2 times contraction while maintaining strength during the tensile test. Short-term aging of 8 – 15 – 25 seconds with repeated quenching of high-carbon steels from 820 °С in cold salted water (6 °С) leads to formation of the structure of ultra-fine-lamellar, submicroplast, globular perlite. There is a three-dimensional nanostructuring of steel that differs from traditional hardening with high temperature tempering by the structure and properties: magnitude of the applied stresses, both at deformation stages and at fracture (increase in σ by 55 %, in σ0.2 – by 17 %, in ψ – 8 times, in αn – by 80 %). Increase in short-time holding up to 40 – 50 sec, with repeated quenching from 820 °C leads, in contrast to the traditional one, to formation of the structure of ultra-small-malt martensite and to appearance in fracture on the slip planes of the patching structure, resembling a honeycomb in shape. After the low temperature tempering during the tensile test, all stages of deformation are presented. In fracture the crushing of pit structure and the absence of brittle tunnel pits are observed, plasticity is improved (δ ~ 1.5 times, ψ – 3 times) at strength preservation.

Comparative analysis of continuous cooling transformation behaviour in CGHAZ of API X-80 and X-65 line pipe steels

Continuous cooling transformation diagrams were obtained for a simulated coarse-grain heat-affected zone of API X-80 and X-65 steels using Gleeble-3800 thermo-mechanical simulator. Dilatometry results showed that variable cooling rates have significant effect on phase transformation behaviour and microstructure of the steels. For X-80 steel, a mixture of bainite/martensite at fast and medium cooling rates and ferrite/pearlite at slow cooling rates was obtained and confirmed by microhardness measurements between 340 and 240 HV, whereas, for X-65 steel soft phase such as ferrite was nucleated along with bainite at moderate (15–100 °C s−1) and pearlite at slow cooling rates (3–10 °C s−1). Dilatometry curve comprises inherent noise, and Loess computational filter was used to filter background noise in the curves. Martensite/bainite and ferrite/pearlite coexisted for most cooling rates, which made it difficult to distinguish transformation kinetics of individual phases. In the first derivative analysis d/dT {(ΔL/Lo)} of dilatometry curves, peaks of individual phases were clearly observed. Peaks of individual phase were separated according to their transformation temperature range, and peak rate temperatures were determined. It was confirmed that the first derivative d/dT {(ΔL/Lo)} analysis described phase transformation behaviour precisely and aided in identification of the transformed phases.

Providing resistance to sulfide stress corrosion cracking of pipelines welded joints by selection of welding parameters

Sulfide stress cracking (SSC) is one of the most dangerous types of pipelines destruction. Thermal impact of the welding process drives to heterogeneity of the microstructure and properties of the metal, which can lead to cracking of pipeline welded joints. Resistance to SSC of welded joints is determined by the thermal cycle of welding and cooling rate in the temperature range of austenite transformation. Due to performed studies based on simulation of welding heating the recommended range of cooling rates of 10–30 ° C/s was established, in which the resistance to SSC of welded joints is ensured. To calculate the cooling rates in coarse grained heat affected zone (CGHAZ) finite-element models of heat distribution were developed for longitudinal multi-electrode submerged arc welding (SAW) and multi-pass girth welding of pipes. Using the developed welding models, it was found that in order to achieve the cooling rate in CGHAZ it is necessary to reduce heat input up to 15-30% during multi-electrode SAW process of longitudinal welds of pipes . For multi-pass girth welding it is necessary to preheat the edges to be welded up to 100-300 °C depending on type of welding (GMAW or SMAW) and pipe wall thickness.

Effect of Heat Treatmemt in (α+γ) Range on Mechanical Properties of Austempered Spheroidal Graphite Cast Iron

It is well known that austempered spheroidal graphite cast iron shows good tensile and elongation. And, retained austenite include in the austempered spheroidal graphite cast iron structure was known to effect for the mechanical properties when TRIP was occurred. On the other hand, fineness and homogeneity of the structure are considered to influence as a factor to improve mechanical property. In this study, the mechanical properties of spherical graphite cast iron was evaluated that it treatmented fineness prior-structure and austenitized at (α + γ) range followed by isothermal treatment at bainitic transformation temperature range. Compared the heat treatment of this study and typical austempering treatment, the mechanical properties with good balance of strength and ductility were obtained in the heat treatment of this study. But, retained austenite that it determined by x-ray diffraction was slightly changed before and after the tensile test. Therfore, it was considered that not TRIP but complex phase effect on mechanical properties.

Shape memory characteristics of (TiZrHf)50Ni25Co10Cu15 high entropy shape memory alloy

A homogenized (TiZrHf)50Ni25Co10Cu15 high entropy shape memory alloy (HESMA) was studied to investigate its transformation behavior and shape memory effect. After solution treatment at 1000 °C, the (TiZrHf)50Ni25Co10Cu15 HESMA still contained Ti2Ni-like second phase and (Ti, Zr, Hf) carbide, and thus was not a single solid solution. It exhibited a broad transformation temperature range with Ms = 36.0 °C and Mf = −80.4 °C. In a three-point bending test, the (TiZrHf)50Ni25Co10Cu15 HESMA showed a reversible strain of 4.8% under 650 MPa, which is similar to the performance of Ti50Ni50 alloy but with a much higher output work density.

Reversible elastocaloric effect at ultra-low temperatures in nanocrystalline shape memory alloys

Superelastic shape memory alloys (SMAs) exhibit a reversible elastocaloric effect that originates from a release/absorption of latent heat associated with a stress-induced martensitic phase transformation. In typical SMAs, the conventional elastocaloric effect will vanish when the operating temperature falls below the temperature range in which martensitic phase transformation can be triggered by stress. We report emergence of an unprecedented elastocaloric effect with a decrease of temperature, well below the temperature range of martensitic phase transformation, in a model nanocrystalline NiTi that preserves slim-hysteresis superelasticity at ultra-low temperatures. The new elastocaloric effect emerges at a temperature of ∼90 K, exhibits an opposite sign than the conventional elastocaloric effect, and intensifies gradually with a decrease of temperature to 18 K. At 18 K, a large adiabatic temperature change ΔTad of +3.4 K is measured upon rapid release of tensile stress. The measured ΔTad are larger and extend over a wider temperature span than the existing electrocaloric, piezocaloric, and barocaloric cryo-refrigeration materials. We show that such low temperature elastocaloric effect originates from an entropic elasticity associated with large non-linear elastic deformations of the nanocrystalline microstructure at ultra-low temperatures. Our study suggests a new avenue to cool ultra-low temperature ambients.

The effect of holding temperature on the strength of the diffusion bond of Ti-alloy and stainless steel through the ultrafine-grained interlayers of Ni and Ni-2%Cr alloy

The manuscript considers the effect of diffusion bonding pressure, as well as holding at different temperatures, on the strength of the joints between titanium alloy and stainless steel, formed by diffusion bonding through ultrafine-grained interlayers of Ni and the Ni-2%Cr alloy. In samples with an interlayer of nickel after holding at 25 °C, the maximum strength of 380 MPa is achieved after diffusion bonding in the pressure range of 4-8 MPa. In samples with an interlayer of Ni-2%Cr, after holding at 25 °C, the highest strength of 490 MPa is achieved after bonding at a pressure of 12 MPa at 700 °C for 20 minutes. In the case of Ni-2%Cr interlayer, the strength of the joint falls more slowly than in the case of Ni interlayer. The mechanical properties of the joints are discussed in terms of the effect of chromium doping of nickel interlayer on the temperature range of transformation of austenite to martensite of the formed TiNi intermetallic compound.

Ferromagnetic-to-Paramagnetic Phase Transition of MnAs Studied by Calorimetry and Magnetic Measurements

Manganese monoarsenide samples have been prepared by the sealed-ampule technique and characterized by X-ray diffraction, differential thermal analysis, and scanning electron microscopy. The hexagonal- to-orthorhombic phase transition of MnAs has been studied using differential scanning calorimetry (DSC) and magnetic measurements. The enthalpy and temperature range of the transition have been determined to be ΔH =–5.6 J/g and 312.5–319 K, respectively. The enthalpy and temperature range of the transition are influenced by the quality of the samples. The samples containing inclusions of the metastable, orthorhombic phase have a lower enthalpy and broader temperature range of the magnetostructural transformation of manganese monoarsenide. It has been demonstrated that DSC is an effective tool for assessing the quality of MnAs samples. Temperature dependences of specific magnetization and magnetic permeability for MnAs lend support to the DSC results. From these data, the Curie temperature of MnAs has been determined to be 40°C, in good agreement with previously reported data.

Kinetics of isothermal B2 → B19′ martensitic transformation in Ti49Ni51 shape memory alloy

The B2 → B19′ martensitic transformation in a quenched Ni-rich NiTi alloy was found to proceed under isothermal conditions at temperatures both within and outside of the transformation temperature range as observed during continuous cooling measurement. The variation in the volume fraction of the martensite that appeared during isothermal holding of quenched Ni-rich NiTi alloy at different temperatures T∗ was measured. It was found that the maximum volume fraction of the martensite (45%) appeared during holding at Ms temperature for 30 min. It was found that the kinetics of the isothermal transformation was different for T∗ > Ms and T∗ < Ms due to the different structure of the alloy before holding that might influence the mechanisms of the martensite formation.

Properties of as-deposited and heat-treated Ni-Mn-Ga magnetic shape memory alloy processed by directed energy deposition

Ni-Mn-Ga magnetic shape memory alloy was processed by laser metal deposition, an additive manufacturing method. Powder used for deposition was crushed from a cast 10M martensite Ni-Mn-Ga ingot. The deposited sample was ferromagnetic and showed a 14M martensite with no detected macroscopic composition differences throughout, except for a thin layer between substrate and deposit. Layer-by-layer deposition resulted in a layered microstructure due to differences in local thermal histories, and the sample's broad transformation temperature range is proposed to originate from the resulting variations in microstructure. Although the sample is clearly polycrystalline, columnar grains span deposition layers, which is potentially favorable to twin boundary motion. After a homogenizing and ordering heat treatment, transformations regained a typical narrow hysteresis and saturation magnetization increased, while grain growth and/or recrystallization took place. The results show the promise of laser-based additive manufacturing processes for production of magnetic shape memory alloys.

The Influence of Thermal History on the Multistage Transformation of NiTi Shape-Memory Alloys

The multistage martensitic phase transformation of a polycrystalline NiTi shape-memory alloy (50.3 at. %Ni–49.7 at. % Ti) has been studied by means of calorimetric measurements. After a conventional thermal treatment followed by successive thermal cycles, the initial two-step forward transformation splits into four-overlapping stages. However, the reverse martensitic transformation maintains the initial two-step sequence, usually assigned to the B19′→R→B2 transformation. The correlation between the forward and reverse steps has been established by means of selected thermal cycles together with an estimation of their enthalpy and thermal hysteresis. These results have also provided information about the storage of the elastic strain energy and the frictional works associated with the variants’ nucleation. Moreover, the study around the forward transformation temperature range by means of uncompleted thermal cycles undoubtedly shows the presence of temperature memory effects in both stages.

Enhanced magnetic refrigeration capacity in Ni-Mn-Ga micro-particles

Bulk magnetic refrigerant materials with first-order character usually possess hysteresis losses and concentrated refrigeration working temperature interval (δTFWHM), which are detrimental for magnetic refrigeration applications. Here, we report on Ni-Mn-Ga micro-particles with size ~38.5–45μm fabricated by grinding and subsequent stress relief annealing (SRA). The microstructure, martensite transformation (MT), hysteresis loss and magnetocaloric effect (MCE) of the SRA particles were studied in comparison with the bulk alloy counterpart. The thermal and magnetic hysteresis losses in SRA particles ~1.3K and ~1.9J/kg were significantly reduced compared to those in the bulk alloy ~5.5K and ~9.7J/kg, respectively, which is related to the reduction of neighboring grain constraints on martensite/austenite phase boundary motion. The MT transformation temperature range of the SRA particles ~26.5K was higher than that of the bulk alloy ~5.5K because of the high specific surface area of the particles and the residual internal stress after SRA. A magneto-structural partly-coupled state was attained in the SRA particles, which is responsible for the wide δTFWHM ~25K and the high refrigeration capacity (RCnet) ~117J/kg. Based on these results, a possible approach for the high magnetic refrigeration efficiency is expected and discussed.