Decrease In Transformation Temperature Research Articles

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.

Structural, magnetic and magnetostrictive properties of the ternary iron–palladium–silicon ferromagnetic shape memory ribbons

The influence of the partial substitution of Fe by Si and thermal treatments on the structural, magnetic and magnetostrictive properties of the Fe67.5Pd30.5Si2 rapidly solidified ribbons has been investigated. A remarkable decrease in the martensite transformation temperature, with ~ 65 K lower than that of the Fe–Pd archetype alloy, is observed in the as-prepared ribbons. The thermal treatments shift the martensite transformation temperatures upward, with approximately 13 K for the higher thermal treatment. Also, these induce an improvement in the crystallinity in these ribbons with high texture and an increase in the crystallite size as a result of reducing the internal defects and stress. The thermodynamic considerations discussed in the frame of the Clapeyron–Clausius relation by using the calorimetric and thermomagnetic measurements (up to 7 T) reveal a weak influence of the magnetic fields on the martensitic transformation temperatures (~ 0.5 K/T). The magnetostriction decrease with temperature under small magnetic fields was discussed, beside an unusual behaviour in the technically saturated domain. This behaviour is based on the coexistence of the ordinary and forced magnetostrictions, the last one increasing faster with the temperature decreasing.

Martensitic Transformation and Metamagnetic Transition in Co-V-(Si, Al) Heusler Alloys

This study investigates the crystal structure, martensitic transformation behavior, magnetic properties, and magnetic-field-induced reverse martensitic transformation of Co64V15(Si21–xAlx) alloys. It was found that by increasing the Al composition, the microstructure changes from the martensite phase to the parent phase. The crystal structures of the martensite and parent phases were determined as D022 and L21, respectively. Thermoanalysis and thermomagnetization measurements were used to determine the martensitic transformation and Curie temperatures. Both the ferromagnetic state of the parent phase and that of the martensite phase were observed. With the increasing Al contents, the martensitic transformation temperatures decrease, whereas the Curie temperatures of both the martensite and parent phases increase. The spontaneous magnetization and its composition dependence were also determined. The magnetic-field-induced reverse martensitic transformation of a Co64V15Si7Al14 alloy under pulsed high magnetic fields was observed. Moreover, using the results of the DSC measurements and the pulsed high magnetization measurements, the temperature dependence of the transformation entropy change of the Co-V-Si-Al alloys was estimated.

Effect of Hf/Zr Ratio on Shape Memory Properties of High Temperature Ni50.3Ti29.7(Hf,Zr)20 Alloys

A series of Ni50.3Ti29.7(Hf,Zr)20 (Zr=0 to 20 at %) high-temperature shape memory alloys were investigated to determine the effect of varying Hf/Zr ratio on shape memory properties. Uniaxial constant-force thermal cycling (UCFTC) tests and electron microscopy were used to determine the effect of composition on microstructure, transformation temperatures, and functional material properties. In general, substitution of Zr for Hf resulted in a slight decrease in transformation temperatures, a negligible change in transformation strain, and an improvement in dimensional stability. Thus, the entire range of NiTiHfZr alloys resulted in acceptable actuation properties across the compositions studied with the additional ability to adjust composition for cost and mass savings.

Thermal preparation and characterization of nanodispersed copper-containing powders produced by non-equilibrium electrochemical oxidation of metals

Physicochemical properties of copper, aluminum and cadmium oxides determine their application in different ways. Electrochemical metal oxidation using alternating current (AC) influences the band gap of prepared copper-containing oxide materials and result in a blue shift. The use of alternating current provides sufficient conditions responsible for nanosized materials formation (grain size: 20–30 nm for separate copper oxidation, 15–20 nm for joint copper and aluminum oxidation). In the present work, XRD and DSC/DTG, SEM analyses were used to characterize the electrolysis products. A decrease of the thermal transformation temperature (Cu2O oxidation at 260°С, copper-aluminum layered double hydroxide (Cu–Al/LDH) decomposition at 140°С) of the products of both separate copper oxidation and joint copper and aluminum (or cadmium) oxidation was established when compared with those of the same oxides prepared by the usual methods. Solution aging of these products after joint electrochemical oxidation of copper and aluminum results in the transformation of Cu2O–AlOOH to Cu–Al/LDH. Products of the joint oxidation of copper and cadmium consist of oxides and hydroxides of copper and cadmium (γ-Cd(OH)2, Cu(OH)2, β-Cd(OH)2, CdО and Cu2O), which are not capable to form LDH. The synthesized copper-containing mixed metal oxide will be investigated elsewhere to obtain their structural characterization and operating performance in catalytic, photocatalytic and other applications.

Features of Austenite Formation in Low-Carbon Steel during High Speed Heating Induced by High-Speed Deformation

An experiment is performed for explosive convergence (collapse) of a cylindrical shell of low carbon steel with a ferrite-perlite structure. It is revealed that during ultra-rapid heating caused by high-speed deformation austenite formation occurs in an unusual sequence: first, free ferrite is converted, then pearlite. The decrease in free ferrite transformation temperature is explained by action of high pressure, as well as by varying degrees of heating steel structural constituents. An effect of barothermal quenching is observed, as a result of which a pearlite-martensite structure forms.

Thermal properties of Na2O\u2013P2O5\u2013Fe2O3 polyphosphate glasses

Iron phosphate glasses are materials that can have many applications like durable matrixes in waste immobilization techniques, biomaterials, optoelectronic devices, etc. Their possible usage is related to their glass network and thermal properties. The influence of Na2O content on thermal properties and crystallization ability of iron phosphate glass of base composition 30 Fe2O3–70 P2O5 mol% were studied. Increasing the content of Na2O causes a decrease in transformation temperature and increase in ΔCp. Characteristic temperatures, thermal stability and crystallizing phases were determined. Increasing content of sodium causes depolarization of iron phosphate glass network which causes a continuous change in ΔCp and glass transformation temperature. Discontinuous change in some glass properties suggests structure rebuilding about 30 mol% of Na2O.

Anomalous effect of Si additions upon the paramagnetic-to-antiferromagnetic transition entropy in fcc high-Mn steels

Fe–Mn based alloys with more 12% at. of Mn are usually called High-Mn steels. Several technological properties of High-Mn steels are explained by the presence of the fcc-hcp martensitic transformation. This transition is strongly affected by the para-to-antiferromagnetic ordering transition present in the fcc structure. The magnetic ordering temperature depends on the alloy chemical composition, and strongly stabilizes the fcc austenitic phase, leading to a decrease of the structural transformation temperatures. In several cases, this effect is strong enough to inhibit the martensitic transformation. In the present study, the effect on the magnetic transition of adding Si to high-Mn Fe–Mn alloys is analyzed. Calorimetric measurements are performed in order to obtain the entropy change associated to the magnetic ordering transition. The results are discussed in the light of the available information in the literature. Striking effects are reported, which indicate that Si addition strongly affects the driving force of the martensitic transition.

Magnetostructural transitions in V-doped MnCoGe compounds

MnCoGe1-xVx (x = 0.005, 0.01, 0.015, 0.02, 0.03, and 0.04) compounds were synthesized and investigated in view of the effect of transition metals on main-group-element sites to the magnetostructural transition. A small amount of V doping results in a decrease of the martensitic transformation temperature, while a further increase of V disturbs the Co-Ge bonds hence destabilizing the MM’X phase. Therefore, the transformation temperature returns to high temperature, and the expected Curie temperature window becomes incomplete. Accordingly, a large magnetic entropy change of about 10 J/kg·K and a refrigerant capacity of about 129.5 J/kg is obtained in the series.

Possibilities of complex experimental study of thermophysical and thermodynamic properties of selected steels

In this paper, selected key thermophysical and thermodynamical properties of steels (three alloys based on Fe–C–Cr) such as solidus (TS) and liquidus (TL) temperatures, peritectic transformation temperatures (TP), heats of fusion (ΔHF), specific heat capacities(cP), surface tension (σ), and wettability expressed by the wetting angle (θ) of liquid steel on alumina substrate were experimentally determined in a high-temperature area up to the temperature of 1600°C. The effect of the temperature and the chemical composition of steel on these properties was investigated using 3D heat flux DSC (Differential Scanning Calorimetry) and a sessile drop method. The interaction of the steel samples with the alumina substrate was studied by SEM, EDX and XRD methods. To assess the influence of the major elements (carbon and chromium), the steels with different carbon and chromium content, which varied in the range of 0.077–0.381wt.% and 0.049–4.990wt.%, were chosen. It was shown that increasing carbon and chromium content led to a decrease in phase transformation temperatures and thermal capacities, as well as an increase in the heat of fusion, surface tension, and wetting angle. Furthermore, the rising temperature increases the thermal capacities and wetting angles. Whereas the surface tension followed the opposite trend.

Relationship between the variation in transformation temperatures, resistivity and dislocation density during thermal cycling of Ni50Ti50 shape memory alloy

AbstractThe aim of the present work is to find the relationship between a decrease in transformation temperature and an increase in dislocation density during thermal cycling of NiTi alloy. The resistivity is used as a measure of the dislocation density variation and it allows one to find a linear dependence of the increment in the transformation temperatures on the dislocation density variation starting with the fifth thermal cycle. It is found that the thermal cycling of NiTi alloy within a temperature range of 473 to 273 K is accompanied by less dislocation density variation than in a range of 413 to 273 K. The dependence of transformation temperatures on increase in dislocation density in the first five cycles is not linear, hence it is concluded that the dislocation density variation is not the unique reason for a decrease in transformation temperature during the thermal cycling of equiatomic NiTi alloy.

Phase transformation and microstructure evolution study in various alloy systems: An insight

Phase transformations occurring in various metals like steel, titanium and super alloysis integrated and presented in this paper. In steels, the microalloying with Nb and Nb-Mo decrease in transformation temperature indicates the acceleration of transformation kineticsin turn which affect the microstructure and continuous cooling transformation (CCT) diagram. Incase of titanium alloys, the thermohydrogen treatment (THT) uses hydrogen as temporary alloying element which facilitatethe alloys to be processed at lower stresses and/or temperatures. Thermomechanical processing sschedules are designed baseds ons the β transus temperature to produce a variety of novel microstructures. In the case of crystalline materials, plastic deformations assisted by dislocations and the concentration of dislocation increases with increasing plastic strainthus becoming a precursor of twinning.Stress-strain curves for different alloys suggested the microstructural changes account for deformation induced twinning and martensite transformation.

Effects of Undercooling and Transformation Time on Microstructure and Strength of Fe–C–Mn–Si Superbainitic Steel

A metallographic method, dilatometry, and X-ray diffraction were applied to investigate the effects of undercooling and holding time on bainitic transformation, microstructure, and strength of Fe–C–Mn–Si superbainitic steel. The results indicate that the strength of the samples decreases and the elongation increases with the isothermal transformation time, resulting in an increase in the product of the tensile strength and the total elongation. Therefore, the prolongation of the transformation time can improve the comprehensive property of the sample. In addition, the morphology of bainite changes from granular bainite to lath-like one with a decline in the isothermal transformation temperature, leading to an increase in strength. The elongation first increases and then decreases with a decrease in the isothermal transformation temperature. Finally, the sample shows the best comprehensive property when it is austempered at an intermediate temperature (350°C). The data presented here are instrumental in optimizing the technology of austempering treatment in industrial production.

The study of the effect of external additives on the synthesis of fumed TiO2 with a high content of rutile structure by the novel natural dropping thermal treatment

The effect of external additives on the synthesis of fumed TiO2 with a high rutile structure content was studied. The focus of this investigation was on the external additive species, agglomerates of the fumed TiO2 before and after the thermal treatment. The transformation ratio from the anatase to rutile structure of the thermally-treated fumed TiO2 was investigated as a function of the fumed TiO2 agglomerate before the thermal treatment. Small agglomerated powders resulted in a decrease of transformation temperature. Two novel results were obtained in this investigation. One was a new fumed TiO2 with a 100% rutile structure having an excellent dispersibility being successfully synthesized by the thermal treatment of AEROXIDE® TiO2 P 25 with a small portion of AEROSIL® R 972. The other was the remarkable acceleration of the transformation from the anatase to rutile structure and grain growth/sintering of the thermally-treated fumed TiO2 observed at a relatively low thermal treatment temperature by the oxidation reaction of calcium stearate as an external additive.

Location dependence of microstructure, phase transformation temperature and mechanical properties on Ni-rich NiTi alloy fabricated by wire arc additive manufacturing

Ni-rich NiTi alloy was successfully fabricated for the first time using wire arc additive manufacturing (WAAM) technique. The distinct anisotropic microstructure with increased amount of Ni4Ti3 but decreased amount of Ni3Ti from lower to upper region leads to obviously decreasing transformation temperature, increasing hardness and tensile strength but reduced ductility.

Correlation between the thermal and superelastic behavior of Ni50-xTi35Zr15Cux shape memory alloys

The alloying effect of Cu on thermal and mechanical behavior in Ni50-xTi35Zr15Cux (x = 0, 3, 5, 10, 15 at.%) shape memory alloys has been systematically investigated in the present study. The addition of Cu changes the lattice parameters and expands the unit cell volume of B2 austenite and B19' martensite. With alloying Cu, the temperature for austenite ↔ martensite transformation tends to decrease, and the transformation from B2 austenite to B19' martensite occurs in a single step mode. The decrease of transformation temperature with alloying Cu is attributed to the chemical stabilization of B2 (destabilization B19') which is reflected by the decrease of the transformation enthalpy. The stabilization of B2 comes along with the increase of valence electron concentration and non-basal shear modulus resisting to transformation into B19'. While, the destabilization of B19' structure is represented by the increase of monoclinic angle. When compared to the ternary Ni49.5Ti50.5-xZrx (x = 0, 3, 5, 10, 15, 18, 20 at. %) and Ni50-xTi50Cux (x = 0, 2.5, 5, 7.5, 10 at. %) alloys, the stabilization of B2 (destabilization B19') is more pronounced in the NiTiZrCu alloys investigated in the present study. Due to the change of thermal and structural properties with alloying Cu, the temperature dependence of stress for inducing martensitic transformation gradually decreases and the superelastic window is enlarged. As a result, the superelastic recovery is enhanced with alloying Cu.

Effects of Ti and Gd for Ga substitution on microstructure, magnetic and mechanical properties of polycrystalline Ni-Mn-Ga magnetic shape memory alloy

Abstract The present study is focused on the influence of Ti and Gd doping at the Ga site on the microstructure, magnetic and mechanical properties of the polycrystalline Ni50Mn25Ga20-xZx (x = 0 or 5, Z = Gd, Ti) magnetic shape memory alloys. Microstructure investigations show that reference Ni50Mn25Ga25 and Ti-doped alloys, both in the as-cast and annealed state, are single phase materials, whereas Gd-doped sample reveals dendritic dual-phase structure with substantial distinction between Gd-rich and Gd-poor regions. Thermomagnetic measurements expose reversible martensitic transition in the Ni50Mn25Ga25 and Ni50Mn25Ga20Ti5 alloys, where Ti addition to NiMnGa composition leads to the decrease of phase transformation temperature from TM = 193 K for reference sample to TM = 172.5 K for Ti-doped material. Furthermore, the Ni50Mn25Ga20Gd5 alloy does not experience fully martensitic transition. Temperatures of magnetic transformation also varies with chemical composition and equals to 379 K, 318 K and 370 K for the annealed Ni50Mn25Ga25, Ni50Mn25Ga20Ti5 and Ni50Mn25Ga20Gd5 alloy, respectively. Mechanical properties investigation based on the nanoindentation measurements shows beneficial influence of doping elements on material hardness. In addition, planar distributions of hardness allow to deconvolute mechanical properties of each individual phase of the Ni50Mn25Ga20Gd5 alloy.

Effect of Hot Rolling Process Parameters on Microstructure Transformation and Microstructure of 45MnSiV Steel

The effects of final rolling temperature, cooling rate and deformation on phase transition point, the duration of the phase transition and the pearlite laminar layer of non-quenched and tempered steel 45MnSiV were studied by simulating the process of rolling and post-rolling cooling on Gleeble-3500 thermal simulator and thermal expansion tester. The results show that: the ferrite and pearlite transformation temperature ranges from 510 °C to 700 °C, and the bainite transformation temperature ranges from 400 °C to 500 °C when the steel is continuously cooled at a final rolling temperature of 950 °C, and the martensite transforming temperature is 300 °C under high cooling rate (&gt; 10 °C/s); The pearlite laminar spacing decreases with the decrease of final rolling temperature. It can be seen that the rolling deformation increases the temperature at which the test steel undergoes a phase change at each cooling rate by comparing the results of deformation and no-deformation test at 950 °C. The effect of time advance on the phase transition zone of ferrite and pearlite is particularly obvious, but the effect on the phase transition temperature and time of the bainite and martensite phase transition is not obvious. When the final rolling temperature remains constant, the Rockwell hardness value of the test steel gradually increases, and the pearlite layer spacing decreases with the decrease of ferrite transformation temperature gradually and the increase of the cooling rate.

The Effect of Anti-Site Disorder on Structural and Magnetic Properties of Ni–Co–Mn–In Alloys: &lt;italic&gt;Ab Initio&lt;/italic&gt; and Monte Carlo Studies

In this paper, we report on the effect of partial structural B2-like disorder on martensitic transformation and thermomagnetization curves in Ni–Co–Mn–In alloy by using the theoretical approach involving first principles and Monte Carlo methods. In case of ground-state electronic calculations, the off-stoichiometric composition was treated by using both supercell and coherent potential approaches. It is found that the increase of a structural disorder degree leads to decrease the energy difference between austenite and martensite phases, which corresponds to a decrease in the martensitic transformation temperature according to $\Delta E \approx k_{B}T_{m}$ . Besides, the magnetic exchange interaction constants for both martensite and austenite are slightly weakened by influence of structural disorder. Using calculated exchange coupling constants in the effective Potts–Blume–Emery–Griffiths Hamiltonian, a set of magnetization curves for the studied system is simulated. It is shown that the account of disorder results in a decrease in the magnetization value around the magnetostructural transformation. Theoretical results reproduce well the experimental ones.