Temperature Region Of Transformation Research Articles

Large elastocaloric effect in as-cast Ni-Mn-Sn-Fe ferromagnetic shape memory alloys

The Ni-Mn-Sn ferromagnetic shape memory alloys exhibit excellent both magnetic field-induced magnetocaloric effect and uniaxial stress-induced elastocaloric effect. However, the intrinsic brittleness and narrow refrigeration temperature regions hinder their practical applications in elastocaloric and magnetocaloric as well other fields. In this work, the as-cast Ni44-xFexMn46Sn10 (x = 1, 2, 3) alloys are prepared directly by cooling solidification. The as-cast alloys improve the mechanical properties and broaden the martensitic transformation temperature region due to the presence of dendrites. The dendrites prevent cracking formation and propagation along the grain boundaries, and the addition of Fe-atoms enriched in the interdendritic region further optimizes the mechanical performance. In addition, the as-cast Ni41Fe3Mn46Sn10 alloy exhibits a large adiabatic temperature variation up to −10.3 K at a moderate stress of 350 MPa. Accordingly, direct cooling solidification of as-cast alloys could be a novel and feasible method for studying elastocaloric cooling technology.

Microstructure and Mechanical Properties of Vacuum Centrifugal Casted Ti-6Al-4V Alloy by Casting and Heat Treatment

The Ti-6Al-4V alloy has been widely used for structural materials due to high specific strength property, however it is difficult to cast because of high reactivity at high temperature. In this study casting characteristics and post-casting heat treatment were studied in the cast Ti alloy. The cast alloy was prepared using high-frequency induction heating and horizontal vacuum centrifugal casting to reduce reaction with crucible and mold by minimizing overheating of the molten metal and casting fast. In the thin casting with a thickness of 2 mm, α’ martensite was observed, and in the casting with a thickness of 4~8 mm, α+β Widmanstätten was observed. As the thickness of the casting increased, the grain size, α-lath width, and α-case thickness increased. Accordingly, as the thickness of the casting increased, the hardness of the matrix decreased and the hardness of the surface increased. In addition, when the casting was heat treated below the β-phase transformation temperature, the α-lath width increased, and when quenched after heat treatment in the β-phase transformation temperature region, an α’ martensite was formed. On the other hand, when the casting was heat treated in the atmosphere, it was confirmed that the elongation was very low because hydrogen and oxygen in the atmosphere were induced. In addition it was confirmed that casting defects can be removed through HIP.

FORC and TFORC analysis of electrodeposited magnetic shape memory nanowires array

Magnetic characterization of ferromagnetic Ni-Fe-Ga shape memory nanowires using a temperature-dependent FORC analysis is shown. The hysteresis loops’ shape indicates a magnetic anisotropy that is governed by the magnetostatic interaction among neighboring nanowires. FORC measurement proved that the Ni-Fe-Ga nanowires' array is a multi-domain and highly interacting system. The FORC analysis shows a minimal coercivity distribution that points to a uniform and homogeneous nanowires' array. The change of the vertical spread of the FORC distribution divergence at about 395 K supports the ferromagnetic shape memory behavior and the structural transformation of the presented nanowires. The previous results are also supported by a unique TFORC analysis, showing structural changes within the transformation temperature region.

Heat treatment behavior of alternator shafts steels

The present paper analyzes properties such as the hardness, the tensile and toughness behaviour and the microstructure for multiple bars of C40 and C45 steel used for shafts in alternators. The materials have been manufactured by rolling processes, followed by normalization treatments or forging processes followed by normalization treatments. Heat treatments are performed on steels to produce different microstructural phases that affect the mechanical properties. Microstructural changes during austenite decomposition depend on the transformation temperature region and cooling rate. Over the years, many experimental studies have been performed to reveal the microstructural changes during isothermal holding or continuous cooling of austenite for various kinds of steels. The isothermal transformation (IT) diagram, which is also called the time-temperature-transformation (TTT) diagram, and the continuous cooling transformation (CCT) diagram have been developed to graphically characterize the decomposition transformation as a function of time and temperature. However, when these diagrams are based on experimental measurements, several limitations are present. Each diagram is limited because the temperatures at which specific transformations occur can vary due to several factors, such as: the chemical composition of steel, the solid solution condition in the austenite prior to cooling and transformation, the presence of precipitates in austenite, the prior austenite grain size, and the applied stresses during the transformation.

Effect of non-isothermal deformation of austenite on phase transformation and microstructure of 22SiMn2TiB steel

Abstract The microstructure evolution and phase transformation of high strength 22SiMn2TiB steel during non-isothermal deformation were investigated in detail. The results indicate that deformation temperature range and amount play a predominant role in phase constitution and volume fraction of the microstructure formed in the process of the non-isothermal compressive deformation. The start martensitic phase transformation temperature M s is monotonously decreased with the deformation amount and slightly influenced by the deformation rate, and not affected by the cooling rate and initial deformation temperature. The diffusional transformation, such as polygonal ferrite and bainitic ferrite, is promoted by the non-isothermal deformation of austenite under specific circumstances where the deformed austenite passes through its transformation temperature region.

Dielectric Property of Functionally Graded Ferroelectric PZT Ceramics

Ferroelectric material possesses a high dielectric constant during ferroelectric to paraelectric phase transition. But, the high dielectric constant exhibits only within a very small temperature window because the phase transition region is narrow. Functionally graded material is suggested to enlarge the phase transition region. Here, both functionally graded ferroelectric ceramics lead titanate zirconate(PZT) and PZT with single composition are prepared. Dielectric property in two systems is measured and compared each other. Functionally graded ceramics PZT possesses a relatively high dielectric constant within a large temperature range. It is due to the FGM PZT has a wide phase transformation temperature region.

XRD study of the transformation characteristics of severely plastic deformed Ni‐Ti SMAs

AbstractSevere plastic deformation (SPD) of Ni‐Ti alloys (Ti‐rich and Ni‐rich) has been performed by High Pressure Torsion (HPT). Aim of the study was to follow the structural evolution in the samples subjected to SPD, in the martensite ↔ austenite phase transformation temperature region. HPT is carried out at room temperature (RT) on two separate Ni‐Ti alloy samples, Ti‐rich alloy with martensite finish temperature (Mf) above and Ni‐rich with austenite finish temperature (Af) below RT. The samples have been studied by in situ X‐ray diffraction (XRD) in order to get information about (i) the transformation characteristics and (ii) the structural evolution after annealing. The results showed that the Ti‐rich alloy retained one step phase transformation (Martensite ↔ Austenite) after HPT, as well as HPT followed by heat treatment. In the HPT condition, Ni‐rich alloy were found to undergo one step phase transformation (Martensite ↔ Austenite), whereas, heat treatment after HPT resulted in the presence of intermediate R‐phase transformation both while heating and cooling in the diffraction chamber. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase transformation and structural study on the severely plastic deformed Ni–Ti alloys

In an effort to obtain nanostructures in bulk Nickel-Titanium (Ni–Ti) shape memory alloys (SMAs) and to carryout their characterisation, samples of the alloys with different compositions have been chosen and subjected to severe plastic deformation (SPD). The selected Ti-rich alloy in the as-received (AR) condition has martensite finish temperature (Mf) above room temperature (RT) and Ni-rich has austenite finish temperature (Af) below RT. SPD method is found to be one of the efficient methods to attain nanostructures. SPD of Ni–Ti alloys (Ti-rich and Ni-rich) have been performed by High Pressure Torsion (HPT) at RT. The changes in the phase transformation temperatures before and after SPD are analyzed by studying the Differential Scanning Calorimeter (DSC) plots. Further, the structural evolution of the samples subjected to SPD in the phase transformation temperature region is studied using in situ X-ray diffraction (XRD) from −180 to +180∘C. Heat treated Ti-rich NiTi samples, previously subjected to HPT, show the same transformation characteristics (transformation temperatures and transformation sequence) as those of the as-received ones. But, Ni-rich alloy shows distinct intermediate R-phase transformation, both while heating and cooling, after subjecting it to HPT followed by heat treatments.

Effect of cyclic stress reduction on the creep behaviour of Al-40 wt%Zn alloy

The effect of cyclic stress reduction on the creep behaviour of A1-40 wt% Zn alloy during transformation was investigated. The influence of cyclic stress reduction of increasing amplitudes (18–47%) and frequencies (0.12–0.29 Hz) was systematically examined at various deformation temperatures (573–633 K). Both transient and steady state creep parameters, β , n and ε s · , were found to be higher under cyclic stress reduction condition than under the static condition for the same peak stress. Increasing temperature increased all the measured parameters. Increasing frequency and cyclic stress reduction increased the parameters n and ε s · but decreased β . The frequency was found to be more effective than the cyclic stress reduction amplitude. Activation energies for the transformation temperature region around 603 K, which were found to be independent on either frequency or amplitude of cyclic stress reduction but depend on the temperature range, characterized a dislocation mechanism involving grain boundary sliding or grain migration.

30 MHz ultrasonic attenuation of TiNi single crystal associated with B2-B19' martensitic transformation and twin boundaries

Ultrasonic attenuation was measured for the first time at megahertz range (30MHz) for a TiNi single crystal, which undergoes B2-B19' (cubic-monoclinic) martensitic transformation. A sharp attenuation peak was found at transformation temperature region (two-phase region), which was associated with the structural transformation and drastic domain wall evolution in the two-phase region. Besides, a very broad peak below the transformation temperature (i.e., in martensite state) was also found, which can be ascribed to twin boundary motion at megahertz range. This broad peak has a high internal friction level of 0.0027. Considering that this alloy also has a high damping capacity at low frequency, we can deduce that TiNi martensite is an excellent high damping material in an extremely wide frequency range and wide temperature range.

The stabilization of transformation pathway in stress induced martensite

It has been recently shown that two way shape memory effect/TWSME/ in shape memory alloys/SMS/ can withstand opposite external forces (the transformation strain induced by thermal change and external stress have opposite signs) up to some level and therefore do work. The reason comes from a strong thermodynamical asymmetry among possible crystallographic martensite variants due to a preceding ``training`` procedure. For example a simple repeated stress cycling proved to be a quite efficient training technique for Cu-based SMA. One of the highly accepted views on TWSME is the idea, that the dislocation configurations formed by stress cycling of SMA are of low energy in repeatedly stress induced variants, while of high energy in other variants. The result is the stabilization of a transformation pathway and the establishment of TWSME in subsequent thermal cycling across the transformation temperature region. When the temperature is lowered below the martensite start temperature M{sub s}, those ``trained`` variants nucleate and grow preferentially even if their growth is opposed by the external bias stress up to some level. Macroscopically, the progress of thermally of stress induced MT can be characterized by the sequence of macroscopic strain states taken by the specimen during the transformation process-transformation pathway.

Thermotropic behavior of hydroxyethyl cellulose acetate

AbstractThermotropic behavior of hydroxyethyl celloulose acetate (HECA) was studied by polarizing microscopy, DSC, and X‐ray diffraction. It was found that HECA was an anisotropic liquid between the temperature from 130 to 185°C on heating, and when the temperature was over 185°C, it became isotropic. In cooling, however, mesophase began to form at about 165°C. The transformation temperature decreased with decreasing molecular weight and the temperature region of transformation decreased with decreasing the distribution of the molecular weight for HECA. The kinetic ability of tranformation appeared the maximum with molecular weight. The HECA liquid crystal is cholesteric and the distance between layers of ordered molecular chains was enlarged when HECA was transformed from liquid crystalline glass to the liquid crystalline state. © 1994 John Wiley & Sons, Inc.

Transformation behaviour of NiTi in relation to thermal cycling and deformation

The transformation behaviour of an equiatomic NiTi alloy was investigated by measurement of the electrical resistivity and by differential scanning calorimetry. The temperature region of transformation depends on the number of thermal cycles and the intermediate phase appears progressively during cooling with increasing thermal cycles, resulting in a two-stage transformation, while the reserse transformation during heating remains a one-stage transformation. The temperature region of transformation is broadened by plastic deformation, and with subsequent annealing the intermediate phase is enhanced, depending on the annealing temperature. Effects of thermal cycling and annealing after deformation on the transformation of NiTi are attributable to transformation-induced defects as dislocations, and it is concluded that the defects make it possible to stabilize the intermediate phase.

Electronic structure and the martensitic transformation in beta -phase Ni-Al alloys: 27Al NMR and specific-heat measurements.

$^{27}\mathrm{Al}$ NMR line shape, Knight shift, relaxation rates together with low-temperature specific heat and magnetic susceptibiltiy were measured as a function of temperature and external magnetic-field in two Ni-rich \ensuremath{\beta}-phase Ni-Al alloys (62% Ni and 63% Ni) undergoing a martensitic phase transformation (MPT) below room temperature. For the purpose of comparison, two further alloys not undergoing the MPT were investigated (50% Ni and 55% Ni). Regarding the electronic properties of the alloys, we find an increase in the density of states at the Fermi level and a decrease of s-character of the conduction electron wave function as a function of the increase of Ni content from the 1:1 Ni-Al alloy. Furthermore, the inhomogeneous broadening of the $^{27}\mathrm{Al}$ linewidth indicates that the additional Ni replacing Al at the ``wrong'' site introduces quasilocalized magnetic states. Regarding the MPT we find a slight decrease of the Knight shift and of the Korringa product (${\mathit{T}}_{1}$T${)}^{\mathrm{\ensuremath{-}}1}$ going from the austenite to the martensite. In the transformation-temperature region, the $^{27}\mathrm{Al}$ NMR line is broadened by the effect of superposition of two signals arising from the austenite and the transforming phase. The deconvolution of the spectrum gives information about the nucleation and growth of the martensite, which appears to be continuous and involving a succession of intermediate states, in contrast with the abrupt nucleation of fully transformed martensite observed in Cu-Zn-Al. No anomalous enhancement of the relaxation rate is observed above ${\mathit{M}}_{\mathit{s}}$ or during the MPT. Although the NMR cannot rule out the presence of static precursor effects a few degrees above the transformation temperature ${\mathit{M}}_{\mathit{s}}$, no evidence was found for the formation of martensitic regions well above ${\mathit{M}}_{\mathit{s}}$ where tweed patterns are observed by electron microscopy.

Kinetics of phase transformation from hBN to cBN in the presence of Mg3BN3catalyst

Abstract Transformation kinetics from hBN to cBN has been studied at 6 GPa with changing reaction temperature and content of catalyst mixed with hBN powder. At lower catalyst content (3–10 wt %), rate of transformation was extraordinary rapid between 1300 and 1350 C. At higher catalyst content (30–50 wt %), rate of transformation was decreased. Normal nucleation kinetics was observed at about 1500 C. The rapid transformation temperature region correlates characteristic behavior of the sintering process.

TiNi形状記憶合金の粉末成形

Using pure titanium and nickele powders, a TiNi shape memory alloy was experimentally manufactured by the vacuum hot pressed powder metallurgy method. An influence of various hot pressed forming process on the homogeneity of alloy structure was investigated to fabricate practical device parts with the shape memory effect.The results are as follows.(1) The chemical diffusion of pure Ti and Ni mixed powder shows a thendency towards inferiority as compared with bulk materials. However, above 1273 K, the diffusion behavior of this powder, which has the liquid phase locally, was closer to that of bulk materials.(2) The homogeneity and density of TiNi alloy varied according to temperature and pressure as well as to the holding time. A density of 96-97% of the ideal value was obtained using a mixed powder of pure Ni and Ti processed by the vacuum hot pressed method at temperature 1323 K, pressure 35 MPa and holding time 3.6 ks.(3) The transformation temperature region (Ms-Af) shows slightly unsharpness as compared with that of molten material. But the result of mechanical and SME test confirmed that these alloys have extremely effective shape memory characteristics, that is, the recovery stress was 75-80% of that of molten material. Finally, as a practical example, we tried the fabrication of a spring washer and a pipe joint tube.