Heating-cooling Rates Research Articles

Structural-phase transformations of 12% chromium ferritic-martensitic steel EP-823

The features of phase transformations of 12 % chromium ferritic-martensitic steel EP-823 under heating and cooling conditions in the temperature range from 30 to 1100 ℃ were studied by the methods of high-temperature X-ray diffraction analysis (XRD) in situ and differential scanning calorimetry (DSC). According to XRD in situ data, upon heating, the temperatures of the beginning and end of the (α → γ) transformation of ferrite (martensite – austenite) are Ac1 ≈ 880 °C, Ac3 ≈ 1000 °C, respectively. Upon cooling, a diffusion (γ → α) transformation occurs with critical points – Аr1 ≈ 860°С (beginning temperature) and Аr3 ≈ 840 °С (end temperature). According to DSC data, during heating, the critical points of the (α → γ) transformation are Ac1 ≈ 840 °C and Ac3 ≈ 900 °C. During cooling, a martensitic (γ → α) transformation is realized with critical points of the beginning of Ms = 344 ℃ and the end of Mf = 212 ℃ of this transformation. The XRD in situ analysis revealed no precipitation of carbide phases under heating and cooling conditions of steel EP-823. Position of the critical points of phase transformations depends on the research method (XRD in situ or DSC), which is determined by the difference in effective (taking into account the time for shooting in the XRD method) heating-cooling rate. The effect of elemental composition on the position of critical points of phase transformations and the formation of structural-phase states of ferritic-martensitic steels is discussed. It is shown that the increased content of ferrite-stabilizing elements (Cr, Mo, Nb) in composition of EP-823 steel, compared with other steels of the same class, expands the region of existence of the ferrite phase, which can contribute to an increase in the temperature of Ac1 .

Effect of continuous retrogression and re-ageing treatment on mechanical properties, corrosion behavior and microstructure of an Al-Zn-Mg-Cu alloy

The mechanical properties, electrical conductivity, corrosion behavior and precipitation evolution of an Al-Zn-Mg-Cu alloy during the continuous retrogression and re-ageing (CRRA) treatment with slow heating-cooling rates are investigated systematically. The results reveal that the partial dissolution of pre-existing precipitates (GP Ⅰ zone and η' phase), and the growth of remaining precipitates, as well as the formation of new η' phase occur in the retrogression heating process, which are similar to the phase transformation in the retrogression stage of the discontinuous retrogression and re-ageing (DRRA) treatment. The isothermal retrogression stage of the CRRA treatment plays a key role for precipitates coarsening. The alloy retrogressed at 190 °C for 10 min and re-aged (RRA190–10 alloy) obtains high strength and excellent corrosion resistance, as well as high electrical conductivity simultaneously. Furthermore, the thermodynamic stability of the precipitates in RRA190–10 alloy (η' phase + η phase) is greater than that of T6 alloy (GP Ⅰ zone + η' phase). The microstructure features related to the property changes are discussed.

Experimental and numerical investigations on radiant floor heating system integrated with macro-encapsulated phase change material

Macro-encapsulated phase change material (PCM) with PCM as core and polymer as shell is reported an extremely stable energy storage medium in radiant floor heating system (RFHS). Herein, RFHS-based rooms with and without macro-encapsulated PCM are built, and the comfort and economy of the corresponding rooms are evaluated. Experimental results show that the room with macro-encapsulated PCM has longer thermal comfort time and slighter temperature deviation than without, according to the comfort standard. Then, a 3D unsteady heat transfer model is established and corrected by experimental data. Results show that macro-encapsulated PCM as temperature buffer layer regulates the heating-cooling rate of room and forms a semi-elliptic air radiant temperature zone. The room with macro-encapsulated PCM has lower daily electricity cost (c‾) of 0.2595 CNY/day and higher comfort time ratio of inside air (t‾) of 80.88%, decreased by 36.71% and increased by 15.43% than without, respectively. The thermal conductivities of shell and core of macro-encapsulated PCM are orthogonally optimized to be respectively 0.25 and 1.00 W/(m·K), with lowest c‾ (0.2480 CNY/day) and higher t‾ (85.47%), decreased by 4.43% and increased by 5.67% than pre-optimization. When the ambient temperature changes within −10∼10 °C, the input power of the room with macro-encapsulated PCM is orthogonally optimized to save c‾ by 35.91%∼39.39% than without.

Development of thermocyclic processing modes for carbon steels used on cold forming tools

In this paper, the effect of thermocyclic processing regimes on carbon tool steels used in cold stamping tools is considered. Thermocyclic processing (TCP) is based on the idea of repeated heating-cooling of steels at a temperature different from conventional heating. In this case, the values of temperature at high and low levels, the heating-cooling rate can be constant. Unlike traditional methods, in thermocyclic processing, each successive cycle occurs with a new structure and phase change. This makes it possible to significantly increase the properties of steel that cannot be achieved with conventional heat treatment due to the combination of different properties that are formed after the final cycle of thermocycling.

Effects of Concentration and Heating/Cooling Rate on Rheological Behavior of Sesamum indicum Seed Hydrocolloid

Hydrocolloids are known as natural hydrophilic biopolymers that can contribute viscosity and gelation in solution, as well as nutritional benefits, thus, they are widely used in the food industry. In our work, hydrocolloid was isolated by aqueous extraction of Sesamum indicum seed at 80 °C and pH 8.0. The chemical composition and functional properties of Sesamum indicum seed hydrocolloid (SISH) were characterized, and the effects of concentration including 1%, 2%, and 3% as well as heating/cooling rate (1, 5, and 10 °C/min) on the rheological behavior of SISH dispersions in aqueous solution were investigated. The viscoelastic properties of SISH dispersions were characterized by small-amplitude oscillatory shear measurement. The resultant SISH consisted of 60.95% carbohydrate and 23.32% protein, and was thus endowed with a relatively high water-holding capacity, solubility, appropriate emulsifying and foaming properties. Rheological results revealed that the aqueous dispersion of SISH exhibited a non-Newtonian shear-thinning flow behavior. The viscoelastic moduli changes were found to be dependent on SISH concentration, temperature, and heating/cooling rate. Increasing SISH concentrations from 1% to 3% promoted the development of stronger cross-link network. The mechanical spectra derived from strain and frequency sweep measurements showed that the storage moduli were always higher than the loss moduli, and the loss tangent was calculated to be above 0.1 and below 1.0. Furthermore, both moduli had slight frequency dependency, and the complex viscosity exhibited an almost linear reduction with the increase of frequency. Therefore, SISH dispersion behaved as a weak gel-like system. The hysteresis of viscoelastic moduli during heating and cooling reduced with decreasing the heating-cooling rates from 10 to 1 °C/min, suggesting that SISH molecules had enough time to develop a stable and thermally irreversible network. Overall, SISH can be regarded as an acceptable hydrocolloid for generating natural food components with intriguing functional and rheological qualities in the formulation of microstructured goods.

Combustion-driven synthesis route for bimetallic Ag–Bi nanoparticle-anchored carbon nanotube electrodes for high-performance supercapacitors

Bimetallic nanostructures within carbon-based materials can overcome the fundamental limits of energy materials, which cannot be obtained using a single material. However, their synthesis involves time-consuming and complex processes that cause phase/interface segregation and non-uniformly distributed metal elements. Herein, we report a facile combustion-driven synthesis for bimetallic Ag–Bi nanoparticle (NP)-anchored carbon nanotube (CNT) electrodes. One-step combustion wave passing through freestanding films comprising Ag2O and Bi powders, nitrocellulose layers within CNTs enables high-density thermochemical reactions in seconds. The rapid heating-cooling rates induce the formation of liquefied Ag–Bi and trapping of metastable Ag–Bi phases at the carbon surfaces, thereby synthesizing homogeneously mixed bimetallic Ag–Bi NPs anchored on the CNTs, along with smaller diameters (∼20 nm) and high distribution density. A supercapacitor electrode employing them exhibits outstanding specific capacitance and retention (1372-1093 Fg-1 at 2-5 mVs−1, and 101.3% of the stabilized capacitance after 10,000 cycles at 100 mVs−1). This was attributed to the large active site surface area from the small diameters and high distribution density of the bimetallic Ag–Bi NPs by low surface energy, and highly stable adhesion to the CNTs. The synthesis strategy can be extended to a scalable fabrication method of various multi-metallic nanostructures for versatile electrochemical electrodes and catalysts.

Electrothermally Driven Nucleation Energy Control of Defective Carbon and Nickel-Cobalt Oxide-Based Electrodes.

Multielement metal/metal oxides/carbon-based support hybrids are promising candidates for high-performance electrodes. However, conventional solid-state synthesis utilizing slow heating-cooling rates is limited by discrepancies in their phase transition temperatures. Herein, we report a rational strategy to control the nucleation energy of defective carbon fibers (DCFs) and Ni-Co-oxide-based electrodes capable of electrochemical activation using electrothermal waves (ETWs). The ETWs, triggered by Joule heating passing through CFs and Ni-Co precursors, induce programmable high-temperature processes via adjustable input powers and durations. The first ETW (∼1500 °C) fabricates the presculpted DCFs, while the second ETW (∼600 °C) directly synthesizes NiCo2O4 spinel nanoparticles on the DCFs. Predesigning DCFs through the Gibbs free energy theory enables tunable control of nucleation energy and solution compatibility with Ni-Co precursors, allowing the morphological and compositional design of the optimal NiCo2O4@DCFs hybrids. Furthermore, they are electrochemically activated to change the morphologies and oxidation states of Ni-Co to more stable wrinkled structures strongly anchored to carbon supports and Ni-Co cations with low oxidation numbers. The activated NiCo2O4@DCFs electrodes exhibit outstanding specific capacitance and long-term cyclic stability (∼1925 F g-1 and ∼115-123% for 20 000 cycles). The ETWs offer a facile yet precise method to predesign carbon supports and subsequently synthesize hybrid electrodes.

High-Temperature Flexible Transparent Heater for Rapid Thermal Annealing of Thin Films

Rapid heating is highly sought-after for compositional control in the sintering-annealing process of materials containing volatile elements. Although several innovative approaches have been reported, suitable processes for the rapid annealing of thin films are still very rare. Herein, we develop a high-temperature rapid heating method for thin-film preparation through employing an indium tin oxide (ITO)/mica-based flexible transparent heater (FTH). The ITO/mica FTH shows an extremely fast thermal response (2 s), high saturation temperature (>830 \ifmmode^\circ\else\textdegree\fi{}C), and high heating-cooling rates (>${10}^{4}$ \ifmmode^\circ\else\textdegree\fi{}C ${\mathrm{min}}^{\ensuremath{-}1}$). Thermodynamic analysis reveals that the excellent heating performance can be largely attributed to the high heat-transfer coefficient and the low thickness of the mica substrates. The remarkable potential of our ITO/mica FTH for practical use is unambiguously evident by the demonstration of water heating and rapid thermal annealing of ferroelectric oxide thin films.

Kakao yağının kristalleşme davranışı üzerinde kayma hızı ve soğutma hızının etkileri

The purpose of this study was to determine the effects of shear and cooling-heating rates on the rheological behavior of cocoa butter. Three different shear rates (25, 50 and 100 s-1) and two different cooling rates (1 and 10 °C.min-1) were applied for crystallization of cocoa butter at 20, 22 and 24°C. Also, effects of shear and heating-cooling rates were monitored during the cooling and heating cycle between 70-20°C. When the cooling rate was 1 °C.min-1, viscosity reached the highest value of 0.6 Pa.s with a shear rate of 25 s-1. A rapid rate of cooling generally led to nucleation at a lower temperature compared to slow cooling. It was observed that crystallization of Form V was improved by shear and induction time of crystallization decreased as the rate of shear increased. The longest induction period was obtained at 24°C. Effect of cooling rate was more significant at low temperatures (P < 0.05). It was also concluded that the crystallization behavior of cocoa butter was dependent on both shear and cooling rates under isothermal conditions.

Stability of Cu-islands formed on Si substrate via ‘dewetting’ under subsequent thermal cycling

Very thin metallic films deposited on a substrate often dewet upon thermal exposure, forming discrete islands of micrometer and nanometer-sized metal particles. Herein, Cu islands on Si substrate, which were formed due to agglomeration (or ‘dewetting’) of Cu thin film at 600 °C, were exposed to thermal cycling, and the ensuing evolution in their morphology was monitored. Thermal cycling was performed between either −25 °C and 150 °C or 25 °C and 400 °C, using different heating and cooling rates. With faster heating-cooling rates, a change in the shape and size of the Cu islands was observed, whereas a slow heating-cooling rate did not induce noticeable effect on their morphology. Furthermore, the formation of new nano- and micro-sized particles, probably through the dewetting of the ultra-thin layer of Cu that was left intact during the initial agglomeration treatment, was observed during the thermal cycling performed at fast rates up to 400 °C. Finite element analysis, incorporating Anand’s viscoplasticity model, revealed the existence of high strain energy density in the vicinity of the particle-Si interface when the thermal cycling is carried at a faster ramp rate, suggesting the pivotal role of thermal stresses, in addition to the maximum temperature, in controlling the morphology of the Cu particles and the dewetting of the residual ultra-thin layer of Cu on Si.

Nucleation kinetics of paramagnetic and diamagnetic metal melts under a high magnetic field

The influence of a high magnetic field (HMF) on the nucleation kinetics of paramagnetic aluminum and diamagnetic zinc melts has been investigated by differential thermal analysis (DTA). It is found that the application of an HMF increases the undercooling of pure aluminum and pure zinc at the same heating-cooling rates. Moreover, the quantitative analysis of activation energy calculated from the DTA results using the Kissinger method manifests that the HMF reduces the activation energy of pure aluminum and pure zinc. Regardless of magnetism, the nucleation frequency under an HMF is higher than that without an HMF. Furthermore, the increase in undercooling under the HMF is mainly attributed to the increase of the contact angle, calculated by the functional relationship between the cooling rate and undercooling. This result is consistent with the increase of the calculated nucleation work for pure aluminum and pure zinc. Additionally, the increase in undercooling caused by the HMF is partly ascribed to the magnetic field-induced suppression of thermal convection in the undercooled melt.

Features of Phase Transformations of Low-activation 12%-Chromium Ferritic-Martensitic Steel Ek-181

An investigation of the features of structural-phase transformations during heating and cooling of the lowactivation 12%-chromium ferritic-martensitic steel EK-181 (Fe–12Cr–2W–V–Ta–B) is performed in the range of temperatures from 30 to 1100°C by the methods of high-temperature X-ray diffraction analysis (XRD) in situ. The critical points of an (α →γ) - transformation (Ас1 ≈ 850°С, Ас3 ≈ 950°С) in the course of steel heating and the temperature interval (910–890°C) of the diffusion-induced transformation of the austenite →α-ferrite type under cooling are determined. It is shown that the position of these points changes depending on the method (XRD or differential scanning calorimetry) of their identification, which is attributed to the differences in their heating-cooling rates. During heating in the range of temperatures of 830–875°C, a significant increase in the volume fraction of М23С6 carbides is observed. Further temperature increase up to 1000–1100°C leads to their complete dissolution. In the course of cooling of the steel specimens from 1100 to 30°C, there is no carbide particle precipitation.

Effect of heating-cooling rates on microstructure evolution of ZrB2-based coatings during oxidation

In this work, the influence of heating-cooling rates on the microstructure of ultra-high temperature ceramics (UHTCs) during oxidation was firstly reported. Four different kinds of processes (low-heating and low-cooling rates, low-heating and high-cooling rates, high-heating and low-cooling rates, high-heating and high-cooling rates) combined with static oxidation at 1500 °C were applied on ZrB2-SiC-WB composite coatings. The results showed that the oxide layers of the samples presented different microstructure characters under different heating-cooling modes. A relatively thick liquid layer with obvious bubble phenomenon was generated under the high-heating rate, while a thin liquid layer without bubbles was formed under the low-heating rate. Certain amount of sheet B2O3 crystals with large sizes were formed under the high-cooling rate, while did not for the low-cooling rate. The formation mechanisms of these unique microstructures were analyzed and the influence of heating and cooling rates was explained.

Thermal shock resistance of continuous carbon fiber reinforced ZrC based ultra-high temperature ceramic composites prepared via Zr-Si alloyed melt infiltration

Thermal shock resistance of continuous carbon fiber reinforced ZrC based ultra-high temperature ceramic matrix composite (C/C-ZrC) from ultra high temperatures (particularly > 1500 °C) to the room temperature was evaluated using a novel self-developed equipment with high heating-cooling rates in controllable atmosphere. Residual strength and mass variation of the as-prepared composite under different thermal shock cycles and temperatures were tested to characterize the thermal shock resistance. Flexural strength of the composite slightly decreases initially without obvious weight loss below 1300 °C, while it decreases by relatively high values at 1300–1900 °C and then sharply decreases over 1900 °C with obvious increase of weight loss. Microstructure of the thermally shocked specimens was examined to reveal the thermal shock damage. Matrix cracking, interfacial debonding and matrix pores were clearly observed, which were the main reasons for the strength degradation of the composite thermally shocked for different cycles and temperatures.

Non-isothermal creep aging behaviors of an Al-Zn-Mg-Cu alloy

To shorten production period and reduce manufacturing cost of high strength aluminum alloy integral panel components, a novel non-isothermal creep age forming (CAF) technology was developed. Taking an Al-Zn-Mg-Cu alloy as the case material, the non-isothermal creep aging (NICA) behavior and the conventional isothermal creep aging (ICA) behavior were comparatively investigated. The corresponding creep deformations, material performance variations and precipitate microstructure evolutions were studied by creep aging tests, mechanical property and corrosion resistance tests, and transmission electron microscope (TEM) and scanning electron microscope (SEM) characterizations. Unlike the ICA which contains short heating and cooling stages and long holding stage, the NICA only has heating and cooling stages, viz., the alloy is heated slowly and the cooling stage begins immediately when reaching the peak temperature. In the NICA process, the precipitates nucleate and grow up in the first half of heating stage, resulting in the reduction of creep rate and enhancement of mechanical strength. The coarsening of precipitates occurs in the second half of heating stage, elevating the creep rate and significantly improving the corrosion resistance. Nearby the peak temperature, the primary precipitates partially dissolve, and the creep rate, mechanical strength and corrosion resistance decrease. Distinctive from the ICA process, a secondary precipitation phenomenon takes place in the cooling stage of NICA, leading to the recovery of high mechanical strength. The higher heating-cooling rate (1 °C/min) is not beneficial to accumulating creep deformation in the shorter creep aging period, while the lower one (0.25 °C/min) causes the loss of mechanical strength due to the excessive dissolution of precipitates. The moderate heating-cooling rate (0.5 °C/min) can lead to larger creep deformation, better mechanical properties and more satisfactory corrosion resistances in the NICA treated alloy, reaching 87.0% of creep strain, 105.7% of yield strength and 97.9% of electric conductivity in the ICA treated alloy. It is worth noting that the time consumption of the optimal NICA process is only 52.9% of the ICA process.

Premelting, Melting, and Degradation Properties of Molten Alkali Nitrates: LiNO3, NaNO3, KNO3, and Binary NaNO3-KNO3

A simultaneous thermal analyzer (STA) was used to observe the transition and degradation events of LiNO3, NaNO3, KNO3, and binary NaNO3-KNO3 salts for potential use as phase change materials (PCMs) and heat transfer fluid (HTF). Samples were heated from 50 °C to 800 °C at 10 °C/min scanning rate in three atmospheres (argon, air, and oxygen) using an STA to observe decomposition behavior. Thermal stability increased for all salts at high partial pressure of O2 ( $$ P_{{{\text{O}}_{2} }} $$ = 1.0) compared to inert argon ( $$ P_{{{\text{O}}_{2} }} $$ = 0). O2, N2, NO, N2O, and NO2 were main evolved gases during nitrate decomposition. NO and O2 started to evolve at approximately the same temperature after melting, indicating that primary and secondary decomposition reactions were concurrent and overlapping. The solid-solid transition, liquidus and solidus temperatures, heat of transition, heat of melting, and heat of solidification were obtained at various heating-cooling rates (1, 2, 4, 5, 6, 8, 10, and 15 °C/min) using an STA. At all heating-cooling rates, a small gap exists between liquidus and solidus temperatures for all samples due to the salts exhibiting supercooling phenomena. This study showed that the degradation point depends on the blanket atmosphere top of the molten salts and that heating rates have a minor effect on transition events (peaks height, peaks width, and transition enthalpies).

Steady and dynamic shear rheological behavior of semi dilute Alyssum homolocarpum seed gum solutions: influence of concentration, temperature and heating-cooling rate.

Alyssum homolocarpum seed gum (AHSG) solution exhibits high viscosity at low shear rates and has anionic features. However there is no information regarding the flow and dynamic properties of this gum in semi-dilute solutions. The present study aimed to investigate the dynamic and steady shear behavior of AHSG in the semi-dilute region. The viscosity profile demonestrated a shear thinning behavior at all temperatures and concentrations. An increase in the AHSG concentration was acompanied by an increase in the pseudoplasticity degree, whereas, by increasing the temperature, the pseudoplasticity of AHSG decreased. At low gum concentration, solutions had more viscosity dependence on temperature. The mechanical spectra obtained from the frequency sweep experiment demonstrated viscoelastic properties for gum solutions. AHSG solutions showed typical weak gel-like behavior, revealing G' greater than G' within the experimental range of frequency (Hz), with slight frequency dependency. The influence of temperature on viscoelastic properties of AHSG solutions was studied during both heating (5-85 °C) and cooling (85-5 °C) processes. The complex viscosity of AHSG was greater compared to the apparent viscosity, indicating the disruption of AHSG network structure under continuous shear rates and deviation from the Cox-Merz rule. During the initial heating, the storage modulus showed a decreasing trend and, with a further increase in temperature, the magnitude of storage modulus increased. The influence of temperature on the storage modulus was considerable when a higher heating rate was applied. AHSG can be applied as a thickening and stabilizing agents in food products that require good stability against temperature. © 2017 Society of Chemical Industry.

The effects of concentration and heating-cooling rate on rheological properties of Plantago lanceolata seed mucilage

In this study, the effect of concentration (0.5, 1, 1.5 and 2%) and heating-cooling rate (1, 5 and 10 °C min−1) on the rheological properties of Plantago lanceolata seed mucilage (PLSM) solutions were investigated. It was observed that the gum dispersions exhibited viscoelastic properties under the given conditions. Mechanical spectra of PLSM were classified as weak gels based on the frequency sweep, complex viscosity (η*) and tan δ results. All variables had significant impacts on the rheological parameters. Chemical and monosaccharide compositions were also determined to provide more structural information. The results revealed that PLSM had high total sugar content (87.35%), and it is likely an arabinoxylomannan-type polysaccharide.

Thermal analysis of molten ternary lithium-sodium-potassium nitrates

The pre-melting, melting and degradation events of a ternary LiNO3-NaNO3-KNO3 (29.63-13.23-57.14 wt%) salt have been investigated using simultaneous thermal analyzer (STA) for potential use as a heat transfer and storage fluid (HTF). This composition was selected based on the thermochemical calculation using the FactSage 7.0 to find the lowest solid-liquid equilibrium temperature. The melting point obtained from STA (122.8 °C) indicated a good agreement with the theoretical value determined by FactSage (120.8 °C), which is 100 °C less compared to binary NaNO3:KNO3 solar salt. The salt was heated from 50 to 800 °C in 3 atm (argon, air, and O2) at three scanning rates (2.5, 5 and 10 °C/min) to investigate its decomposition limits. At 10 °C/min scanning rate the salt started to degrade rapidly at 545 °C in argon, 571 °C in air and 600 °C in oxygen, respectively. In argon at a higher heating rate (10 °C/min), the salt degradation was delayed by 42 °C. The main gaseous products of decomposition were found to be O2, N2, NO, N2O and NO2. After melting, at higher temperature the evolution of O2 and NO indicated that primary and secondary decomposition reactions are concurrent and overlapping. The heating-cooling rates have low effect on the onset of melting and offset freezing temperatures but, have an effect on peaks height, peaks width, and transition enthalpies. The α/β transition (75 °C peak temperature) during cooling only appeared at the 1 °C/min cooling rate. The ternary salt offers 70 °C greater operating temperature range compared to binary solar salt in argon. At a higher partial pressure of O2, decomposition was delayed by 55 °C (at 10 °C/min), which could be used to increase the power cycle efficiency of solar electricity generation.

Internal Friction and Dynamic Modulus in High Temperature Ru-Nb Shape Memory Intermetallics

ABSTRACTNowadays, aeronautic and aerospace are the more demanding sectors for shape memory alloys (SMA) after the bio-medical one. In particular the interest has been recently focused on very high temperature SMA, which would be able of working as sensors and actuators in the hot areas of the engines and exaust devices.In the present work we undertook a study of the Ru-Nb SMA Intermetallics, which undergo two succesive martensitic transformations around 1050 K and 1180 K respectively, depending on composition. This study has been focused on measurements of internal friction spectra and dynamic modulus variation up to 1700 K, which have been carried out in a sub-resonant torsion mechanical spectrometer.The internal friction and dynamic modulus have been studied as a function of the heating-cooling rate and the frequency in order to compare experimental behaviour with theoretical models for martensitic transformations. In addition to the internal friction peaks linked to both martensitic transformations we have also observed a complex relaxation process around 950 K, which seems to be linked to the interaction of the martensite interfaces with structural defects. An analysis and discusion of the potential microscopic mechanisms are also presented.