Low-temperature Martensitic Phase Research Articles

Effect of Segregation Band on the Microstructure and Properties of a Wind Power Steel before and after Simulated Welding

This article uses scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD) to study the effect of C and Mn segregation on the microstructure and mechanical properties of high-strength steel with 20 mm thickness used for wind power before and after simulated welding. A Gleeble-3500 (GTC, Dynamic Systems Inc., Poestenkill, NY, USA) was used to study the microstructure evolution of the simulated coarse-grained heat-affected zone (CGHAZ) of experimental steel under different welding heat inputs (10, 14, 20, 30 and 50 kJ/cm) and its relationship with low-temperature impact toughness (−60 °C). The results indicate that alloy element segregation, especially Mn segregation, significantly affects the impact toughness scatter of the steel matrix, as it induces the formation of low-temperature martensite or hard phase, such as M/A (martensite/austenite) constituent. In addition, segregation also reduces the low-temperature impact toughness of the simulated welding samples and increases the fluctuation range. For high-strength steel with yield strength higher than 460 MPa used for wind power generation, there is an optimal welding heat input (~20 kJ/cm), which enables the simulated coarse-grained heat-affected zone (CGHAZ) to obtain the highest impact toughness due to the formation of lath bainite (LB) and the finest crystallographic block units. Excessive or insufficient heat input can induce the formation of coarse granular bainite (GB) or lath martensite (LM), leading to a larger size of crystallographic block units, reducing the hindering effect of brittle crack propagation and deteriorating low-temperature impact toughness.

First-principles study on the effect of Ni addition on the stability of B2 Ti50Ru50 – a supercell approach

Shape memory alloys (SMA's) are a special kind of metal alloys with unique structural properties, such as shape memory effect (SME) and pseudo-elasticity (SE) that were well-observed in TiNi alloy, which commercially trades as Nitinol. This polymorphic crystal behaviour arose from the reversible phase transition between an ordered high-temperature B2 and low-temperature martensite phase (L10/B19/B19′). Due to the high ductility of Ru, this work employed a supercell approach to track the effect of nickel addition on highly stable B2-Ti50Ru50 with the intent to stimulate SME. Thermodynamic, mechanical, electronic structure and lattice dynamic characteristics were estimated using first-principles calculations. The phonons spectra of B2 Ti50Ru50 consisted of only positive frequencies, but the addition of Ni above 18.75 at. % resulted in positive and negative frequencies, indicative of a possible martensitic transformation, which is a strong character for SME.

Magneto-structural coupling induced multiple magnetic transitions and magnetocaloric effect of Ni43Mn46Sn9Ge2 alloy

Multiple magnetic and structural phase transitions and magnetocaloric effect of Ni43Mn46Sn9Ge2 Heusler alloy were detailedly investigated. The martensitic and inverse-martensitic transitions occur at temperatures slightly below room temperature, which induces a ferromagnetic - antiferromagnetic transition and large inverse magnetocaloric effect. A paramagnetic - ferromagnetic and antiferromagnetic - ferromagnetic transition was observed in high-temperature austenitic and low-temperature martensitic phase, respectively. The evident thermomagnetic irreversibility at low temperatures of the martensitic phase suggests a reentrant spin glass state, which originates from the frustration of the competing ferromagnetic and antiferromagnetic spins and results in the visible exchange bias, large coercivity and double-shifted hysteresis behaviors. A T-μ0H phase diagram was finally constructed.

Detection of structural phase transition in SMART microwires using Magneto-Impedance sensing

• Correlation between the structural transition and magneto-impedance is shown. • A well-defined variation (6%) of magneto-impedance effect is appeared. • Magneto-Impedance effect can be used for phase\strain detection in SMA. We report on the possibility to use magneto-impedance effect to study the structural phase transformation of glass coated Ni 2 FeGa microwires. These microwires show martensitic phase transition within the temperature range from 300 to 350 K. Hysteresis loops confirm the variation of easy magnetization axis direction connected to martensitic transformation . Being parallel to the wire’s axis in high-temperature austenite phase, it is tilted a little bit out of the easy axis for low temperature martensite phase. The temperature dependence of magnetoimpedance effect shows well-defined variation (6%) of impedance that appears within the temperature range of structural transformation as a result of the easy axis variation. These properties give the microwire function of very sensitive SMART actuators since they have also sensing functionality due to the possibility to use magneto-impedance for sensing the structural transformation.

Doping site induced alteration of incommensurate antiferromagnetic ordering in Fe-doped MnNiGe alloys

The magnetic structure of Fe-doped MnNiGe alloys of nominal compositions ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ has been explored through detailed neutron powder diffraction (NPD) study in ambient and high-pressure (6 kbar) conditions. Both these alloys crystallize with ${\mathrm{Ni}}_{2}\mathrm{In}$-type hexagonal structure (with $P{6}_{3}/mmc$ space group) at room temperature and undergo a martensitic transition to low-temperature TiNiSi-type orthorhombic structure (with space group $Pnma$). Both the alloys show incommensurate antiferromagnetic ordering at the low-temperature martensitic phase. However, the modulation of the magnetic structures depends strongly on the doping site of Fe. The incommensurate propagation vector $\mathbf{k}$ for ${\mathrm{MnNi}}_{0.75}{\mathrm{Fe}}_{0.25}\mathrm{Ge}$ and ${\mathrm{Mn}}_{0.85}{\mathrm{Fe}}_{0.15}\mathrm{NiGe}$ alloys are found to be (0.1790(1),0,0) and (0.1543(3),0,0), respectively at 1.5 K, and these remain almost unchanged with increasing sample temperature under ambient conditions. The application of the external pressure results in a significant effect on both martensitic transition temperature and low-temperature magnetic structure. The propagation vectors $\mathbf{k}$ for both the alloys show a monotonic decrease with increasing sample temperature in the presence of external pressure. Interestingly, no sign of the magnetically ordered hexagonal austenite phase was observed in any alloys down to the lowest measurement temperature.

Long- and short-range order in the Ni52Co2Fe20Ga26 ferromagnetic Heusler alloy

The crystalline structure and Fe local environment in a Co-doped Ni–Fe–Ga Heusler alloy, prepared by the melt-spinning technique, were investigated by X-ray diffraction (XRD) and EXAFS at room and low temperatures. The characteristic temperatures of the austenite–martensite phase transitions were determined by differential scanning calorimetry via cooling and heating cycles of the alloy ribbons. As shown by room-temperature XRD, the austenitic phase of the alloy has the chemically ordered L21 Heusler structure. This was confirmed by EXAFS, although this technique was not able to conclusively distinguish between the L21 and B2 structures of the austenite for the analyzed alloy. The low-temperature martensitic phase and its structural evolution towards austenite with increasing temperature were studied by high-energy X-ray diffraction, which evinced the martensite modulation. However, the Fe environment could be fitted by EXAFS with the tetragonal L10 structure of the non-modulated martensite. This proves that the martensite modulation has structural effects on a long-range scale, without significant changes in the short-range order around the atoms. The changes in the local structure around iron on martensitic transformation were correlated with changes in the electronic structure, described by XANES spectroscopy at the Fe K edge.

In-situ strain measurement of shape memory alloy fiber during the austenitic and martensitic phase transformation

Fibers made from shape memory alloys perform strain recovery during phase transformation of crystal structure. Controlled parameters such as heating time, holding temperature, and cooling time dominate the micro deformation of shape memory alloy fiber (SMAF). Mechanical contraction occurs at high temperature (austenitic phase), and strain-relief happens at low temperature (martensitic phase). But the rapid change of temperature causes unfinished or incomplete phase transformation, which induces residual strain within the SMAF. The accumulated defects shorten the specimen’s life during cyclic loading and lower the limit of fatigue failure. This study uses digital image correlation to provide the in-situ response temperature-strain relations during the phase transformation and quantizes the residual strain within the SMAF. Experiments accurately control the holding temperature of SMAF in three conditions, which are (a) under, (b) around, and (c) beyond the phase transformation temperature of the specimen, and systematically analyzes the self-accommodation and strain-recovery of SMAF in the three experiments. The in-situ measurement of microscale deformation can help the end-user optimize the control parameters and increase the life and performance of SMAF.

Martensitic transition assisted modification of the antiferromagnetic ordering in Ge-site doped MnNiGe alloys

The magnetic equiatomic alloys of the nominal compositions ${\mathrm{MnNiGe}}_{0.9}{\mathrm{Al}}_{0.1}$ and ${\mathrm{MnNiGe}}_{0.928}{\mathrm{Ga}}_{0.072}$ have been investigated through the neutron powder diffraction (NPD) technique. Both these alloys undergo a martensitic phase transition from the high-temperature hexagonal (${\mathrm{Ni}}_{2}\mathrm{In}$ type, with space group $P{6}_{3}/mmc$) austenite phase to the low-temperature orthorhombic (TiNiSi type, with space group $Pnma$) martensite phase. Our NPD analysis confirmed an incommensurate antiferromagnetic ordering of the low-temperature martensitic phase with helical modulation. The incommensurate propagation vector $\mathbf{k}$ is found to be (0.2065(1),0,0) and (0.2088(4),0,0) for Al and Ga doped alloys, respectively, at 1.5 K and shows a monotonic increase with increasing sample temperature. On the other hand, a commensurate antiferromagnetic ordering was observed for the high-temperature austenite phase. The incomplete martensitic transition allowed both these incommensurate and commensurate antiferromagnetic structures to coexist down to the lowest temperature of measurement (1.5 K).

Specific Features in Mechanical Behavior of Ring-Shaped Force Drives Operating in the Two-Way Shape Memory Effect Regime

We have analyzed the possibility of improving deformation–force properties of ring-shaped drives made of titanium nickelide and operating in the regime of two-way shape memory effect (TWSME). For this purpose, thermocycling of an oval sample was used with the transformation of the material from the low-temperature martensite phase to the high-temperature austenite phase in free state and cooling via the temperature interval of the transformation under a load. The possibility of generation of force in ring-shaped structures changing shape during heating as well as during cooling due to the TWSME process is demonstrated. The reported results indicate asymmetry of these properties relative to the direction of temperature variation. It is shown that the cooling rate substantially affects the deformation and force properties of ring-shaped drives.

Martensitic Transition, Magnetic, Microstructural and Exchange Bias Properties of Melt Spun Ribbons of Mn-Ni-Sn Shape Memory Heusler Alloy

In the present report, we have studied the structural, microstructural, magnetic, exchange bias (EB) properties and magnetoresistance (MR) in Mn rich (Mn at. ∼50%) Mn50Ni50−xSnx (x = 10) shape memory Heusler alloy ribbons, prepared using the melt spinning method. These ribbons were found to exhibit a first order structural (i.e., martensitic) transition at around 205 K from the high temperature austenite to the low temperature martensite phase. The martensitic transition occurs at comparatively lower temperatures in these ribbons than in the same bulk alloy. Curie temperature of the austenite phase (TCA) is found to be larger i.e., around room temperature than that of bulk alloy. A significant EB field (HEB) of around 960 Oe has been observed at 2 K for these ribbons, which is found to be comparable to that reported for other Heusler systems. The presence of the EB effect in these ribbons is attributed to the coexistence of FM/AFM exchange interactions in the martensite phase. They are also found to show a maximum negative MR of around 12% near the martensitic transition, for 50 kOe field change. Investigation of DC magnetization, AC susceptibility measurements and the observation of training effect (i.e., characteristic feature of EB) strongly corroborates with the coexistence of FM/AFM exchange interactions in the martensite phase of these ribbons, which results in the large EB. The effects of temperature and magnetic field on the EB properties and MR have also been studied here.

Особенности механического поведения силовых приводов кольцевой формы, действующих в режиме обратимой памяти формы

The possibility of increasing the deformation-power properties of TiNi ring-shaped drives operating in the two-way shape memory effect (TWSME) has been studied. For this purpose, thermal cycling of an oval sample was used with the transfer of material from the low-temperature martensite phase to the high-temperature austenitic phase in the free state and cooling through the transformation temperature range under load. The possibility of force generation in ring-shaped structures, the shape change of which is caused by the TWSME process, both during heating and during cooling, is shown. Data on the asymmetry of these properties with respect to the direction of temperature change are presented. The significant influence of the cooling rate on the deformation-power properties of the ring-shaped drives is shown.

Temperature versus composition phase diagram and temperature evolution of structure and modulation of Ni2MnGa1-xInx single crystals

A series of single crystal Ni2MnGa1-xInx samples, varying in compositions between x = 0 to 0.15, were grown by Bridgman method. Critical temperatures were obtained from the magnetisation and electrical resistivity measurements. A Curie temperature and the temperatures of pre-martensitic and martensitic transformation decrease with increasing indium content. The decrease rate is faster than the rate published previously on polycrystalline samples, since no martensitic transformation was observed in our samples with the indium content x > 0.05. The results of X-ray diffraction measurements of low temperature martensitic phase show significant decrease of monoclinic lattice parameter a and increase of monoclinic lattice parameter c and γ angle with respect to increasing indium content. Composition dependence was observed also for the modulation vector in martensitic and pre-martensitic phase, where its x and y coordinates gain values around 3/7 and 1/3 respectively.

Temperature Dependence of Anisotropy in Ti and Gd Doped NiMnGa-Based Multifunctional Ferromagnetic Shape Memory Alloys.

The temperature dependence of magnetocrystalline anisotropy was investigated in detail for the polycrystalline Ni50Mn25Ga25, Ni50Mn25Ga20Ti5 and Ni50Mn25Ga20Gd5 ferromagnetic shape memory alloys in the temperature range of 50–400 K. The effective anisotropy constant was estimated from a series of high field magnetization curves based on the fitting procedure according to the law of approach to magnetic saturation. The low temperature martensitic phase was found to have a significantly higher anisotropy energy in comparison to a high temperature austenitic phase, which was observed through a sudden, distinct drop of anisotropy energy. The calculated values of the effective anisotropy constant were comparable to the results published by other authors. Moreover, the strong influence of chemical composition on the first-order phase transition and the second-order ferromagnetic to the paramagnetic transition was revealed. Finally, the strong coupling between the temperature dependence of the coercive field and the temperature dependence of magnetocrystalline anisotropy was also shown and discussed in the present study.

Magnetic field dependence of the martensitic transition and magnetocaloric effects in Ni49BiMn35In15

The structural, magnetic, and magnetocaloric properties of the Bi-doped Heusler alloy Ni49BiMn35In15 have been investigated using room temperature X-ray diffraction (XRD) and magnetization measurements in a temperature interval of 5-400 K. The alloy at room temperature was found to be in a mixture of a high temperature austenite phase (AP) and a low temperature martensite phase (MP). A drastic shift in the martensitic transition temperature at the rate of 16 K/T from 197 K to lower temperatures was observed. A kinetic arrest phenomenon of the AP was observed in the magnetization and electrical resistivity measurements during field-cooled (FC) measurements at 5T. A metamagnetic behavior characterized by a jump in magnetization in the isothermal M(H) curves near TM was observed. The maximum value of the magnetic entropy change and refrigerant capacity at Curie temperature were found to be 5.5 Jkg-1K-1 and 312 Jkg-1 for μoΔH = 5T, respectively. A large magnetoresistance value of -56% was found near the martensitic transition.

Magnetic ordering of the martensite phase in Ni–Co–Mn–Sn-based ferromagnetic shape memory alloys

The magnetic state of low temperature martensite phase in Co-substituted Ni–Mn–Sn-based ferromagnetic shape memory alloys (FSMAs) has been investigated, in view of numerous conflicting reports of occurrences of spin glass (SG), superparamagnetism (SPM) or long range anti-ferromagnetic (AF) ordering. Combination of DC magnetization, AC susceptibility and small angle neutron scattering (SANS) studies provide clear evidence for AF order in the martensitic phase of Ni45Co5Mn38Sn12 alloy and rule out SPM and SG orders. Identical studies on another alloy of close composition, Ni44Co6Mn40Sn10, point to the presence of SG order in the martensitic phase and the absence of SPM behavior, contrary to earlier reports. SANS results do show the presence of nanometre-sized clusters but they are found to grow in size from 3 nm at 30 K to 11 nm at 300 K, and do not correlate with magnetism in these alloys.

Magnetocaloric effect and large refrigerant capacity at paramagnetic to ferromagnetic transition in Ni50Mn33In17

The structural and magnetic properties of the austenite phase of Ni50Mn33In17 are investigated by means of X-ray diffraction and DC magnetization measurements. The room temperature XRD measurements confirmed the L21 Heusler cubic structure belonging to space group Fm3m with some leftover low temperature martensite phase which disappeared by heating the material upto 400 K. A large normal magnetocaloric effect was observed over a wide range of temperature in the vicinity of the paramagnetic to ferromagnetic transition with magnetic entropy change Δ SM of ∼6.4 J/kg-K around 300 K for a field change of 0–60 kOe. For the same magnetic field variation, a large refrigerant capacity of ∼300 J/kg is also observed. The obtained results show that the paramagnetic to ferromagnetic transition in Ni50Mn33In17 may be more suited for magnetic refrigeration applications than the related magnetostructural transition that occurs at a lower temperature.

Structural, magnetic and magnetocaloric effect studies of Ni42.5(Fe, Co, Ni, Cu)0.5Mn46Sn11 Heusler alloys

We investigated the structural and magnetic properties of Ni42.5(Fe, Co, Ni, Cu)0.5Mn46Sn11 alloys fabricated by arc melting. Substitution of Ni by Fe, Co and Cu causes lattice expansions consistent with increasing atomic sizes. The zero-field cooled and field cooled results show second-order magnetic transition at the high-temperature austenite phase to a first-order magnetic transition in the low-temperature martensite phase. The substitution of Ni by Fe and Co increases the austenite Curie temperature TCA from 282 K to 289 K and 294 K respectively while Cu reduces it to 278 K. The martensitic transition temperature TM increased from 221 K to 241 K for Fe substitution and decreased to 210 K and 209 K for Co and Cu respectively. The coercive field HC increased significantly from 457 Oe for Ni at 100 K to 729 Oe for Co at 80 K. The increase to 763 Oe for Fe and 769 Oe for Cu occurred at the same temperature of 40 K. We attribute such increases to domain wall pinning effects due to the inclusions of Fe, Co and Cu. The HC exhibited an anomalous temperature dependence in all the samples. The exchange bias field HEX also showed a significant enhancement below 40 K from 196 Oe for Ni to 476 Oe, 430 Oe and 434 Oe for Fe, Co, and Cu substitutions respectively. The fits to the temperature dependence of the HC reveal significant changes in the competition between ferromagnetic and antiferromagnetic interactions. The peak magnetic entropy change ΔSMpk has a linear dependence on the magnetic field H2/3. The highest value of 28.8 J kg-1 K−1 for ΔSM is obtained in the first order magnetic transition compared to 3.0 J kg-1 K−1 in the second order transition. We report an effective cooling power of 155 J kg-1 in the second order magnetic transition.

Effect of Bi substitution on the magnetic and magnetocaloric properties of Ni50Mn35In15-xBix Heusler alloys

The structural, magnetic, magnetocaloric, and transport properties of Ni50Mn35In15-xBix (x = 0, 0.25, 0.5, 1, 1.5) compounds has been studied through X-ray diffraction (XRD), differential scanning calorimetry, and magnetization measurements. A mixture of high temperature austenite phase (AP) and low temperature martensitic phase (MP) was observed from the XRD at room temperature. The saturation magnetization MS at 10 K was found to decrease with increasing Bi content. A shift in the martensitic transition temperature (TM) relative to the parent compound was observed with a maximum shift of ∼ 36 K for x = 1.5. Abnormal shifts in TC and TM to higher temperatures were observed at high field for x ≥ 0.5. Large magnetic entropy changes (ΔSM) of about 40 J/kg K (x = 0) and 34 J/kg K (x = 0.25) were observed at TM with H = 5 T, which reduced significantly for higher Bi concentrations. The doping of small amounts of Bi in the In sites increased the peak width of the ΔSM curves at the second order transition, leading to larger values of relative cooling power. A significant magnetoresistance (-30%) was observed near TM with ΔH = 5T for x = 0.5.

Adaptive tuned vibration absorber based on magnetorheological elastomer-shape memory alloy composite

Abstract Shape memory alloy (SMA) is an attractive smart material which could be used as stiffness tuning element in adaptive tuned vibration absorber (ATVA). The sharp modulus change in SMA material during phase transformation creates difficulties for smooth tuning to track forcing frequency to minimize vibrations of primary system. However, high hysteresis damping at low temperature martensitic phase degrades performance of vibration absorber. This paper deals with the study of dynamic response of system in which SMA and magnetorheological elastomer (MRE) are combined together to act as a smart spring- mass-damper system in a tuned vibration absorber. This composite is used as two way stiffness tuning element in ATVA for smooth and continuous tuning and to minimize the adverse effect at low temperature by increasing equivalent stiffness. The stiffnesses of SMA element and MRE are varied respectively by changing temperature and strength of external magnetic field. The two way stiffness tuning ability and adaptivity have been demonstrated analytically and experimentally. The experimental results show good agreement with analytical results. The proposed composite is able to shift the stiffness consequently the natural frequency of primary system as well as reduce the vibration level of primary system by substantial mount.

A Combinatorial Study for Interdiffusion, Crystallography and Mechanical Behavior of Ni-Mn-Ga Alloys

The ferromagnetic shape memory and magnetocaloric properties of NiMnGa alloys are closely related to the martensitic transformation from high temperature austenitic phase to low temperature martensitic phase. The transformation temperature and the resulting microstructure and crystallography of the martensites can be very complex, but are crucial to the optimization of the material performance. A combinatorial study with a series of solid-to-solid diffusion couples and various characterization techniques, including scanning electron microscopy, focused ion beam, transmission electron microscopy, electron probe microanalysis, and nanoindentation, was carried out to investigate the microstructural and crystallographic development, and mechanical properties in NiMnGa alloys. Both austenitic and martensitic phases were found at room temperature in each diffusion couple with a clear interphase boundary. Crystallographic variations in martensitic phase, including non-modulated (NM) martensite and modulated (5M or 7M) martensite, were found in the diffusion couples. All martensitic microstructure consists of variants with different orientations and the twinning relationship. A decrease of reduced elastic modulus (Er) was observed with Ni substituting for Ga in the austenitic phase. However, an opposite trend of an increase in Er was found in the martensitic phase. The softening of the elastic constants near the vicinity of martensitic transformation contributed to a sharp decrease in Er near the interphase boundary. The measured Er had a larger scatter for the martensitic phase than that for the austenitic phase.