Premartensitic Transformation Research Articles

An in-situ study of low thermal expansion and internal stress evolution in FeMn alloys

The mechanism of low thermal expansion phenomenon in antiferromagnetic FeMn alloys needs to be revealed. In this work, the evolution of lattice thermal expansion and internal stress in FeMn alloys was investigated by in-situ synchrotron X-ray diffraction during thermal cycling. Preceding the martensitic transformation, the coefficient of lattice thermal expansion (CLTE) decreased and the full width at half maximum (FWHM) of the diffraction peaks broadened abnormally. The evolution of the lattice thermal expansion and the FWHM were both reversible and isotropic. The additional reflection spots around the fundamental diffraction spots in the electron diffraction patterns of austenite and the dispersed nano-scale martensite embryos in austenite observed in the dark field TEM images proves the existence of premartensitic transformation. Similar phenomena were found in FeMn Invar alloy, which indicate that the Invar effect is closely related to the internal stress evolution caused by premartensitic transformation. • In this paper, many abnormal phenomena were observed in the Fe Mn alloys. The lattice thermal expansion behaviors and internal stress evolution in FeMn alloys was investigated by in situ synchrotron X-ray diffraction. The correlation between low thermal expansion property and premartensitic transformation were revealed by comparing the similar abnormal phenomena during Invar effect and the premartensitic transformation. The highlights are as follows: • During the Invar effect of Fe 27Mn alloy and preceding the martensitic transformation of Fe 24Mn alloy, the coefficient of thermal expansion decreased and the FWHM of diffraction peaks of austenite increased abnormally. The evolution of thermal lattice strain and FWHM was reversible and isotropic. • Transmission electron microscopy observations confirmed that there are massive martensite embryos embedded in austenite phase during Invar effect and premartensitic transformation. The existence of martensite embryos caused the abnormal increase of FWHM and may be related to the low thermal expansion behavior of FeMn alloy. • These similar phenomena imply a close relationship between the mechanism of Invar effect and the premartensitic transformation in FeMn alloy. • Our results may help to reveal the relationship between Invar effect and premartensitic transformation in antiferromagnetic Invar alloys, and lead to an in-depth understanding of the mechanism of low expansion property.

Dependence of martensite transformation temperature on magnetic field in Ni2MnGa and Ni2MnGa0.95In0.05 single crystals

The shift of martensite transformation temperatures with magnetic field is an essential parameter for magnetic shape memory effect as well as for magnetocalorics. We compare the dependence of martensite transformation temperature on magnetic field in Ni2MnGa and Ni2MnGa0.95In0.05 single crystals. Choosing the stoichiometric and isoelectronic compound enables to observe the effect of In alloying free from the influence of off-stoichiometry. The martensite start temperature, Ms, decreases several kelvins in low magnetic fields (< 1 T). In high field (> 2 T), Ms increases with the magnetic field and the Ms(H) dependence is about linear with the slope of 0.26 K/T (Ni2MnGa) and 0.36 K/T (Ni2MnGa0.95In0.05). Modest 1.25 at% indium alloying decreases not only transformation temperature Ms by about 100 K but also considerably decreases the latent heat of martensite transformation from 1.99 kJ/kg (Ni2MnGa) to 0.29 kJ/kg (Ni2MnGa0.95In0.05) determined from Clausius-Clapeyron equation. The latent heat of premartensitic transformation also decreases, approximately five times.

Study of the Correlation between Premartensitic Transformation and Invar Effect in Femn Alloys by Synchrotron High-Energy X-Ray Diffraction

Study of the Correlation between Premartensitic Transformation and Invar Effect in Femn Alloys by Synchrotron High-Energy X-Ray Diffraction

Study of the Correlation between Premartensitic Transformation and Invar Effect in Femn Alloys by Synchrotron High-Energy X-Ray Diffraction

Study of the Correlation between Premartensitic Transformation and Invar Effect in Femn Alloys by Synchrotron High-Energy X-Ray Diffraction

Magnetic and Magnetostrictive Properties of Ni50Mn20Ga27Cu3 Rapidly Quenched Ribbons.

The influence of the rapid solidification technique and heat treatment on the martensitic transformation, magnetic properties, thermo- and magnetic induced strain and electrical resistivity is investigated for the Cu doped NiMnGa Heusler-based ferromagnetic shape memory ribbons. The martensitic transformation temperatures are unexpectedly low (below 90 K—which can be attributed to the disordered texture as well as to the uncertainty in the elements substituted by the Cu), preceded by a premartensitic transformation (starting at around 190 K). A thermal treatment slightly increases the transformation as well as the Curie temperatures. Additionally, the thermal treatment promotes a higher magnetization value of the austenite phase and a lower one in the martensite. The shift of the martensitic transformation temperatures induced by the applied magnetic field, quantified from thermo-magnetic and thermo-magnetic induced strain measurements, is measured to have a positive value of about 1 K/T, and is then used to calculate the transformation entropy of the ribbons. The magnetostriction measurements suggest a rotational mechanism in low fields for the thermal treated samples and a saturation tendency at higher magnetic fields, except for the temperatures close to the phase transition temperatures (saturation is not reached at 5 T), where a linear volume magnetostriction cannot be ruled out. Resistivity and magnetoresistance properties have also been measured for all the samples.

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.

Magnetic Domain Walls and Macroscopic Magnetization-Related Elastic and Anelastic Effects during Premartensitic Transition in Ni₂MnGa.

The temperature and field dependences of internal friction and Young’s modulus are studied using a high-resolution ultrasonic (90 kHz) technique in stoichiometric ferromagnetic Ni2MnGa shape memory alloy close to the premartensitic transformation temperature, TPM, in the demagnetized state and under moderate fields. Several new effects observed like an apparent Young´s modulus softening close to TPM under moderate fields, instead of the hardening outside this range, as well as existing controversies in the apparent elastic and anelastic properties of Ni2MnGa close to TPM are explained by microeddy and macroeddy current relaxations that to date have been disregarded.

Internal friction behavior of Ni50.5Mn25Ga24.5 alloy with cellular microstructure

Ni50.5Mn25Ga24.5 alloy with cellular microstructure was fabricated. The effect of the cellular microstructure on the internal friction (IF) behavior associated with martensitic transformation (MT), premartensitic transformation (PMT) and twin boundary motion (TBM) was investigated. Compared with conventional Ni–Mn–Ga alloys, the temperature span of the IFMT peak is significantly expanded, the IFPMT is weakened, and no IFTBM is detected. Both IFMT and IFPMT in the cellular microstructure exhibit unique frequency dispersion feature which has never been observed in conventional Ni–Mn–Ga alloys. The effect of point defects associated with the cellular microstructure is considered to understand the observed results.

Microstructure of as-cast single and twin roller melt-spun Ni2MnGa ribbons

Stoichiometric Ni2MnGa alloys are processed by two rapid solidification techniques – single-roller (SR) and twin-roller (TR) melt spinning – and the resulting microstructures and magnetic properties determined. Samples processed at tangential wheel speeds of 10m/s (V10) and 15m/s (V15) are studied in the as-cast condition to analyze the influence of the production methods on the microstructure. Important aspects like the resulting phases, their crystallographic texture, magnetic properties, martensitic transformation temperatures and Curie temperatures are compared. In addition, the magnetization mechanism involving twin boundary motion is explored. Our results indicate that the TR method provides lower cooling rates, thicker samples, higher internal stresses and larger MnS precipitates. However, the quenching rate is mainly determined by the tangential wheel velocity. TR samples also exhibit [100] texture normal to the ribbon plane but in a lesser extent than SR ribbons. Martensitic transformation temperatures are higher in samples V15 (~150K) than in V10 (~100K), with no clear difference between the SR and TR modes. This behavior is explained by considering distinct degrees of disorder in the L21 austenite phase resulting from quenching. The hysteresis of the transformation, defined as the difference Af−MS, takes similar values in the four samples analyzed. Pre-martensitic transformation temperatures are also slightly higher in samples V15, (230±3) K, than in samples V10, (222±3) K, as the magnitude of the Hopkinson effect, in good agreement with a higher residual stress level in TR ribbons. In the martensitic state, all ribbons exhibit hysteresis loops characteristic of a magnetization mechanism involving twin boundary motion. The switching magnetic fields for the onset of Type I twin boundary motion result between 220mT and 365mT, values equivalent to twinning stresses of about 1MPa. It is concluded that both procedures, SR and TR melt spinning, provide microstructures favoring magnetic field induced twin variant reorientation.

Mechanical anomaly observed in Ni-Mn-Ga alloys by nanoindentation

Reduced elastic modulus (Er) and hardness (H) for NiMnGa alloys were investigated using nanoindentation over a large compositional range generated via diffusion couple annealing. Diffusion couples were assembled from selected starting alloys and annealed at 900 °C for 120 h. Compositions generated through diffusion couples, as examined by electron probe micro-analyzer, covered large part of the β phase region. Microstructural analyses by scanning electron microscopy revealed that, for each diffusion couple, an interface separating austenite and martensite was created due to variation of the martensitic transformation temperature at different compositions. Nanoindentation measurements were carried out across the interdiffusion zone for each diffusion couple, and a correspondence between Er or H and composition is established. The measured Er and H had larger scatter for the martensitic phase than for the austenitic phase. A decrease of Er and H was observed with Mn or Ni substituting for Ga, and Ni substituting for Mn in the austenitic phase. However, an opposite trend was found in the martensitic phase: an increase of Er and H was observed with Mn or Ni substituting for Ga, and Ni substituting for Mn. A minimum value of Er and H was always observed near the interface between austenite and martensite. The softening of the elastic constants due to pre-martensitic transformation contributed to the sharp decreases in Er and H near the interface region. The results demonstrate that the influence of lattice softening is significant over a large compositional range on the mechanical properties of NiMnGa alloys and potentially in other materials that have a martensitic transformation.

Magnetoelastic properties in polycrystalline ferromagnetic shape memory Heusler alloys

The influence of the heat treatments on the martensitic transformation, magnetic properties and thermo- and magnetic induced strain on Ni50Fe20Ga27Cu3 ferromagnetic shape memory alloy prepared as ribbons by melt–spinning technique are investigated. The degree of atomic order as effect of different thermal treatments produces important changes in the magneto-crystalline anisotropy of the martensite phase. The anomalies evidenced in the thermo-and magnetic- strain curves are discussed and correlated with the thermo-magnetic data. The transformation-induced strains with and without magnetic field have been measured, the results setting out the influence of the pre-martensitic transformation.

Crystallography and morphology of antiphase boundary-like structure induced by martensitic transformation in Ti–Pd–Fe alloy

The antiphase boundary (APB)-like structure of both 9R and B19 martensites in the Ti–Pd–Fe alloy was investigated by means of transmission electron microscopy. Some APB-like structures with curved and wide contrasts along the (001)9R basal plane are observed in 9R martensitic plates. The atomic displacement on the APB-like structure reflects the atomic movement stemming from the microdomains formed as a pre-martensitic transformation. The displacement vector of the APB-like structure in the B19 martensite can be expressed as R=〈1/30−1/2〉B19. The density of APB-like contrasts increases by the substitution of Fe for Pd in Ti–Pd–Fe alloy.

Effect of lateral pre-compression on the compressive behavior of a CuZr-based bulk metallic glass composite containing B2-CuZr phase

The effect of pre-loading on the compressive behavior of a ductile Cu48Zr48Al4 bulk metallic glass composite (BMGC) containing a B2-CuZr phase was investigated by a lateral pre-compression method. It was found that noticeable plasticity and fracture strength degradations occurred for the BMGC after the lateral pre-compression. The degraded mechanical properties of the BMGC were correlated with the martensitic transformation from B2-CuZr phase to B19′-CuZr phase induced by the pre-compression. Based on the compressive behavior of the B2-CuZr and B19′-CuZr phases, influences of the two crystalline phases on the shear band multiplication in the BMGC were compared. It is expected that the pre-martensitic transformation of the B2-CuZr phase during the pre-compression would reduce the contribution of the martensitic transformation on the mechanical properties of the BMGC during subsequent compression, causing degradations of its plasticity and work-hardening-like characteristics. The results suggest that mechanical prehistory of the CuZr-based BMGC may be an important factor on its subsequent mechanical behavior.

Internal friction associated with the premartensitic transformation and twin boundary motion of Ni50+xMn25−xGa25 (x = 0−2) alloys

The internal friction (IF) during the martensitic transformation (MT) has been extensively studied in NiMnGa alloys. In this paper, temperature dependence of the IF associated with the premartensitic transformation (PMT) and twin boundary motion (TBM) was investigated in Ni50+xMn25−xGa25 (x = 0−2) alloys. Both the composition and frequency had no obvious effect on the IF peak position of the PMT and TBM. With increasing frequency from 0.1 Hz to 5 Hz, the IF peak height corresponding to the TBM was significantly decreased, but was kept constant during the PMT. The observed phenomena were discussed in terms of the different microscopic mechanisms of the TBM and PMT.

ChemInform Abstract: Influence of Long‐Range Atomic Order on the Structural and Magnetic Properties of Ni—Mn—Ga Ferromagnetic Shape Memory Alloys

AbstractReview: 67 refs.

Spontaneous Magnetization and Magnon-TA Phonon Interaction during the Premartensitic Transformation in Ni2 MnGa Alloy

The magnetic anomaly associated with the premartensitic transformation (PT) in Heusler alloy Ni2MnGa is investigated by using the Green's function technique based on the magnon-TA phonon interaction. The results of the numerical calculations suggest that the magnetic anomaly during the PT may originate from the strong magnon-TA phonon interaction, because this kind of coupling effect will destroy the magnetic ordering in some local zones and reduce the magnetization of the system. Magnon-magnon interaction plays a positive role in enhancing the magnetization and tends to reduce the PT temperature while this kind of interaction has little effect on the characteristic TA phonon-wavevector corresponding to the anomaly of spontaneous magnetization at the PT temperature.

Effect of directional solidification rate on the solidified morphologies and phase transformations of Ni50.5Mn25Ga24.5 alloy

The effects of directional solidification rate on the solidified morphologies and phase transformations were studied in polycrystalline Ni50.5Mn25Ga24.5 alloy prepared by induction zone melting method. With the directional solidification rate increasing from 10mm/h to 800mm/h, the solidified morphologies of the austenite phase evolved from planar, to cellular, and then to dendritic. The premartensitic transformation was only observed in the planar austenite, and was suppressed in the cellular and dendritic morphologies. The martensitic transformation was observed in all austenitic morphologies. Increasing the directional solidification rate, the martensitic transformation temperatures were slightly decreased, and the transformation hysteresis was expanded. The local energy barrier related to the compositional segregation from the cellular and dendritic austenitic morphologies suppressed the premartensitic transformation and had little influence on martensitic transformation.

Effects of aging treatment on martensitic transformation of Fe–Ni–Co–Al–Ta–B alloys

Abstract The Fe–Ni–Co–Al–Ta–B (notated as NCATB) SMA recently reported by Tanaka et al. exhibits a huge superelastic strain of over 13% through the carefully adjusting martensite transformation from non-thermoelastic to thermoelastic and introduction of a strong recrystallization {0 3 5}〈1 0 0〉 texture by thermomechanical treatment. However, the superelasticity mechanism of this promising novel alloy is still not very clear. In this paper, the effects of aging treatment on martensitic transformation of the metastable NCATB alloy were investigated by means of hardness tests, XRD, 3DAP, and DMA. The hardness tests demonstrated a conventional aging hardening process when the samples were aged isothermally at 923 K. The 3DAP results indicated that the main composition of the γ′ precipitates was Ni, Al and Ta. The DMA results showed that the martensite transformation became more easily with increasing aging time. When the samples were aged at 923 K for 24 h, a premartensitic transformation was also observed in the DMA tests.

In Situ Three-Dimensional Reciprocal-Space Mapping of Diffuse Scattering Intensity Distribution and Data Analysis for Precursor Phenomenon in Shape-Memory Alloy

Diffuse scattering contains rich information on various structural disorders, thus providing a useful means to study the nanoscale structural deviations from the average crystal structures determined by Bragg peak analysis. Extraction of maximal information from diffuse scattering requires concerted efforts in high-quality three-dimensional (3D) data measurement, quantitative data analysis and visualization, theoretical interpretation, and computer simulations. Such an endeavor is undertaken to study the correlated dynamic atomic position fluctuations caused by thermal vibrations (phonons) in precursor state of shape-memory alloys. High-quality 3D diffuse scattering intensity data around representative Bragg peaks are collected by using in situ high-energy synchrotron x-ray diffraction and two-dimensional digital x-ray detector (image plate). Computational algorithms and codes are developed to construct the 3D reciprocal-space map of diffuse scattering intensity distribution from the measured data, which are further visualized and quantitatively analyzed to reveal in situ physical behaviors. Diffuse scattering intensity distribution is explicitly formulated in terms of atomic position fluctuations to interpret the experimental observations and identify the most relevant physical mechanisms, which help set up reduced structural models with minimal parameters to be efficiently determined by computer simulations. Such combined procedures are demonstrated by a study of phonon softening phenomenon in precursor state and premartensitic transformation of Ni-Mn-Ga shape-memory alloy.

Unusual magnetic anisotropy in the ferromagnetic shape-memory alloy Ni50Fe23Ga27

Unusual magnetic anisotropy of the ferromagnetic shape-memory alloy Ni50Fe23Ga27 has been observed. The anisotropy of the austenite becomes very large, even larger than that of the martensite in ribbon samples. Lowering the temperature from 300 K to 80 K, the saturation field of the austenite is dramatically increased from 200 Oe up to 6 kOe. This high-anisotropy behavior clearly highlights the demagnetization effect of the martensitic transformation. The physical mechanism is attributed to a collective effect coming from the atomic disorder, the premartensitic transformation, and the off-stoichiometric Ga-rich composition of the alloys.