Добірка наукової літератури з теми "Binary Heusler Alloy"

Although Heusler alloys have been known for more than a century, but since the last decade there has been a quantum jump in research in this area. Heusler alloys show remarkable properties, such as ferromagnetic shape memory effect, magnetocaloric effect, half metallicity, and most recently it has been shown that it can be used for direct conversion of heat into electricity. Heusler alloys Ni-Mn-Z (Z=Ga, Al, In, Sn, Sb), show a reversible martensitic transformation and unusual magnetic properties. Other classes of intermetallic Heusler alloy families that are half metallic (such as the half Heusler alloys Ni-Mn-Sb and the full Heusler alloy Co2MnGe) are attractive because of their high Curie temperature and structural similarity to binary semiconductors. Unlike Ni-Mn-Ga, Ni-Mn-In and Ni-Mn-Sn transform from ferromagnetic austenite to non-ferromagnetic martensite. As is consistent with the Clausius-Clapeyron equation, the martensitic phase transformation can be manipulated by a magnetic field, leading to possible applications of these materials enabling the magnetic shape memory effect, energy conversion and solid state refrigeration. In this paper, we summarize the salient features of Heusler alloys, like the structure, magnetic properties and potential application of this family of alloys in industry.

Abstract Heusler alloys are a huge family of binary, ternary and quaternary compounds and contain a wide range of unique properties, which made Heusler compounds to be the efficient materials for diverse applications. When it comes to the commercial applications mechanical properties are worth of check on and turn out to be the significant factor in the processing and final use of the materials. These properties make the study proficient by examine the nature of material under external pressure. In this study, we estimated mechanical properties of Rh2MnZn full-Heusler alloy using the full-potential linearized augmented plane wave method (FP-LAPW) method within the density functional theory (DFT). We have obtained C11, C12 and C44 elastic constants due to cubic symmetry using Charpin method implemented in Wien2k code. Further, using these elastic constants mechanical properties such as bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, anisotropic factor and Cauchy’s pressure are calculated. These moduli depicted hardness, ductility and elastic anisotropy of the alloy.

AbstractIn this work, detailed first-principles calculations within the generalised gradient approximation (GGA) of electronic, structural, magnetic, and optical properties of Ni,Ti, and Al-based Heusler alloys are presented. The lattice parameter of C1b with space group F4̅3m (216) NiTiAl alloys is predicted and that of Ni2TiAl is in close agreement with available results. The band dispersion along the high symmetry points W→L→Γ→X→W→K in Ni2TiAl and NiTiAl Heusler alloys are also reported. NiTiAl alloy has a direct band gap of 1.60 eV at Γ point as a result of strong hybridization between the d state of the lower and higher valence of both the Ti and Ni atoms. The calculated real part of the dielectric function confirmed the band gap of 1.60 eV in NiTiAl alloys. The present calculations revealed the paramagnetic state of NiTiAl. From the band structure calculations, Ni2TiAl with higher Fermi level exhibits metallic properties as in the case of both NiAl and Ni3Al binary systems.

Hypothetical half-Heusler (HH) ternary alloy of CoVSn has already been computationally investigated for possible spintronics and thermoelectric applications. We report the experimental realization of this compound and the characterizations of its thermoelectric properties. The material was synthesized by a solid-state reaction of the stoichiometric amounts of the elements via powder metallurgy (30 h mechanical milling and annealing at 900 °C for 20 h) and spark plasma sintering (SPS). The temperature-dependent ternary thermodynamic phase diagram of Co-V-Sn was further calculated. The phase diagram and detailed analysis of the synthesized material revealed the formation of the non-stoichiometry HH CoVSn, mixed with the binary intermetallic phases of SnV3, Co2Sn, and Co3V. The combination of X-ray diffraction, energy-dispersive X-ray spectroscopy, and thermoelectric transport properties confirmed the formation of a multi-phase compound. The analysis revealed the predicted thermoelectric features (zT = 0.53) of the highly doped CoVSn to be compromised by the formation of intermetallic phases (zT ≈ 0.007) during synthesis. The additional phases changed the properties from p- to overall n-type thermoelectric characteristics.

The isothermal section of the Ni-Mn-Sb ternary system at 773 K was measured by means of 117 alloys which were analyzed by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), and electron probe microanalysis (EPMA) techniques. The existence of 7 binary compounds, namely NiMn, Mn2Sb, MnSb, NiSb2, NiSb, Ni5Sb2, Ni3Sb and 2 ternary compounds, namely Ni2MnSb and NiMnSb were confirmed for this isothermal section. The four binary compounds Ni3Sb (Cu3Ti structure, Pmmn space group), Ni5Sb2 (Ni5Sb2-type structure, C2 space group), NiSb2 (FeS2-type structure, Pnnm space group) and Mn2Sb (Cu2Sb-type structure, P4/nmm space group) in the binary systems Ni-Sb and Mn-Sb were stoichiometric compounds, the homogeneity ranges of which were negligible. However the five single phases in the Ni-Mn system and the two binary compounds MnSb and NiSb showed more or less homogeneity ranges formed by substitution of Mn and Sb for Ni atom. The Heusler compound ? (Ni2MnSb) has L21-type ordered structure with space group Fm-3m, a = 0.6017 nm. And the crystal structure for the Half-Heusler compound ? (NiMnSb) is C1b-type (F-43m) with a = 0.5961 nm. The approximate homogeneity ranges of the two ternary compounds ? and ? at 773 K were investigated.

The aim of the present thesis work was to synthesize and explore the phase transformation behavior of a binary Heusler alloy based on AlMn system. The intermetallic phase with the stoichiometry AlMn, which is usually termed as τ phase, is metastable and the only phase in the system that exhibits ferromagnetic behavior. Thus, good magnetic properties are directly related to the amount of τ phase present in the alloy. Due to its metastability, synthesis of τ phase in bulk form has always been a challenging task for the materials scientists. In this work, we have demonstrated a possible route for synthesizing complete τ phase (i.e., without any other nonmagnetic phases) in bulk form. This has been achieved without any addition of τ phase stabilizers such as C, B etc. We have carried out several heating and cooling (including isothermal) experiments to understand the exact nature of phase transitions that can yield hundred percent τ phase. The thesis is divided into 6 chapters. Chapter 1 deals with a brief introduction on magnetism and types of magnetic behavior with several examples. Following this, we have given a brief review on Heusler alloys based on Mn with a special emphasis on AlMn binary alloy. Chapter 2 discusses the experimental techniques employed during the present investigation. Vacuum arc melting/casting unit was used to make the alloys. Microstructural features were studied using scanning electron microscopy (SEM) while the phase identification was carried out using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The thermal analysis of the samples was carried out using differential scanning calorimetry (DSC) while a vibrating sample magnetometer (VSM) used for magnetic measurements. Chapter 3 outlines the detailed procedure for synthesis of complete τ phase in bulk form without any addition of stabilizers. This was achieved through a controlled solidification of a Al45Mn55 at.% alloy composition and subsequent heat treatment. To obtain the domain for the formation of the τ phase from high temperature ε phase, isothermal transformation experiments were carried out, that helps to generate a complete TTT diagram. The τ phase start and end times were obtained through magnetic and X-ray measurements. Subsequently, the obtained TTT curve was converted to CCT curve by using standard Avrami method. The τ phase formation is a process of solid - solid phase transformation that depends on the cooling rate (i.e. heat extraction rate during cooling). Hence, thermal modeling was carried out to predict the heat extraction rate for different diameter copper mold (2 to 12 mm) containing hot solids and the obtained cooling rate curves were overlapped with the calculated CCT diagram. Hence we can approximately estimate the diameter of the mould to be used for obtaining τ phase directly during casting. We found that 10 mm diameter casting is suitable to get complete τ phase. This has been further experimentally verified. A saturation magnetization of 128 emu/gm at room temperature was measured for this 10 mm sample containing only τ phase. This represents the highest value reported till now in this system. The Curie point for this phase was found to be 395 oC. Additionally, the cast rod exhibits compressive strength of 1170 MPa with > 10% compressive ductility that is higher than other existing permanent magnets. Chapter 4 discusses the nature of phase transformation behavior of ferromagnetic Al45Mn55 at.% alloy ( phase) during heating to high temperature. Experiments were carried out non-isothermally using differential scanning calorimetry (DSC) to evaluate the structural changes during heating. The progressive structural (or phase) changes with temperature and time were recorded and analyzed. The DSC heating and cooling curves exhibit endothermic and exothermic peaks, which reflect the phase or structural changes and have been discussed in detail. To identify the phases responsible for the transformation peaks, the samples were heated in a muffle furnace at a rate of 10 °C/min upto the temperatures just below and above the peaks followed by rapid quenching in water to arrest the phase. In situ X-ray diffraction was also performed to correlate and confirm the phase transitions that are seen during heating in DSC. The present study confirms transformation of τ→β+γ2→β+γbcc→ and ε→τ→β+γ2→β+γbcc→. We found that the γbcc phase (high temperature phase) cannot be retained during quenching experiments and hence was not detected by earlier investigators. Therefore, this study provides a more complete understanding of the τ phase decomposition. Chapter 5 demonstrates the effect of mechanical ball milling on the evolution of phases with starting materials having  phase (10 mm cast rod) as well as  phase. A planetary ball mill P7 was used for milling and the samples were collected at regular intervals of time. We observed no effect of milling on the magnetic properties of ε phase since it is a nonmagnetic phase. But subsequent annealing at 350 °C for 30 min after milling results in structural change and exhibits magnetic response. The phase transitions were found to depend on prior milling history. The saturation magnetization and coercivity for 4 h milled (and annealed to 350 oC for 30 min) was measured to be 23 emu/gm and 5 kOe respectively. In the case of  phase as a starting material, we found no decomposition upto 9 h of milling even though the particle size reduction was observed with increasing milling time. Additionally we found that after 3 hours of milling, the saturation magnetization value reduces to 23 emu/gm and coercivity increases to 5.2 kOe. Further milling causes decrease in both the values. Annealing of the 3 h milled powder at 350 °C for 30 min, resulted in slight decrement in coercivity (Hc = 5 kOe) but significant increase in saturation magnetization (32 emu/g) value. Experimental results suggest that magnetization reversal is domain nucleation controlled and that the nonmagnetic phases present can act as the pinning sites. The final chapter summarizes the major conclusions of the present work.