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Small-sized materials with large surface to volume ratio favor heat transfer during magnetic refrigeration cycling and thus may help enhancing the refrigeration efficiency. Here, high Fe content Ni44.9Fe4.3Mn38.3Sn12.5 polycrystalline microwires were prepared by a melt-extraction technique. The as-extracted microwires were annealed at 1173 K for 60 min, leading to significant grain growth and formation of a secondary Fe-rich γ phase. The annealed microwire exhibits larger magnetization difference (ΔM) between the austenite and martensite phases and smaller thermal hysteresis compared to the as-extracted microwire. The annealed microwire possesses a magnetic transition to austenite at 299 K, followed by a martensitic transformation (MT) from a ferromagnetic austenite to a weak-magnetic martensite at 208 K upon cooling. Under a magnetic field of 50 kOe, the annealed microwires show a maximum magnetic entropy change ΔSm of 6.9 J/kg·K and an effective refrigeration capacity RCeff of 78.0 J/kg over a broad working temperature span ΔTFWHM of 20 K around the MT. In addition, magnetic transition of the austenite gives rise to ΔSm −3.7 J/kg·K and RCmag 232.5 J/kg with ΔTFWHM of 85 K under 50 kOe. Keywords: Ferromagnetic shape memory alloys, Microwires, Martensitic transformation, Magnetocaloric effect, Ni-Fe-Mn-Sn

Ni-Mn-Sn alloys exhibit high magnetocaloric effect (MCE) during the magnetic-field-induced martensite to austenite phase transformation. However, the low magnetization of the austenite, narrow working temperature interval and high magnetic hysteresis reduced their magnetic refrigeration capacity. Here, the effects of Fe doping on the structural and magnetic transitions of Ni 50-x Fe x Mn 38 Sn 12 alloys were investigated. The doping of Fe higher than 2.9% induced the occurrence of a secondary γ phase, creating a dual phase alloy. The martensite transformation temperature significantly decreased with the occurrence of the secondary phase. However, the Curie temperatures of both austenite and martensite changed slightly with the formation of the secondary phase. As a result, the magnetostructural coupling, i.e. overlap of the structural and magnetic transitions between the ferromagnetic austenite and the paramagnetic martensite, occurred in the alloys with x = 0.5 - ∼4.2. Furthermore, the magnetization of the austenite and martensite both increased with increasing Fe content from x = 0 to x = 5.5, with the magnetization difference between the austenite and martensite saturating at x = 4.2. The dual phase Ni 45.8 Fe 4.2 Mn 38 Sn 12 alloy, exhibiting a magnetic field dependence of martensite transformation temperature ΔT M / μΔH - 0.28 K/kOe, showed a wider working temperature interval than that of the un-doped Ni 50 Mn 38 Sn 12 alloy. This demonstrated that the Ni 45.8 Fe 4.2 Mn 38 Sn 12 alloy might exhibit a high magnetic entropy change, wide working temperature interval and low magnetic hysteresis, and thus may be a promising magnetic refrigeration material.

The effects of Fe doping on the microstructure, martensite transformation and superelasticity in melt-extracted Ni50Mn25Ga25−xFex (x=1–6) microwires were investigated. The unique solidification process during melt-extraction creates the micron-sized diameter wires with small grains and semicircular cross-section. At ambient temperature Ni50Mn25Ga25−xFex (x 5 are martensitic phases with seven-layered modulated (7M) structure. The results point out that martensite transformation temperatures are strongly related to Fe content due to the change of valence electron concentration (e/a). Reversible superelastic strains of 0.92% and 0.75% are obtained in Ni50Mn25Ga21Fe4 and Ni50Mn25Ga20Fe5 microwires, respectively. It is demonstrated that the temperature dependence of stress-induced martensite (SIM) stress follows the Clausius–Clapeyron relation. The temperature dependence of SIM stress in Fe-doped Ni–Mn–Ga microwires is 10.5 MPa/K.

Ni50Mn25Ga25–xFex (1 ≤ x ≤ 6) microwires with a diameter of 30–50 µm were prepared by melt-extraction. The effects of Fe doping on the martensite transformation temperature, Curie point and magnetic properties before and after chemical ordering treatments were investigated. Experimental results show that Ni50Mn25Ga25–xFex microwires exhibit typical thermoelastic martensite transformation and magnetic transition. Transformation temperatures of the microwires increase with Fe content with a rate of 27.3 K/at%. Thermal hysteresis during martensite transformation is lowered after Fe doping. The Curie point of Ni50Mn25Ga25–xFex (1 ≤ x ≤ 6) microwires keeps stable at 390 K, which is 25 K higher compared with Ni2MnGa ternary alloy. After chemical ordering heat treatment, the microwires exhibit a typical ferromagnetic transition and a higher saturation magnetization at low field compared to as-extracted microwires.

We report a systematic study of a new class of melt-extracted Ni49.7Mn25.0Ga19.8Fe5.5 polycrystalline microwires in terms of fabrication, microstructural characterization, and evaluation of shape memory effects (SMEs). The as-extracted microwire has enhanced ductility due to its fine grains with diameters of 1–3 µm. The total strain in its martensite state reached 2.14% as loading at 350 MPa and 100% plastic deformation strain recovery was achieved after heating process. During thermo-cycling tests, a fully recoverable transformation strain of 1.5% was obtained under a tensile bias-stress of 468 MPa. It can be therefore concluded that Fe-doped microwires display the least strain dependence on stress compared with other alloys such as NiTi and NiMnGa, and could be useful in devices required constant stress output.

Co68.15Fe4.35Si12.25B15.25 (at%) amorphous microwires with a smooth surface and a circular cross-section were fabricated by the glass-coated melt spinning method. Their mechanical properties were evaluated through tensile tests of the glass-coated amorphous microwires, and their fracture reliability was estimated using two- and three-parameter Weibull analysis. X-ray diffraction and transmission electron microscopy results showed that these glass-coated Co-based microwires were mostly amorphous. The coated Co-based microwires exhibit a tensile strength of 1145 to 2457 MPa, with a mean value of 1727 MPa and a variance of 445 MPa. Weibull statistical analysis showed that the tensile two-parameter Weibull modulus of the amorphous microwires is 4.16 and the three-parameter Weibull modulus is 1.61 with a threshold value as high as 942 MPa. These results indicate that the fabricated microwires exhibit good tensile properties and fracture reliability, and thus appear to be good candidates for electronics reliability engineering applications.

In this work, we report a comprehensive investigation on mechanical and giant magneto-impedance (GMI) properties of melt-extracted Co-rich amorphous microwires with the nominal composition of Co69.25Fe4.25Si13B13.5−xNbx (x = 0, 1, 2, 3, 4). Results show that the tensile strength and GMI properties gradually increase with the substitution of B by Nb at first, and then decrease with a further Nb substitution. The melt-extracted microwires with the substitution of B by 3% Nb exhibit a maximum tensile strength σm of 4000 ± 50 MPa owing to the formation of nanosized crystals in the amorphous matrix. In addition, the substitution of B by 1% Nb assures an excellent GMI property. At 5 MHz, the maximum GMI ratio [ΔZ/Zmax]max for the melt-extracted microwires reaches as high as 450 ± 10% versus 380 ± 10% for traditional CoFeSiB microwires and the field-response sensitivity increases from 350 ± 10% Oe−1 to 700 ± 10% Oe−1 for 1% Nb substitution. It is thus concluded that both mechanical and GMI properties of the melt-extracted Co-based amorphous microwires can be greatly improved by properly adding Nb, and this is promising for many engineering applications, including high-resolution magnetic, stress and biomedical sensors.

Melt-extracted amorphous Co68.15Fe4.35Si12.25B15.25 microwires were drawn at ambient temperature into different diameters and the dependence of tensile properties and microstructural evolution on the degree of deformation was investigated. The cold-drawn wire with 51% area cross-section reduction exhibits a tensile ductility of 1.64%, with a tensile strength exceeding 4000 MPa. Deformation-induced Co-rich nanocrystals were observed in the cold-drawn wires and proved to be capable of arresting the quick extension of shear bands and leading to the enhanced global plasticity of the microwires.

The bamboo-structured Ni45Co5Mn36.8In13.2 fibers with martensitic transition temperature at room temperature were fabricated by melt spinning followed by annealing. A magnetic-field-induced reverse martensitic transformation from martensite to austenite with a shift of transformation temperature of 2.6 K/T was obtained in the microfibers. Different from the intrinsic brittleness evidenced in bulk alloy, the microfibers stored at 320 K (47 °C) were easily flexible, which is due to the fact that the formation of bamboo structures reduces the mismatch stress/strain near grain boundaries.

Amorphous Co68.15Fe4.35Si12.25B15.25 wires with smooth surface and circular cross section were fabricated by melt extraction technology using a copper wheel with a knife-edge cross section angle of 60 deg. The effect of some process parameters such as wheel circumference velocity, molten alloy feed rate, and temperature on the geometry and weight, i.e., melt extracted layer thickness, of wire was examined carefully. An optimum process parameter to produce high-quality circular wires was presented. A high resolution CCD video camera recorder was used to monitor the changing of the surface shape of molten alloy contacting the wheel tip under different conditions. It was found that the mechanism of the wire formation during the optimum process condition was controlled by the momentum mechanism, while in the low wheel speed region, heat transfer turned out to be a dominant factor. Some characteristics of the circular wires such as amorphous nature and tensile strength were also studied.

In order to study the effect of wire bonding by Cu electroplating on giant magneto-impedance (GMI) stability of microwires for potential sensor application. The electroplated microstructure was observed by scanning electron microscopy (SEM) and GMI was studied by a precision impedance analyzer placed in a magnetically shielded space. Experimental results indicated that after electroplating at a current intensity of 395.0 A/dm 2 holding for 60 s with a two-terminal electroplated length of 4mm of the microwire, the Cu layer with a homogeneous and compact granular structure has higher adhesive strength between the wire and the Cu layer, better wettability between wire and SAC, lower contact resistance and coefficient of thermal expansion (CTE). Consequently, the wire bonding by Cu electroplating could enhance the conductivity and impedance stability under different applied field. It can, therefore, be concluded that the wire bonding by a Cu electroplating technique is suitable for electronic packaging for sensor applications.

The phase evolution in the transition region of a bulk Zr56.6Cu17.3Ni12.5Al9.6Ti4 amorphous alloy wedge-ingot upon annealing has been studied. The result shows that the primary crystallized metastable phase and the amorphous matrix were found to crystallize into different products. Concerning the solidification process, it is deduced that the understanding of the competitive relationships among the intermetallic phases and the amorphous phase can be described by the phase analyses of solidification, rather than the widespread investigations of the crystallization by annealing.

In order to clarify the competing relationships among the intermetallic precipitation phases during the cooling of a bulk amorphous Zr 56.6 Cu 17.3 Ni 12.5 Al 9.6 Ti 4 alloy, the wedge-shaped ingots of this alloy were made by casting the melt into a wedge-shaped copper mould, and then the zones with different cross-sections were studied by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that these phases are very different in both the morphology and sizes, and they are dependent on the thickness of wedge ingot, i.e., the cooling rates. It is therefore concluded that the Zr 2 Ni phase is the strongest competing rival to the amorphous matrix in this alloy. These findings support that compared to Zr–Ni pair, Zr–Cu pair has a larger glass forming tendency. The glass forming ability can be improved by inhibiting the precipitation of Zr 2 Ni phase by, for example, decreasing the Ni content in this alloy.

This paper presents a new way of designing bulk metallic glasses in Cu–Ti–Zr–Ni system based upon binary deep eutectic. By conventional copper mold casting method, these alloys can be quenched into single amorphous structure with the diameter ranging from 3 to 5 mm. Experimental results show that among the alloy series designed Cu 52.55 Ti 30.05 Zr 11.4 Ni 6 and Cu 53.1 Ti 31.4 Zr 9.5 Ni 6 possess the relatively higher T rg of 0.603 and 0.598, respectively, which contribute to their good glass forming ability, and Cu 52.55 Ti 30.05 Zr 11.4 Ni 6 exhibits a relatively higher T g (694 K) and Δ T x (52 K), which indicate its good thermal stability.

Ni48Mn26.4Ga19.7Fe5.9 microwires with grain size of 1-3 micron were successfully fabricated by melt-extraction. The superelastic effects in the microwires under various temperatures and loads were systematically demonstrated. The as-extracted microwires displayed partial superelasticity when attended at relatively high temperature. The critical stress for stress-induced martensite formation increases linearly with temperature and follows the Clausius-Clapeyron relationship. The temperature dependence of the as-extracted polycrystalline Ni48Mn26.4Ga19.7Fe5.9 microwires is 16.4 MPa/K, which is higher compared with Ni-Mn-Ga single crystals. In addition, the as-extracted microwires display excellent shape memory behavior with the recovery strain and recovery ratio of 1.26% and 86%, respectively, when the total strain reaches 1.47% at 310 K.

The relative glass forming ability (GFA) of bulk amorphous Zr 60 Cu 20 Ni 8 Al 7 Hf 3 Ti 2 (A 1 ) and Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 (Vit105) alloys has been investigated by comparing the maximum thickness of amorphous region of wedge-shaped samples, as well as the evaluated critical cooling rates using differential thermal analysis (DTA). The results show that the maximum thickness of amorphous region of wedge-shaped samples are d m =2 mm for A 1 , d m =3.5 mm for Vit105, and the evaluated critical cooling rates are R c =33.5 and 4.5 K/s for A 1 and Vit105, respectively. It is suggested that the glass forming ability of A 1 is smaller than that of well known Vit105 alloy, though A 1 has the significant supercooled liquid region of over 140 K. Therefore, it is concluded that a large supercooled liquid region Δ T x does not necessarily mean good glass forming ability of an amorphous alloy.

The nanocrystallization kinetics of Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 (at.%) bulk metallic glass was investigated by differential scanning calorimetry (DSC) in the mode of continuous heating and isothermal annealing. In the case of continuous heating, three exothermic crystallization peaks can be observed, and the peak temperatures display a strong dependence on the heating rates, which can be fitted by a first order decay equation. The activation energies for crystallization are estimated to be E p1 =166.83±8.85 kJ/mol, E p2 =256.15±9.34 kJ/mol, E p3 =174.36±12.56 kJ/mol, which correspond to peak temperatures of T p1 , T p2 , and T p3 , respectively, indicating the formation of different crystallization phases at different stages. In the case of isothermal annealing, the crystallization products under isothermal annealing were observed and determined by transmission electron microscopy (TEM). The precipitation phases of the sample heated at higher temperatures were identified by X-ray diffraction (XRD), showing that the body-centered tetragonal (bct) Zr 2 Cu and hexagonal ZrBe 2 are the primary phases, in spite of the presence of other phases. These results are consistent with the complexity of the DSC curves obtained during the continuous heating and isothermal annealing.

The crystallization processes of Zr55Al10Ni5Cu30 (at.%) bulk metallic glass from the amorphous state and supercooled liquid region during the continuous heating and isothermal annealing courses were investigated. The apparent activation energy derived from the Kissinger plots for glass transition Eg is higher than that for crystallization Ep and Ex calculated by the peak temperature Tp and onset crystallization temperature Tx during the continuous heating process. The isothermal activation energy obtained using Arrhenius equation shows that it increases accompanying the proceeding of the crystallization transformation. The incubation time of crystallization becomes longer and the crystallization process becomes slower as the annealing temperature is reducing during the isothermal process. The crystallization mechanism was studied using Kolmogorov–Johnson–Mehl–Avrami (KJMA) equation. The results indicate that the Avrami exponent n≈2 at the initial crystallization stage, and it alters gradually following the increasing crystallization volume fraction.

A class of biphase nanocrystalline/amorphous Gd(50+5x)Al(30−5x)Co20 (x = 0, 1, 2) microwires fabricated directly by melt-extraction is reported. High resolution transmission electron microscopy and Fourier function transform based analysis indicate the presence of a volume fraction (∼20%) of ∼10 nm sized nanocrystallities uniformly embedded in an amorphous matrix. The microwires possess excellent magnetocaloric properties, with large values of the isothermal entropy change (−ΔSM ∼ 9.7 J kg−1 K−1), the adiabatic temperature change (ΔTad ∼ 5.2 K), and the refrigerant capacity (RC ∼ 654 J kg−1) for a field change of 5 T. The addition of Gd significantly alters TC while preserving large values of the ΔSM and RC. The nanocrystallites allow for enhanced RC as well as a broader operating temperature span of a magnetic bed for energy-efficient magnetic refrigeration.