J.G. Li

Effect of grain constraint on the field requirements for magnetocaloric effect in Ni45Co5Mn40Sn10 melt-spun ribbons

The influence of grain constraint on the magnetic field levels required to complete the isothermal martensitic transformation in magnetic shape memory alloys has been demonstrated for a NiCoMnSn alloy, and the magnetocaloric performance of an optimally heat treated alloy was quantified. Ni45CoxMn45-xSn10 melt spun ribbons with x = 2, 4, 5, and 6 were characterized. The x = 5 sample was determined to exhibit the lowest transformation thermal hysteresis (7 K) and transformation temperature range during transformation from paramagnetic austenite to nonmagnetic martensite, as well as a large latent heat of transformation (45 J kg-1 K-1). For this composition, it was found that increasing the grain size to thickness ratio of the ribbons from 0.2 to 1.2, through select heat treatments, resulted in a decrease in the magnetic field required to induce the martensitic transformation by about 3 T due to the corresponding reduction in the martensitic transformation temperature range. This decrease in the field requirement ultimately led to a larger magnetocaloric entropy change achieved under relatively smaller magnetic field levels. The giant inverse magnetocaloric effect of the optimized alloy was measured and showed that up to 25 J kg-1 K-1 was generated by driving the martensitic transition with magnetic fields up to 7 T.

An approach to the bulk textured Fe81Ga19 rods with large magnetostriction

Bulk textured Fe81Ga19 alloy with large magnetostiction of up to 830ppm is fabricated using the technique of rapid directional solidification by triggering the undercooled Fe81Ga19 melts. The saturation magnetostriction is about two times as large as the maximum of the earlier report on Fe–Ga bulk samples. The large magnetostriction is ascribed to the high concentration of Ga–Ga atom pairs created by rapid solidification and their preferential orientation in the [100] textured rod.

Design in Ni–Mn–In magnetic shape-memory alloy using compositional maps

Using empirical data, a map of composition versus magnetic properties was developed to design Ni-Mn-In magnetic shape-memory alloys. By the aid of this composition map, five alloys with desired properties were designed. A linear composition dependence of martensitic transformation temperature (Tm) and alloy was revealed. A transformation from austenite to martensite could be achieved at an electron concentration of ∼7.9. As such, a series of magnetic shape-memory alloys, which work at different conditions, can be designated under the guidance of a series of contour maps derived from Tm and thermal hysteresis.

Superheated liquid fragility and thermodynamic refinement for evaluation of metallic glass-forming ability

Based on the super-Arrhenius equation and Angell’s fragility concept [J. Non-Cryst. Solids 131, 13 (1991)], the expression of the fragility parameter for superheated liquid is deduced as M=E∞∕kBTl, where E∞ is the activation energy, kB the Boltzmann constant, and Tl the liquidus temperature. It exhibits a negative correlation with the glass-forming ability (GFA) of the referenced metallic glasses in the same system rather than in the different systems, while the parameter e based on order-disorder competition is just the opposite. The refined fragility parameter M* (=M∕e) gives a much better reflection of the GFA for the metallic glasses.

Applications of the directional solidification in magnetic shape memory alloys

A zone melting liquid metal cooling (ZMLMC) method of directional solidification was applied to prepare highly-oriented Ni52Fe17Ga27Co4 magnetic shape memory alloys. At high temperature gradient and low growth velocity, the well-developed preferred orientation for coarse columnar crystals was obtained. Such a structure leads to a large complete pseudoelastic recovery of 5% at 348 K. Moreover, the pseudoelastic behaviours and the kinetics of the martensitic transformation (MT) are significantly affected by the intersection angle between the loading direction and the grain boundaries.

Uniform Droplets Generation Using a Direct-Mode Jet Pulse Spray System

A novel method of preparation of uniform droplets using a direct-mode jet pulse spray system has been developed. By means of the periodical switch-on between a membrane-like rotating distribution exit and a micronozzle, a fluid stream was stimulated to be disintegrated into monodisperse droplets. The effects of the rotational speed (n) and thickness (H) of the distribution exit and flow rate of the dispersed phase (Q) on droplet characteristics were investigated. It was found that the uniformity of droplets would be enhanced with increased rotational speed and decreased thickness of the distribution exit, even for a relatively high flow rate of the dispersed phase. A relatively steady value of the coefficient of variation in size less than 10% could be obtained under the conditions of n = 350 rpm and H = 3 mm at Q = 7.5–8.7 mL/min. The influence of vortices was characterized by the Taylor number and parameter K, showing that the parameter K is capable of more precisely predicting the turbulence transition. Therefore, the system has effectively attained an improvement not only in uniformity but also in productivity of droplets generation.

Glass Formation and Mechanical Properties of Ti–Cu–Ni Alloys with High Ti Content

A series of Ti–Cu–Ni alloys with Ti content as high as 50–70 at. % expected to possess potential high glass-forming ability (GFA) was designed according to the e criterion (Xia, M. X., Zhang, S. G., Ma, C. L., and Li, J. G., “Evaluation of Glass-Forming Ability for Metallic Glasses Based on Order-Disorder Competition,” Appl. Phys. Lett. Vol. 89, 2006, pp. 091917-1–091917-3) and were prepared by melt spinning and suck casting methods. The samples were examined by X-ray diffractometry, differential scanning calorimetry, optical microscopy, scanning electronic microscopy, and quasistatic compression test. The GFA of the melt-spun ribbons is enhanced with increasing e. Ti58Cu32Ni10 alloy with the maximum designed e value of 0.542 exhibits best GFA with a glass transition temperature of 627 K and a wide supercooled liquid region of 45 K. However, this alloy failed to form a fully glassy rod of 1 mm in diameter. Room temperature compression tests reveal that the 1 mm diameter Ti58Cu32Ni10 glass composite exhibits work-hardening characteristic, with ultimate compressive stress of 2418 MPa, yielding stress σ0.2 of 1448 MPa and about 7.8 % plastic strain. The combination of high strength and ductility was attributed to a dendritic TiCu(Ni) network embedded in the hard glass matrix.