Q. Zheng

Magnetocaloric effect and magnetic-field-induced shape recovery effect at room temperature in ferromagnetic Heusler alloy Ni?Mn?Sb

The martensitic transition, magnetocaloric effect ( MCE) and shape memory effect ( MSE) of ferromagnetic Heusler alloys Ni50Mn50-x Sb-x (x = 12, 13 and 14) have been investigated. A large positive magnetic entropy change Delta SM was observed in the vicinity of the martensitic transition. The maximum value of Delta SM is 9.1 J kg(-1) K-1 in Ni50Mn37Sb13 at 287K for a magnetic field change of 5 T. This change originates from the first-order transition from a low-temperature weak-magnetic martensitic phase to a high-temperature ferromagnetic parent phase. A magnetic-field-induced shape recovery strain of about 15 ppm at room temperature and at a relatively low magnetic field ( 1.2 T) was observed to accompany the reverse martensitic transformation. The large field-induced MCE and MSE in the NiMnSb system make it a promising material for room-temperature application.

Large magnetocaloric effect and enhanced magnetic refrigeration in ternary Gd-based bulk metallic glasses

The magnetocaloric effect and refrigeration capacity (RC) of Gd(55)Co(20)Al(25) and Gd(55)Ni(25)Al(20) bulk metallic glasses (BMGs) have been investigated. Large magnetic entropy changes Delta S(M) of 11.2 and 10.8 J kg(-1) K(-1) and large RC values of 846 and 920 J kg(-1) are obtained for Gd(55)Co(20)Al(25) and Gd(55)Ni(25)Al(20), respectively, at a field change of 7 T. The RC value (640 J kg(-1) at 5 T or 920 J kg(-1) at 7 T) of Gd(55)Ni(25)Al(20) BMG is larger than that reported for all magnetocaloric materials, including crystalline and amorphous materials measured under the same conditions. The large RC value is due to the broad Delta S(M) peak (more than 100 K), which is caused by the disordered structure of an amorphous material. The large Delta S(M) and RC values make these Gd-based ternary BMGs attractive candidates for magnetic refrigeration applications

High glass-forming ability correlated with fragility of Mg–Cu(Ag)–Gd alloys

We report bulk metallic glasses with critical diameter (D(c)) in the 20-27 mm range over a relatively wide composition range in the Mg-Cu-Ag-Gd quaternary system. Such an extraordinary glass-forming ability is correlated with the relatively strong liquid behavior of these alloys in terms of Angells [Science 267, 1924 (1995)] fragility concept. The relaxation time of the ternary Mg(61)Cu(28)Gd(11) and quaternary Mg(59.5)Cu(22.9)Ag(6.6)Gd(11) alloys was measured. In terms of the fragility parameter D(*), the Mg(59.5)Cu(22.9)Ag(6.6)Gd(11) alloy with a critical diameter of 27 mm under copper mold casting has a D(*) of 25, higher than all the bulk metallic glass-forming alloys reported so far. The implications of these findings are discussed. (c) 2007 American Institute of Physics.

Amorphous Mg-based metal foams with ductile hollow spheres

To date, high compressive ductility and energy absorption have been achieved in amorphous metal foams based on high-toughness Pd- and Zr-based metallic glasses and are known to result from two extrinsic toughening mechanisms: bending of struts and shear band arrest by pores. We study here a syntactic amorphous metallic foam produced by infiltration of a bed of hollow crystalline iron spheres with a low-toughness Mg-based metallic glass (Mg60Cu21Ag7Gd12). After quenching to vitrify the matrix, this foam exhibits much higher compressive failure strain and energy absorption than the monolithic metallic glass, an improvement which is assigned to an intrinsic mechanism, i.e., composite toughening by the network of hollow ductile iron spheres.

Enhanced magnetic refrigeration properties in Mn-rich Ni-Mn-Sn ribbons by optimal annealing

The influence of annealing time on temperature range of martensitic phase transition (ΔTA-M), thermal hysteresis (ΔThys), magnetic hysteresis loss (ΔMhys), magnetic entropy change (ΔSM) and relative refrigeration capacity (RC) of the Mn-rich Ni43Mn46Sn11 melt spun ribbons have been systematically studied. By optimal annealing, an extremely large ΔSM of 43.2u2009J.kg−1K−1 and a maximum RC of 221.0u2009J.kg−1 could be obtained respectively in a field change of 5u2009T. Both ΔTA-M and ΔThys decreases after annealing, while ΔMhys and ΔSM first dramatically increase to a maximum then degenerates as increase of annealing time. A large effective cooling capacity (RCeff) of 115.4u2009J.kg−1 was achieved in 60u2009min annealed ribbons, which increased 75% compared with that unannealed ribbons. The evolution of magnetic properties and magnetocaloric effect has been discussed and proved by atomic ordering degree, microstructure and composition analysis.

Thermal stability of phases in a NiCoCrAlY coating alloy

[Liang, J. J.; Wei, H.; Hou, G. C.; Zheng, Q.; Sun, X. F.; Guan, H. R.; Hu, Z. Q.] Chinese Acad Sci, Inst Met Res, Superalloys Div, Shenyang 110016, Peoples R China. [Liang, J. J.] Chinese Acad Sci, Grad Sch, Beijing 100039, Peoples R China.;Wei, H (reprint author), Chinese Acad Sci, Inst Met Res, Superalloys Div, Shenyang 110016, Peoples R China;hwei@imr.ac.cn

Magnetocaloric effect in high Gd content Gd-Fe-Al based amorphous/nanocrystalline systems with enhanced Curie temperature and refrigeration capacity

The Gd-Fe-Al amorphous/nanocrystalline composites were successfully designed and obtained with both high Curie temperature (Tc) and large magnetic entropy change (ΔSM). The Tc can be tuned from 172 to 280 K and refrigeration capacity (RC) has a value between 690 and 867 J/kg under a field change of 0–5 T by changing the Gd contents and the formation of Gd nanocrystallites. And, ΔSM in Gd-Fe-Al amorphous/nanocrystalline composites reached a value of 7.2 J kg−1 K−1 under a field change of 0–5 T. The high RC in Gd-Fe-Al system were ascribed to the widening full width at half maximum (δFWHM) up to 240 K of the magnetic entropy change (ΔSMmax) peak because of the combination contribution of amorphous matrix and the precipitated Gd-riched nanocrystalline. Our research would shed light on how to design attractive candidates for magnetic refrigeration materials with high performance at near room temperature.

Low beryllium content Zr-based bulk metallic glass composite with plasticity and work hardenability

A modified Zr-based bulk metallic glass matrix composite Zr47.67Cu40Ti3.66Ni2.66Be6 has been produced by increasing the contents of elements of Zr and Cu with higher Poisson ratio and reducing the contents of Ti, Ni, and Be elements with lower Poisson ratio based on famous metallic glass former Vitreloy 1. A compressive yielding strength of 1804u2009MPa, fracture strength of 1938u2009MPa and 3.5% plastic strain was obtained for obtained metallic glass composite. Also, work-hardening behavior was observed during compressive experiment which was ascribed to the interaction of the in situ precipitated CuZr phase and shear bands.

Preparation and Magnetic Properties of Anisotropic SmCo 5 /Co Composite Particles

Anisotropic SmCo5/Co nanocomposite powders have been prepared by electroless Co deposition on commercial SmCo5 powders with hydrazine as reducer. The Co particles are mainly in the range of 8–27xa0nm and form dense/continuous soft magnetic coatings on the surface of SmCo5 powders. Exchange coupling happened between the coated Co soft magnetic particles and the SmCo5 hard phase. As a result, SmCo5/Co nanocomposite powders with remanence of 73.58xa0emu/g and energy product of 13.74 MGOe were obtained in the optimum condition, as compared with those of 70.52xa0emu/g and 13.40 MGOe for uncoated SmCo5 powders. The effects of Co adding amount on Co particle size, coating morphology, and magnetic properties of SmCo5/Co products were investigated.

Self‐Assembly of CoPt Magnetic Nanoparticle Arrays and its Underlying Forces

Nanoparticles covered with surfactants are often used to study particle motion patterns and self-assembly processes in solutions. Surfactants have influence on the interparticle interactions and therefore on the particle motion tracks and final patterns. In this study, CoPt nanoparticles are synthesized in aqueous solution without any surfactant. In situ transmission electron microscopy observation is performed to monitor the self-assemble process. Two types of magnetic nanoparticle superlattice arrays are formed: hexagonal equal distance superlattice arrays when particle size is 3 nm, and tight unequal distance superlattice arrays when particle size is 4.5 nm. It is interesting to observe that two small arrays merge into a large one through rotational and translational movements. A Monte Carlo simulation is carried out which successfully restores the whole process. It is identified that the underlying forces are van der Waals and magnetic dipolar interactions. The latter is responsible for orientation of each particle during the whole process. This investigation leads to a better understanding of the formation mechanism of magnetic nanoparticle superlattice arrays.