H.Q. Ye

High temperature embrittlement of NiAl alloy induced by hot isostatic pressing (HIPing) and aging

acad sinica, inst met res, shenyang 110015, peoples r china. acad sinica, inst met res, atom imaging solids lab, shenyang 110015, peoples r china.;cui, cy (reprint author), acad sinica, inst met res, shenyang 110015, peoples r china

Preliminary investigation of directionally solidified NiAl–28Cr–5.5Mo–0.5Hf composite

An in-situ NiAl/Cr(Mo)-Hf eutectic composite has been successfully fabricated using the Bridgman technique. The microstructure was analyzed using scanning electron microscopy (SEM) with energy disperse spectroscopy and transmission electron microscopy (TEM). The mechanical behavior of the composite, including compressive and tensile properties at room and elevated temperature, was studied. It was found that the strengths of the composite are higher than that of NiAl alloys. Subsequently, the mechanism of improving the strength was analyzed

Effect of Ho additions on the microstructure and mechanical properties of Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf alloys

Abstract The effect of Ho additions on the microstructure and compressive properties of the alloy Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf (at.%) was investigated by using scanning electron microscopy, X-ray diffraction, transmission electron microscopy and by performing compression tests. The results show that Ho additions cause remarkable microstructural refinement. The addition of 0.1 at.% Ho to the alloy increases its strength and ductility. When the amount of Ho is increased to 0.2 at.%, the strength and ductility decreases. Ho addition inhibits the formation of the (Ti, Nb)5Si3 phase and the amount of the (Ti, Nb)5Si3 phase decreases, even disappears, with increasing Ho content. In addition, the fracture surfaces of Ho-free alloys deformed at room temperature in compression tests exhibit cleavage for both the (Nb, Ti) solid solution and the (Nb, Ti)5Si3 intermetallic compound, while limited local plastic strain of the (Nb, Ti) solid solution is observed in Ho-doped alloys. The effect and mechanism of strengthening and plastic deformation of Ho additions to the alloy was discussed in view of the microstructure.

An analysis of growth direction of β phase precipitates in a TiAlW alloy

beta phase precipitates at the lamellar interface after aging heat treatment in a TiAlW alloy. The orientation relationship (OR) between the beta and gamma phase is (gamma)// (beta), (111)(gamma)//(110)(beta). Six different beta phase variants can be observed along the [111](gamma) zone axis. The OR between beta and gamma phase and the growth directions of beta precipitates are predicted from the invariant line strain model

Microstructure and compressive properties of NiAl-Cr(Mo)-Dy near eutectic alloy prepared by suction casting

Abstract The NiAl–Cr(Mo) eutectic alloy doped with Dy was prepared by suction casting technique and its microstructure and mechanical properties were investigated. It is found that with the addition of Dy, the Ni5Dy phase is formed along the NiAl/Cr(Mo) phase boundary in the intercellular region. By the suction casting method, the microstructure of the alloy get well optimisation which can be characterised by the fine interlamellar spacing, high proportion of eutectic cell area and fine homogeneous distributed Ni5Dy phase. The compression test results reveal that the room temperature and high temperature mechanical properties of the suction cast alloy improve significantly, compared with the conventionally cast alloy.

Brittle-to-ductile transition in multiphase NiAl alloy

The microstructure of a directionally solidified (DS) NiAl/Cr(Mo,Hf) alloy was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). This alloy was composed of NiAl, Cr(Mo) and Ni2AlHf phases. Tensile testing of this multiphase NiAl alloy was performed from room temperature to 1323 K at various strain rates from 1.04×10−4 s−1 to 1.04×10−2 s−1. It was found that the brittle-to-ductile transition temperature (BDTT) of this alloy was higher than values reported for many of NiAl-based alloys and dependent on the strain rate. Regardless of strain rate, at temperatures just above the BDTT, the fracture morphology changed from NiAls cleavage and debonding along the NiAl/Cr(Mo) interface to completely ductile in nature. It was also found that the apparent activation energy responsible for BDT is 463 kJ mol−1, which may be responsible for the higher BDTT.

Microstructural characteristics of Ti-48Al-2Cr alloy

The microstructural characteristics of two-phase α2+γ titanium aluminide before and after deformation are investigated by transmission electron microscopy (TEM). The orientation relationships associated with the γ-γ combinations can be divided into three types: true twin, order domain, and pseudo-twin. Of the three orientation relationships, true twin accounts for more than 50%, which is most likely due to a minimization of interfacial energy related to misfit and interaction energy of interface atoms. During deformation, lamellar boundaries cause different resistance to the propagation of intersecting twin. Experimental observations and theoretical analysis consistently show that the resistance has the following sequence: F120°

Thermal decomposition of mechanically alloyed nanocrystalline fcc Fe60Cu40

A ferromagnetic and supersaturated fee Fe60Cu40 solid solution was prepared by mechanical alloying (MA). The phase transformations of the as-milled Fe60Cu40 powder upon heating to 1400 degrees C and subsequently cooling to room temperature were characterized by differential thermal analysis (DTA) and thermal magnetic measurement. The fee Fe60Cu40 solid solution decomposes into alpha-Fe(Cu) + gamma-Fe(Cu) + Cu(Fe) upon heating from 300 to 460 degrees C, and on further heating, alpha-Fe(Cu) transforms to gamma-Fe(Cu) at 640 --> 760 degrees C; during cooling, the reverse transformation occurs from 800 --> 640 degrees C (obtained from thermomagnetic measurement) or from 700 --> 622 degrees C (obtained from DTA). The gamma reversible arrow alpha transformation in mechanically alloyed Fe60Cu40 nanocrystalline occurs in a wide temperature range; the transformation temperature is higher than that of the martensite transformation in as-cast Fe-Cu alloys, but is much lower than that of the allotropic transformation of pure Fe. These differences may be caused by the different fabrication process, the nonequilibrium microstructure of MA, as well as the inhomogeneous grain size in alpha-Fe(Cu). High resolution transmission electron microscope (HRTEM) observations carried out in the specimen after the DTA run show that N-W or K-S orientation relationships exist between alpha-Fe(Cu) and Cu(Fe), which also represent the orientation relationship between alpha-Fe(Cu) and gamma-Fe(Cu) due to excellent coherency between gamma-Fe(Cu) and Cu(Fe). The grain size of the alpha-Fe(Cu) is inhomogeneous and varies from 50-600 mm. Energy dispersive x-ray spectroscopy (EDXS) result shows that the Cu content in these alpha-Fe(Cu) grains reaches as high as 9.5 at. % even after DTA heating to 1400 degrees C, which is even higher than the maximum solubility of Cu in gamma-Fe above 1094 degrees C. This may be caused by the small grain size of alpha-Fe(Cu).

Porous octahedral PdCu nanocages as highly efficient electrocatalysts for the methanol oxidation reaction

Porous octahedral PdCu alloy nanocages are prepared by templating against Cu2O together with the disproportionation and displacement reactions at room temperature. Through tuning the molar ratios of the Pd/Cu precursors (PdCl42−/Cu2O), the morphology and the electrocatalysis of the octahedral PdCu nanocages can be controlled. The PdCu-5 (Pd/Cu precursor ratio of 1 : 5) nanocages exhibit superior electrocatalytic activity (1090 mA mgPd−1) towards the methanol oxidation reaction, 2.9 times higher than that of the commercial Pd/C catalyst, with remarkable durability. The combined characterization by SEM, TEM, XRD and XPS demonstrated that the excellent performance and durability of the PdCu-5 nanocages can be ascribed to their excellent hollow and porous structures as well as the electronic effect between the noble metal Pd and the cheap metal Cu. Herein, this kind of nanomaterial with novel cage-like structures is a promising catalyst for direct methanol fuel cells (DMFCs).

Hollow PdCo alloy nanospheres with mesoporous shells as high-performance catalysts for methanol oxidation

Monodisperse hollow mesoporous PdCo alloy nanospheres are prepared via a simple galvanic replacement reaction. The as-prepared PdCo hollow nanospheres have small diameter, such as Pd78Co22 nanospheres of diameter about 25 nm and mesoporous shells about 4-5 nm. The Pd78Co22 hollow mesoporous nanospheres possess the largest electrochemical active surface areas (ECSA, 53.91 m2 g-1), mass activity (1488 mA mg-1) and specific activity (2.76 mA cm-2) towards to methanol oxidation relative to the Pd68Co32, Pd92Co8 hollow mesoporous nanospheres and commercial Pd/C catalysts. Moreover, the activity of Pd78Co22 after long-term stability tests is still the best and even better than those of fresh Pd68Co32 and commercial Pd/C catalysts. The PdCo catalysts not only effectively reduce the Pd usage by forming hollow structure, but also fully realize the Pd-Co alloying effects for enhancing the methanol oxidation catalytic performance.