J.T. Guo

Microstructure, interface and mechanical property of the DS NiAl/Cr(Mo,Hf) composite

A NiAl–Cr(Mo) composite containing Hf was directionally solidified under Ar atmosphere in Al2O3–SiO2 ceramic mold by the standard Bridgman method. The microstructure in the as-grown and hot isostatic pressing (HIP) conditions was studied using SEM, HREM and TEM equipped with EDXS. The composite was mainly composed of three phases, i.e. lamellar Cr(Mo), NiAl matrix and semi-continuously distributed Ni2AlHf. The common growth direction of NiAl and Cr(Mo) is , and the interface between them was atomically flat with no transition layer. Fine G-phase precipitates were formed in the presence of Si which was from the ceramic shell molds used during the directional solidification. The G-phase precipitates were cuboidal in shape and coherent with NiAl matrix. After the HIPed and aged treatment, the G-phase precipitates completely disappeared and the amount of intercellular Ni2AlHf phase was partially reduced. The mechanical properties were measured by tensile and compressive tests within the temperature range of 25∼1100°C. The BDTT of the HIPed and aged materials is significantly higher than the as-grown composite. The yield stress of the composite was higher than that of many NiAl-based alloys and possible strengthening mechanism were discussed.

Orientation relationship and interfacial structure between ζ-Ti5Si3 precipitates and γ-TiAl intermetallics

Abstract The orientation relationship (OR) and the interface structure between Ti 5 Si 3 precipitates and the γ -TiAl phase have been investigated systematically. The habit plane of Ti 5 Si 3 in TiAl was determined to be (0001) ζ ‖(111) γ . However, there is no low-index direction of the two phases parallel to each other in the plane. This “abnormal” OR has been predicted precisely from a recently developed geometrical method, in which the overlap of reciprocal lattice points of two adjoining crystals is utilized to obtain the optimum OR. In spite of the significant difference in crystal structure between TiAl and Ti 5 Si 3 , the interface was found to be semi-coherent with good matching and has the largest possible displacement shift complete lattice corresponding to the bicrystal. The energy of the interface is also discussed. The translational state between the two lattices and the chemistry of the terminating plane of Ti 5 Si 3 have been determined by using high-resolution transmission electron microscopy.

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

Effect of Ag alloying on microstructure, mechanical and electrical properties of NiAl intermetallic compound

In this paper, the effects of Ag on microstructure, mechanical and electrical properties of NiAl intermetallic compound were investigated. The present results show that the NiAl-Ag alloys consisted of two phases: beta-NiAl and Ag-rich solid solution. The amount of the Ag-rich phase increased with increasing Ag content. Ag has very low solubility in NiAl and they form pseudo-binary monotectic systems. The addition of 0.5-1at.% Ag to NiAl increased its strength while the addition of more than 1at.% Ag decreased its strength. The strengthening and weakening effects come from solid solution hardening and second ductile phase softening. In addition, Ag alloying can improve NiAls room temperature compressive ductility. The NiAl-Ag alloy has high hardness and electrical conductivity that make it an attractive candidate for electrical contact material

Grain growth and kinetics for nanocrystalline NiAl

The ordered intermetallic NiAl has emerged as a suitable candidate materials for high temperature application because of its high melting point, low density, excellent oxidation resistance, high thermal conductivity, high temperature thermodynamic stability and high stiffness. However, lack of room temperature ductility and toughness are its major shortcomings. As an important new class of nonequilibrium materials, nanocrystalline materials have drawn wide attention in recent years. The interest in these materials stems partly from their potential to transform nominally brittle compounds into ductile materials. However, a problem common to all nanocrystalline materials is their lack of thermal stability. Investigations of the grain size stability have been reported in various nanocrystalline materials, including pure metals, oxides, compounds, and composites. However, reports are still lacking on condensed ordered intermetallics, especially on NiAl. In this work, isothermal and isochronal annealing were carried out on a nanocrystalline NiAl compact prepared by mechanical alloying and vacuum heat pressing, with an effort to investigate its thermal stability.

Synthesis of NiAl–TiC nanocomposite by mechanical alloying elemental powders

A NiAl-TiC nanocomposite has been synthesized by mechanical alloying from Ni, Al, Ti, and C powders. During milling, an abrupt reaction occurred, resulting in simultaneous formation of NiAl and TIC phases. It is suggested that two separate exothermic explosive reactions, i.e, Ni + Al --> NiAl and Ti + C --> TiC, were involved. However, the reactions were incomplete with the existence of a small amount of elemental powders. Prolonged milling led to a gradual formation of NiAl and TiC as well as grain refinement. The final grain size for TiC was 3.5 times as large as that for NiAl. The formation mechanism of the NiAl-TiC nanocomposite during mechanical alloying was also discussed

Tensile creep behaviour of NiAl-Cr(Zr) multiphase intermetallic alloy

Abstract The microstructure of a multiphase NiAl-33.5Cr-0.5Zr intermetallic alloy was examined by SEM with energy dispersive spectroscopy and TEM. The tensile creep behaviour of the hot isostatically pressed NiAl-33.5Cr-0.5Zr alloy was studied. The results of the creep test indicated that all of the creep curves under the present test have similar characteristics: a short primary creep stage, a dominant tertiary creep stage, and nearly identical creep strains (~45%). The apparent stress exponent and the apparent activation energy were analysed and discussed. The mechanism of the creep deformation was also analysed by the observation of TEM.

Deformation mechanism for high temperature creep of a directionally solidified nickel-base superalloy

Modern nickel-base superalloys, which contain a high volume fraction of hard cubical {gamma}{prime} precipitates embedded coherently in a softer {gamma} matrix, are used as turbine blade material due to their good resistance to creep deformation at high temperature. In this present study, the authors will analyze the relationship between the steady-state creep rate and the applied stress of DZ17G, a modern DS cast nickel-base superalloy used for producing turbine blades and vanes for aeroengine applications. On the basis of TEM observations and calculations of the threshold stress for different deformation mechanism, the authors finally propose the deformation mechanism for creep of DZ17G superalloy at high temperature.

Effect of Nd on microstructure and mechanical properties of NiAl-based intermetallic alloy

Abstract The influence of Nd on the microstructure and mechanical properties of NiAl–28Cr–5.5Mo–0.5Hf alloy was investigated. The study suggests that appropriate addition of Nd produces an improvement of the compressive strength and ductility of the alloy. In addition, the microstructure of the alloys is also refined gradually with increasing Nd content.

On the orientation relationship between Ti5Si3 precipitates and B2 phase in a Ti-47Al-2W-0.5Si alloy

Acad Sinica, Inst Met Res, Lab Atom Imaging Solid, Shenyang 110015, Peoples R China. CNR, TEMPE, I-20133 Milan, Italy.;Yu, R (reprint author), Acad Sinica, Inst Met Res, Lab Atom Imaging Solid, Shenyang 110015, Peoples R China