Elemental Cr Powders Research Articles

Improving sintering kinetics and compositional homogeneity of Inconel 625 superalloy open-cell foams made by suspension impregnation method

Open-cell Inconel 625 superalloy foams were prepared by suspension impregnation method which involves replication of pure nickel foam ligaments with a mixed powder suspension and subsequent heat treatments. The objective of the present study is to find a suitable method ensuring complete densification of the superalloy foam ligaments without using low-melting additives and achieving uniform chemical composition at moderate sintering temperatures. Foams prepared using the above methodology were evaluated for the microstructure and compositional variation across the foam ligaments by scanning electron microscopy (SEM) coupled with electron dispersive spectroscopy (EDS). Quasi-static compressive properties of the foams were evaluated. It is shown that if elemental powders of Cr, Mo and Ni are used as additives to the superalloy powder suspension, high ligament density with uniform chemical composition conforming to the Inconel 625 specification is achieved at moderate sintering temperatures. These results are attributed to the formation of low melting eutectics and to acceleration of the diffusion kinetics due to the non-equilibrium thermodynamic conditions created by large compositional gradients in the presence of elemental powders.

Cr3C2–20%Ni cermets prepared by high energy milling and reactive sintering, and their mechanical properties

Cr3C2–20%Ni cermets were fabricated with Cr, C and Ni mixed-powder by high energy milling and reactive carburising sintering. The elemental powders of Cr, C and Ni were mixed with proper ratio (Cr/C = 3:2 atomic ratio) and milled to the nanometre crystallite sizes (about 20–30 nm). Specimens were cold-isostatically pressed at 200 MPa and sintered at 1453, 1503 and 1553 K with vacuum atmosphere for 1 h, respectively. It was shown that the relative density of the sintered specimens increases first and then decreases slightly with increasing sintering temperature. The maximum values of the hardness (86.7HRA) and bending strength (1140 MPa) were achieved at 1503 K.

Effects of milling and subsequent consolidation treatment on the microstructural properties and hardness of the nanocrystalline chromium carbide powders

The influence of milling and subsequent consolidation treatments on the microstructural properties and hardness of the fabricated Cr 3C 2, Cr 7C 3 and Cr 23C 6 ceramic powders are investigated. For this reason, the elemental powders of Cr and C were mixed with proper ratio and then milled to the nanometer crystallite sizes (between 6 and 20 nm) and then were consolidated by using uniaxial cold press and subsequent heat treatment (at 1100 °C for 1 h) in Argon atmosphere. Microstructures of consolidated samples were characterized using X-ray diffraction (XRD) and microhardness measurements. A drastic increase in crystallite size of the samples was observed due to the effect of heat treatment. However, the as-consolidated samples still maintained their nanocrystalline characteristic with an average grain size of less than 100 nm. Besides, a very high hardness of 25 GPa was achieved for the Cr 3C 2 composition. This high hardness is attributed to the formation of carbide phases in the consolidated samples.

Microstructural and Mechanical Characteristics of in Situ Synthesized Chromium-Nickel-Graphite Composites

Cr-C-Ni composites were synthesized in situ from elemental powders of Cr, Ni and C by high energy milling followed by reactive sintering. The milled powders with the grain size in nano-scale were pressed to compacts and sintered. During the thermal treatment that was performed after the pressing the chromium carbide was formed first and then the Cr3C2-Ni cermets were sintered in one cycle. The interface between the binder phase and the carbide grains of the in situ composite has a good bonding strength as it is not contaminated with oxidation films or other detrimental surface reactions. The microstructure of such cermets is fine-grained and more homogeneous than that of cermets produced by conventional method. As a result the cermets have higher hardness, transverse rupture strength and 1.2-2 times higher wear and erosion resistance.

固体潤滑材分散青銅系焼結軸受の開発 第１報 青銅圧粉体へのＣｒ添加による焼結ち密化促進について

In order to develop Graphite dispersed sintered bronze bearing material standing in the use of a higher load application, effects of Cr addition on sintering properties and microstructures of the bronze matrix were investigated. Mixed powders were prepared from elemental Cr powders and atomized 90Cu-10Sn powders, of which particle sizes were 106-50μm and 1-38μm, respectively. Compacts pressed at 390 MPa were sintered at 670-1030 K for 0.3-180 ks in H2. With increasing sintering time at 1030 K, pores decrease rapidly with the refinement of grain size in the bronze matrix surrounding Cr particle. This indicates that the densification of the bronze is promoted by the diffusion of Cr.