Initial Powder Composition Research Articles

Mechanical properties of hot isostatically pressed nanograin iron and iron alloy powders

Hot isostatic pressing (hipping) was performed on a variety of iron and iron alloy powders produced by attrition ball milling. Microhardness and compression tests were used to determine the mechanical properties. Fracture morphology was studied by scanning electron microscopy. For each specific alloy, increasing the hipping temperature causes an increase in density up to a maximum. Hipping at still higher temperatures improves the bonding without any change in density. The minimum temperature required for obtaining the maximum density depends on the initial powder composition. While iron powder processed in argon (Fe [Ar]) reached a maximum density at 580°C, an identically processed iron carbon alloy (Fe–2C [N]) reached its maximum density above 850°C. Two types of compression strain–stress curves were obtained: up to a certain temperature, most powders showed an increase in compression yield stress with temperature. Hipping above this point causes a decrease in the compression yield stress, and an increase in elongation. This behaviour is entirely different than that of a stainless steel powder (Fe–18Cr–8Ni), which shows substantial work hardening for hipping temperatures ranging between 965 and 1050°C.

Electron-microscopic study of morphology and phase composition of lithium-titanium ferrites

In the present paper, the phase composition and morphology of lithium-titanium ferrospinel, synthesized by ceramic technology and subjected to thermal (T) and radiation-thermal (RT) annealing at a temperature of 1373 K, are studied. RT annealing of samples was performed by heating radiation of high-power electron beam pulses with energy 1.8 MeV. Bulk-density powders and powders compacted under a pressure of 1200 kg/cm2 were used as samples. The results obtained indicate the polyphase composition of initial powders. Annealing decreases the relative content of the secondary LiFeO2-based phase and increases the relative content of the master LiFe5O8-based phase. The efficiency of this process depends on the powder compactness and the heating regime. RT annealing significantly increases the rate of dissolution of the LiFeO2-based phase. An impact diffraction analysis has shown that the secondary phases have polycrystalline structures, whereas the LiFe5O8-based phase is formed by monocrystalline particles. Annealing of the compacted powder by an electron beam completely eliminates polycrystalline aggregates with ultradisperse grained structure.

Extrusion and selected engineering properties of Mo–Si–B intermetallics

In this study, an extrusion process has been developed to produce defect free, high-density rods of Mo–Si–B material. An initial powder composition (53.5 vol.%, 91 wt.%) of 66 vol.% Mo5Si3Bx (T1)–16 vol.% MoB–18 vol.% MoSi2 was mixed with a paraffin-wax based binder (46.5 vol.%, 9 wt.%) and extruded using a twin-screw extruder. Following binder removal by a combination process of wicking and thermal degradation, the material was sintered at 1800°C. The bulk density of the sintered material was 90–92% of theoretical. Thorough binder removal was evidenced by low impurity levels: 258±6 ppm carbon and 772±10 ppm oxygen. The material demonstrated excellent high temperature oxidation resistance. The calculated parabolic rate constant is 1.1×10−2 mg2/cm4/h at 1600°C. The extruded material was also successfully tested as a resistance heating element. These materials show promise for the development of heating elements with enhanced performance compared to current MoSi2-based heating elements.

Grain core study of Fe1-xCrx nanograins obtained by mechanical alloying

Mechanical alloying (MA) of Fe1-xCrx powder mixtures was performed over a wide range of concentration. Both x-ray diffraction and TEM analyses show that, after 30 hours of milling, the powder particles consist of nanocrystalline grains less than 10 nm in size. The kinetics of mixing is studied by Mössbauer spectrometry. This is the first time that the FeCr mixing state has been studied as a function of the milling conditions and the initial powder composition. This mixing state is defined by a parameter (d) calculated from the hyperfine field values. The alloying process is weakly composition dependent (for x40 at.% Cr), but is linked to the energy input to the powder. Especially an energy threshold must be transferred to the powder to reach a complete alloying. By studying the hyperfine field distributions of the Mössbauer spectra (for Cr<40 at.%), it seems that the Fe1-xCrx nanograin cores are quite homogeneous in composition and have hyperfine parameters close to those of the bulk alloys.

Effect of the extent of nitriding silicon on the binding properties in the silicon nitride-phosphate binder system

con. The chemical composition of the initial powders is shown in Table 1 and their specific surface, determined by the method of low-temperature adsorption of nitrogen, is given in Table 2.

Effect of the fractional composition of initial powders on the structure and properties of high-speed steels

1. With increasing particle size of the powder fractions, microhardness of steel 10R6M5 increases, microhardness of steel R0M2F3 decreases; this has to do with the difference in phase composition of these steels. The structure of steel 10R6M5 obtained from powder of the coarse fractions (>250 μm) consists chiefly of martensite and austenite; the structure of the fine fractions consists of δ-ferrite. The structure of steel R0M2F3 does not depend on the powder size, and it is austenite and martensite; the latter was not detected by the method of optical metallography. The carbide phase in the investigated steels does not depend on the size of the fractions and consists of carbides MC and M6C (steel 10R6M5) and MC and M2C (steel R0M2F3). 2. After annealing and subsequent heat treatment differences in the structure and properties of powdered steels 10R6M5 and R0M2F3 were not detected, nor was any effect of the size of the fractions on the properties discovered. 3. Specimens obtained by compacting by rolling of powder of different fractions are close to each other in properties and structure; differences in the properties of the initial powders of different fractions disappear after rolling.