Stable Boride Research Articles

Characterization of rapidly solidified ultrahigh boron steels

Rapidly solidified powders of two binary FeB alloys and two boron-containing tool steels exhibiting a fine lamellar eutectic microstructure were investigated in both rapidly solidified and consolidation conditions. Both binary alloys contained ferrite and the metastable boride Fe 3B, the latter of which was transformed into the stable boride Fe 2B upon annealing at 610°C. The hardness of the as-quenched powders was very high (up to 1050 HV) because of high volume fractions of both phases. A loss of hardness with increasing annealing temperature was observed and may be attributed to the transformation of the metastable boride. Both tool steel powders showed the presence of austenite and stable borides. Fine, uniform microstructures consisting of 2–3 μm grains were developed in all materials after consolidation at temperatures ranging from 800 to 1150°C. When compression tested at 900°C, all materials exhibited a low stress exponent of 2–3, which may be explained in terms of grain boundary sliding. It appears that the borides containing chromium, nickel and/or molybdenum in both tool steels are harder than iron-borides at high temperatures, making the tool steels superior to the binary alloys over the entire temperature range up to 1100°C.

Microstructure and mechanical properties of melt atomized and rapidly solidified ultrahigh boron alloy steels

Abstract Rapidly solidified high alloy steel powders with boron contents up to 4.2wt.% and particle sizes of less than 80 μm have been produced by argon melt atomization. The microstructure consists of metastable and stable borides with dendritic or complex shapes within the α- or γ-solid solution matrix. After consolidation by extrusion the materials exhibit a fine-grained equiaxed microstructure with grain sizes of less than 5 μm. These particle-reinforced ultrahigh boron steels reveal superior elastic and strength properties at room and elevated temperature. In addition, structural superplasticity with large elongation to failure ( > 500%) occurs at higher temperatures of about 1000°C. This is due to grain boundary sliding accommodated by lattice and grain boundary diffusion.

Synthesis of niobium boride powder by solid state reaction between niobium and amorphous boron

Niobium boride powders were synthesized by solid state reaction between niobium metal powder and amorphous boron powder. The formation of niobium borides was found to be dependent on temperature. Single phases of the stable borides, NbB and NbB 2, were formed by heating mixed powders corresponding to the stoichiometric compositions at 1000 °C for 60 min. A single phase of Nb 3B 4 was obtained at a higher temperature of 1800 °C as a result of the promoted diffusion of boron atoms in niobium metal. All synthesized powders were well dispersed and had particle sizes of 5–10 μm. The sinterability of the synthesized NbB 2 powder was evaluated at high pressure (4 GPa) and temperature (1600 °C) for 15 min; a single-phase sintered compact of NbB 2 was formed with a relative density of 98% and a Vickers microhardness of 2600 kg mm −2.

Microstructural refinement and associated strength of copper alloys obtained by mechanical alloying

Abstract Several dispersoid-containing copper alloys have been produced by ball milling mixtures of copper and other powders followed by hot extrusion of the milled powders. In all cases the added material is quickly dispersed to very fine size within the copper. This dispersion forms more quickly at lower dispersoid fractions and for slightly soluble additions, such as chromium and niobium, and less rapidly for stable boride additions. Particle stability, both during extrusion as well as during subsequent heat treatments, is controlled by a volume diffusion coarsening process. The grain size is determined uniquely by grain boundary pinning by the particles. The strengths of the materials obtained are completely explained by an Orowan-type dislocation model, and the grain size dependence of strength is shown to be a consequence of the interdependence of particle distribution and grain size.

Influence of alloyed elements on electrolytic boriding of medium-carbon steel

1. All the alloyed elements decrease the rate of increase of the thickness of the borided layer. 2. Elements which increase the binding strength in the austenite lattice have the most important decelerating effect on the diffusion of boron. They decrease the diffusion rate of carbon and deform the lattice of the metalsolvent (γ-Fe) very little. 3. The elements which form stable borides increase the concentration of FeB in the layer and decrease the amount of the second boride (Fe2B). 4. With increasing degrees of alloying of the steel and with increasing saturation temperatures the structure of the boride needles becomes more complex.

The Thermodynamic Stability of Refractory Borides

Reactions between nitrogen gas and metal borides and between graphite and metal borides have been studied to obtain qualitative information on the relative stabilities of the metal boride phases. At high temperatures many borides are inert toward carbon or nitrogen. The most stable borides are found to be those of Ti, Zr, Nb, and Ta of the fourth and fifth groups of the periodic table. Data are summarized in terms of limits to the heats of formation.