Abstract
An accurate knowledge of several metal-boron phase diagrams is important to evaluation of higher order systems such as metal-silicon-boron ternaries. The refinement and reassessment of phase diagram data is a continuous work, thus the reevaluation of metal-boron systems provides the possibility to confirm previous data from an investigation using higher purity materials and better analytical techniques. This work presents results of rigorous microstructural characterization of as-cast hafnium-boron alloys which are significant to assess the liquid composition associated to most of the invariant reactions of this system. Alloys were prepared by arc melting high purity hafnium (minimum 99.8%) and boron (minimum 99.5%) slices under argon atmosphere in water-cooled copper crucible with non consumable tungsten electrode and titanium getter. The phases were identified by scanning electron microscopy, using back-scattered electron image mode and X-ray diffraction. In general, a good agreement was found between our data and those from the currently accepted Hafnium-Boron phase diagram. The phases identified are αHfSS and B-RhomSS, the intermediate compounds HfB and HfB2 and the liquide L. The reactions are the eutectic L ⇔ αHfSS + HfB and L ⇔ HfB2 + B-Rhom, the peritectic L + HfB2 ⇔ HfB and the congruent formation of HfB2.
Highlights
Metal-silicon-boron (Me–Si–B) alloys have been intensively studied due to their potential for the development of high temperature structural materials[1,2,3,4,5]
The diffractogram of the Hf88B12 alloy (Figure 2a) has indicated the presence of αHf solid solution and HfB phases while the micrograph of this alloy (Figure 3a) shows essentially a typical eutectic microstructure composed of αHfSS and HfB phases, which confirms the Rudy and Windisch’s proposal[10] (Figure 1) for the composition of the liquid that participates of the reaction
L ⇔ αHfSS + HfB at approximately 13 at.% B, close to the composition calculated by Rogl and Potter[11] and Bitterman and Rogl[14], respectively, at 14.4 at.% B and 15 at.% B
Summary
Metal-silicon-boron (Me–Si–B) alloys have been intensively studied due to their potential for the development of high temperature structural materials[1,2,3,4,5] Considering that these materials are highly demanded in service, multicomponent based alloys seems to be the only possibility to satisfy all the requirements for structural integrity[6]. We have verified inconsistencies in binary systems such as Nb–B7, V–B8 and Ta–B9, what demonstrates that the refinement and reassessment of phase diagram data is a continuous work. In this investigation, the microstructural characterization of as-cast (AC) hafnium–boron (Hf–B) alloys has been carried out. It contributes to understand the solidification pathway of more complexes hafnium–silicon–boron (Hf– Si–B) alloys, helping the goal of establishing the liquidus projection of this ternary system
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