Microstructural Characterization of As-cast Hf – B Alloys

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 αHf SS and B-Rhom SS , the intermediate compounds HfB and HfB 2 and the liquide L. The reactions are the eutectic L ⇔ αHf SS + HfB and L ⇔ HfB 2 + B-Rhom, the peritectic L + HfB 2 ⇔ HfB and the congruent formation of HfB 2 .


Introduction
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 .In this sense, phase diagram information becomes extremely important.We have investigated several Me-Si-B systems from the point of view of phase stability and, as part of this work, the evaluation of binaries metal-silicon (Me-Si) and metal-boron (Me-B) became necessary.We have verified inconsistencies in binary systems such as Nb-B 7 , V-B 8 and Ta-B 9 , 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.Revisiting the Hf-B system provides the opportunity to confirm previous data from an investigation using higher purity materials and better analytical techniques.Among other issues, 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.
The currently accepted Hf-B phase diagram, from the work of Rudy and Windisch 10 , based on results of XRD analysis by means of Debye-Scherrer photographs and metallography of the samples, is shown in Figure 1.This diagram indicates the stability of the phases βHf bodycentered cubic (BCC), αHf hexagonal compact (HCP), rhomboedric boron (B-Rhom) and liquid (L), as well as the intermediate phases HfB and HfB 2 .
Rogl and Potter 11 assessmed the Hf-B system based on the experimental results of Rudy and Windisch 10 and Portnoi and Romashov 12,13 .Bitterman and Rogl 14 repeated the same assessment into the study of the Hf-B-C ternary system.Table 1 shows the proposals of Rudy and Windisch 10 , Rogl and Potter 11 , Portnoi and Romashov 12 and Bitterman and Rogl 14 for the reactions in the Hf-B system.been calculated the composition interval for each alloy from the mass losses associated to the melting steps, supposing that all mass losses were either from Hf or B volatilization.The composition adopted for each alloy is expressed by the mean value of this interval.The alloys were characterized via scanning electron microscope in the back-scattered electron mode, and X-ray diffraction were performed in a Shimadzu XRD6000 diffractometer, at room temperature, with CuKα radiation and graphite monochromator.For the analysis via scanning electron microscope, the alloys were prepared following standard metallographic procedures: hot mounting in resin; grinding in the sequence #220-#4000 with SiC paper; and polishing with colloidal silica suspension (OP-S).The images were obtained in a LEO 1450VP  instrument.For the X-ray diffraction experiment the samples were mechanically ground and sieved to below 80 mesh.The measurement conditions were: 10° < 2θ 90°; 0.05° (2θ step) and 2 seconds integration time.The phases in each sample were identified based on the simulated diffractions patterns obtained from the program PowderCell for Windows ® (version 2.3) 15 using crystallographic data shown in Table 2.

Results and Discussion
Table 3 shows the phases present in each alloy as well as the type of invariant reaction observed in each microstructure.
The alloy with composition 12 at.%B intended to verify the composition of the liquid in the eutectic transformation L ⇔ αHf SS + HfB in Hf-rich region, as proposed by Rudy and Windisch 10 .The diffractogram of the Hf88B12 alloy (Figure 2a) has indicated the presence of αHf solid solution (αHf SS ) and HfB phases while the micrograph of this alloy (Figure 3a) shows essentially a typical eutectic microstructure composed of αHf SS 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 ⇔ αHf SS + 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.
The alloys with compositions between 20 at.%B and 50 at.%B intended to verify the composition of the liquid in the peritetic transformation L + HfB 2 ⇔ HfB, as proposed by Rudy and Windisch 10 .The diffractogram of the Hf77B23 alloy (Figure 2b) has indicated the presence of αHf SS and HfB phases.It is noted a relative increase in the intensity of the HfB peaks compared to the previous alloy.The micrograph of this alloy (Figure 3b) indicates primary precipitation of HfB and the αHf SS + HfB eutectic microstructure in the remaining region, in agreement with the previous result.
The diffractograms of the Hf75B25 (Figure 2c) and Hf50B50 (Figure 2d) alloys have indicated the presence of αHf SS , HfB and HfB 2 phases.In the microstructure of the Hf75B25 alloy (Figure 3c), precipitation of HfB is observed with a typical αHf SS + HfB eutectic microstructure in the remaining region.Due to the small amount of HfB 2 phase and a low contrast with respect to HfB, it is not possible to point the HfB 2 phase in the micrograph.On the other hand, the microstructure of the Hf50B50 alloy (Figure 3d) shows clearly the primary precipitation of HfB 2 , which is involved by HfB, an evidence of the peritectic formation of HfB, with a typical αHf SS + HfB eutectic microstructure in the remaining region.The absence of HfB 2 phase in the Hf77B23 alloy (Figure 2b) and its presence in the Hf75B25 alloy (Figure 2c) indicates that the transition from HfB primary to HfB 2 primary should occur between 23 at.%B and 25 at.%B, which correspond to the liquid composition of the peritectic reaction L + HfB 2 ⇔ HfB.This is in agreement with the proposal of Rudy and Windisch 10 at approximately 24 at.%B (Figure 1) and slightly in disagreement with the composition calculated by Rogl and Potter 11 and Bitterman and Rogl 14 , respectively, 22.5 at.%B and 22 at.%B.
The alloy with composition 66.6 at.%B intended to verify the composition of the liquid in the congruent transformation L ⇔ HfB 2 , as proposed of Rudy and Windisch 10 .The diffractogram of the Hf33.4B66.6 alloy (Figure 2e) has indicated the presence of the HfB 2 phase and minor amounts of αHf SS and HfB.In agreement with these results, the microstructure of this alloy (Figure 3e) shows large amount of primary HfB 2 , the αHf SS and HfB phases being present in the last regions to solidify.In addition, these results confirm the congruent formation of HfB 2 as considered for Rudy and Windisch 10 (Figure 1), Rogl and Potter 11 and Bitterman and Rogl 14 .
The alloys with compositions between 70 at.%B and 100 at.%B intended to verify the composition of the liquid in the eutectic transformation L HfB 2 + B-Rhom SS , as proposed of Rudy and Windisch 10 .The diffractograms of the Hf7.7B92.3alloy (Figure 2f) and Hf01B99 alloy (Figure 2g) have indicated the presence of HfB 2 and B-Rhom SS phases.In the microstructure of the Hf7.7B92.3alloy (Figure 3f), primary precipitation of HfB 2 is observed with a typical HfB 2 + B-Rhom SS eutectic microstructure in the remaining region.On the other hand, the Hf01B99 alloy (Figure 3g) presents a large amount of primary B-Rhom SS phase, and the presence of the HfB 2 phase in the last parts to solidify, possibly formed by the HfB 2 + B-Rhom SS eutectic.These results indicate that the liquid composition of the L HfB 2 + B-Rhom SS eutectic reaction should be around 99 at.%B, as proposed by Rudy and Windisch 10 , Figure 1, and Bitterman and Rogl 14 , but slightly in disagreement with the composition calculated by Rogl and Potter 11 at 97.3 at.%B.

Conclusions
In this investigation we have carried out a detailed microstructural characterization of as-cast Hf-B alloys which allowed the evaluation of the invariant reactions involving the liquid phase in this system.The phases identified are αHf SS and B-Rhom SS , the intermediate HfB and HfB 2 compounds and the liquid L. The reactions are the eutectic L ⇔ αHf SS + HfB and L ⇔ HfB 2 + B-Rhom SS , the peritectic L + HfB 2 ⇔ HfB and the congruent formation of HfB 2 .In general, a good agreement was found between our data and those of the currently accepted Hf-B phase diagram as proposed by Rudy and Windisch 10 .

Table 1 .
14oposals of Rudy and Windisch 10 , Rogl and Potter 11 , Portnoi and Romashov12Bitterman and Rogl14for the reactions in the Hf-B system.

Reaction Composition (% at. B) Temperature (°C) Reference Remarks
Figure 1.Hf-B Phase Diagram proposed by Rudy and Windisch 10 .

Table 2 .
Crystalographic data of the phases of the Hf-B system.

Table 3 .
Alloy compositions, identified phases in each alloy and type of invariant reaction observed.