Bulks of Al-B-C obtained by reactively spark plasma sintering and impact properties by Split Hopkinson Pressure Bar

O Vasylkiv,H Borodianska,T S Suzuki,A Kuncser,M A Grigoroscuta,P Li,D Demirskyi,I Pasuk,P Badica

doi:10.1038/s41598-019-55888-z

Abstract

Mixtures of B4C, α-AlB12 and B powders were reactively spark plasma sintered at 1800 °C. Crystalline and amorphous boron powders were used. Samples were tested for their impact behavior by the Split Hopkinson Pressure Bar method. When the ratio R = B4C/α-AlB12 ≥ 1.3 for a constant B-amount, the major phase in the samples was the orthorhombic AlB24C4, and when R < 1 the amount of AlB24C4 significantly decreased. Predictions that AlB24C4 has the best mechanical impact properties since it is the most compact and close to the ideal cubic packing among the Al-B-C phases containing B12-type icosahedra were partially confirmed. Namely, the highest values of the Vickers hardness (32.4 GPa), dynamic strength (1323 MPa), strain and toughness were determined for the samples with R = 1.3, i.e., for the samples with a high amount of AlB24C4. However, the existence of a maximum, detectable especially in the dynamic strength vs. R, indicated the additional influence of the phases and the composite’s microstructure in the samples. The type of boron does not influence the dependencies of the indicated mechanical parameters with R, but the curves are shifted to slightly higher values for the samples in which amorphous boron was used.

Highlights

Compounds, such as borides, with a strongly covalent character show excellent wear resistance, hardness, refractoriness, and chemical inertness properties
The following observations are of interest: (i) The α-AlB12 peaks in the x-ray diffraction (XRD) pattern of the raw powder cannot be visualized in the patterns of the sintered samples
This suggests that this phase is consumed during the SPS heating processes reacting with B4C and B to form Al boride (AlB31), borocarbide (AlB24C4, Al0.3B13.3C1.3, Al3BC), oxide (Al2O3) or boroxide (Al4B2O9) phases

Summary

Introduction

Compounds, such as borides, with a strongly covalent character show excellent wear resistance, hardness, refractoriness, and chemical inertness properties. The most studied is the α-AlB12 phase composed of B12 icosahedra This material can serve as a model for other structurally alike phases, e.g., AlB24C4 The crystal structure of the BC phases, namely α-AlB12 (Al0.083B), AlB12C2 (Al0.5B6C or Al0.083BC0.167) or AlB24C4 (Al0.25B6C or Al0.0416BC0.167) can be viewed as a stack of B12-like icosahedra packed into the rhombohedral, tetragonal, and orthorhombic unit cells, respectively[2]. A more compact and as close as possible to a cubic ideal packing is expected to provide the best impact resistance properties, but no evidence has been presented to support this idea. The stoichiometry, crystal structure and stability domains of different B12-like icosahedra-containing phases, including B4C, need further clarifications. Another problem is the quality of the available raw materials. The use of amorphous boron promotes slightly higher values of the mechanical parameters without influencing their dependence on R

Methods

Results

Conclusion