Abstract
Tungsten tetraboride (WB4) is experimentally considered as potentially superhard material and is therefore expected to have highly structural stability and enhanced resistance against plastic deformation and failure. The examinations of bond-deformation mechanism suggest a significantly soft bond-deformation pattern induced by ionic W-B bonding for nominal WB4 in experiments, largely responsible for the limitation of its strength and structural integrity. Computations on the structures and mechanical properties for WB4 show a novel thermodynamically favored MoB4-type phase with excellent mechanical properties and remarkable incompressibility along c direction. The illustrations of nonstoichiometry and x-ray diffraction spectra rationalize the experimental observation of nominal composition WB4 as defective tungsten borides (W1-xB3 (x<0.25) or WB4-x (x>0.25)). The results provide new insight into the real structural and mechanical properties of tungsten borides.
Highlights
Transition metal borides are of particular interest with outstanding mechanical properties,[5,6,7,8,9,10] among which tungsten tetraboride (WB4) is one of the most promising candidates for potentially superhard materials
The heat of formation ( Hf) of candidate WB4 phases was calculated by the equation of Hf =Etotal(WBx) (Etotal(W) + x Etotal (B)), in which the cubic tungsten (W) and a-B12 are adopted for the calculations
For the experimentally observed nominal WB4 (n-WB4) (see Fig. 3(a) and 3(b)), an interesting finding is that the ideal tensile strength is calculated to be 5.8 GPa along [210] direction, which is the lowest values predicted in transition metal borides so far, even smaller than that of pure iron (12.6 GPa).[30]
Summary
The studies of novel hard or superhard materials, in particular, on the synthesis and design, have been intensified for decades due to the technological and industrial applications.[1,2,3,4] Transition metal borides are of particular interest with outstanding mechanical properties,[5,6,7,8,9,10] among which tungsten tetraboride (WB4) is one of the most promising candidates for potentially superhard materials. (Received 21 December 2011; accepted 8 February 2012; published online 12 March 2012)
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