On the fracture toughness of fine-grained Mo-3Si-1B (wt.%) alloys at ambient to elevated (1300 °C) temperatures

Mo‐Si‐B Alloys for Ultrahigh‐Temperature Structural Applications

“…Lemberg et al123 confirmed the importance of microstructural size scale and morphology with their study of the fracture behavior of two ultra‐fine‐grained Mo‐3Si‐1B alloys, one mechanically alloyed and the other reaction synthesized, comparing their results to the earlier work of Kruzic et al117 on coarse grained Mo‐3Si‐1B alloys. Each of the Lemberg alloys contained ∼55 vol.% continuous α ‐Mo and but unlike the Kruzic alloys, the grain size was far smaller, on the order of 5 μm.…”

“…§ The initiation toughnesses at 1300 °C reported by Lemberg et al123 represent equivalent K values which would have been obtained had samples large enough to satisfy the requirements of linear elasticity. Instead, J ‐integral (i.e., nonlinear elastic fracture toughness) values were estimated based on crack tip opening displacements at crack initiation, as given by Eq.…”

“…Hot pressing of blended powders provides a relatively simple means of creating a two‐ or three‐phase microstructure without the pitfalls of direct solidification; for this reason it is now widely used in processing of Mo‐Si‐B alloys. The technique has been used to successfully compact and sinter mixtures of molybdenum silicides, Mo and B,81, 113 precast Mo‐Si‐B powders,82–87 elemental Mo, Si and B114 as well as rapidly‐solidified,18, 19, 32, 95–101 surface‐modified,115–118 reaction synthesized15, 119–123 or mechanically alloyed4, 10, 11, 14, 123–137 Mo‐Si‐B powders. Heat treatment is required after hot pressing to further homogenize the material.…”

“…5, corresponding to plane stress loading of a perfectly plastic material, whereas a more accurate value for their material is 0.586. Accordingly, the more appropriate recalculated toughness values given by Lemberg et al123 is also given here, in parenthesis;…”

“…The room temperature fracture behavior of Mo‐Si‐B alloys has been thoroughly investigated by Choe et al,78, 79, 85 Kruzic et al,116–118 Lemberg et al123 and Alur et al99 for various alloy compositions, phase distributions and processing methods. Choe et al85 studied the room temperature fracture behavior of two cast alloys (Mo‐4.2Si‐1.1B and Mo‐12.1Nb‐4.2Si‐1.1B) and two hot‐pressed powder alloys (both Mo‐6.1Si‐1.2B, differing starting powders).…”