The Propagation of Slip- and Spark-Induced Cracks in Polycrystalline Molybdenum

Abstract

The effect of temperature on the fracture stress of as-recrystallized cast polycrystalline molybdenum specimens and those containing spark- and slip-induced cracks has been investigated in the range 4·2–230 K at strain rates of ∼ 1·5 × 10⁻⁴ s^−l. The method of cracking that gave the most reproducible results was a 1% precompression at 77 K. Spark-cracks did not alter the ductile–brittle transition temperature of the material; the effect of the precompression was to lower it by ∼ 14 K. In all cases microscopic plastic deformation preceded failure. The brittle-fracture stress of the spark-cracked specimens decreased from ∼ 660 MN/m² at 210 K to ∼ 200 MN/m² at 4·2 K. The fracture (effective surface) energy calculated according to the Griffith–Orowan formula from the 4·2 K test is 5·7 J/m², compared with estimates of true surface energy using Gilman's model of 2·6–4·5 J/m². The brittle-fracture stress of precompressed specimens remained approximately constant at 650 ± 50 MN/m² and was identified with the crack-propagation stress, σ _P . This value agrees well with a determination of 640 ± 20 MN/m², made by an independent method, on another batch of cast polycrystalline molybdenum (also of mean grain dia. 27 μm).