Surface oxides softening in tungsten and molybdenum single crystals

Athermal-to-thermal transition defines the temperature range over which surface film softening occurs in bcc metals. Tungsten and molybdenum should exhibit substantial softening at temperatures in the vicinity of room temperature. For tungsten deformed at strain rates of the order 10-4 s-1 the transition temperature is above 6000K (16); for molybdenum the corresponding temperature is approximately 4500K (17). Mechanical testing of these metals, coated and uncoated, at room temperature should disclose if softening exists in these metals. Experiments on oxide-coated single crystals of tungsten and molybdenum were performed to test this idea. Surface film softening occurs for Mo and the effect is of the same order of magnitude (100 MN/m2 out of 300 to 400 MN/m2) as in W