Gradient Nanostructured Tungsten and the Thermal Shock Response

Abstract

In service environments, the surface of tungsten is damaged first. To enhance the surface toughness, strength, and hardness of tungsten, a lamellar gradient surface layer (up to about 50 μm) from nano to microscale was produced on as-rolled pure tungsten using a dry sliding method called surface mechanical attrition treatment. The dry-sliding temperature and time were established based on the evolution of grain size from room temperature to 750 °C and the coefficient of friction. During the refinement of surface grains with temperature, above the ductile-brittle transition temperature(DBTT) of tungsten, the grains rapidly changed their orientation and gradually changed into a fiber-based structure during deformation. 500°C is a suitable operating temperature, for dynamic recrystallization occurred at 750 °C. The COF decreased with friction time and stabilized at about 50 min, indicating near-saturation in surface refinement. Repetitive thermal loads of 10–30 MW/m 2 were exerted on the gradient layer surface, and corresponding response was analyzed. Heat load of 30 MW/m 2 is the threshold for grain fast growth of tungsten. The nano-gradient W sample undergoes relatively slow recrystallization and grain growth, in contrast to holes and protrusion structure as well as recrystallization in the as-rolled W sample.

Keywords: Dry sliding, Coefficient of friction, Gradient layer, tungsten