Mechanical properties and fatigue crack growth in tungsten deposited by RF-plasma

Abstract

Fracture toughness and fatigue crack growth rates of free-standing thick W deposits produced in a protective atmosphere using radio frequency inductively-coupled plasma process were investigated. Stress-strain behavior of the deposits was studied using four-point bending and their fracture toughness was estimated from three-point bending following the ASTM E1820 standard. The fatigue crack growth rates were measured in symmetrical bending using a resonance approach. The results were compared with commercially available bulk tungsten prepared by powder metallurgy that served as a reference. The results showed that the deposits had significantly lower stress-strain properties: the elastic modulus and tensile strength ranged from 30% to 50% of the reference bulk values. The crack growth rate tests together with fractographic analysis suggested that a static failure by inter-granular decohesion predetermines the fatigue properties of the deposits and causes high slopes of the corresponding fatigue crack growth rate curves. Contrary to this, the reference sheet failed trans-granularly. The inter-granular failure mode of the deposits was caused by grain boundary embrittlement, a phenomenon probably related to oxides. The lowest obtained oxygen concentration in deposit was nearly by two orders of magnitude higher, than in reference sheet. The distribution of oxygen was studied by TEM and the amorphous and crystalline oxide phases were identified at splat interfaces. The presence of oxygen in the deposits produced by the inherently non-oxidizing RF technology calls for control of oxygen content in the feedstock powder. The performed characterization provided baseline data of mechanical and fatigue properties of RF plasma sprayed tungsten.