Mechanical and Fatigue Properties of Tungsten Heavy Alloy Prepared by RF-Plasma

Abstract

Tungsten heavy alloy (WHA) of W-Ni composition was deposited from a blend of standard thermal spray powders using radio frequency inductively coupled plasma torch (RF-ICP) in a protective atmosphere. The deposit (RF WHA) contained a fully developed WHA structure; i.e.; spherical W particles embedded in a Ni-rich matrix. The bending tensile strength Rm; bending yield strength Rp;0.2; and elastic modulus of the deposit were compared with two W-Ni-Co references fabricated by powder metallurgy (PM WHA) via sintered and quenched (PMSQ); and forged and annealed (PM-FA). While the RF deposit properties are comparable with the PM-SQ reference; the PMFA exhibited higher mechanical properties. The deposit showed very limited ductility A < 3%. The fatigue crack growth rate in the deposit measured in bending (R < -1) was comparable to the PM-SQ reference material in the near-threshold region whereas the forged PM-FA had significantly better fatigue performance. In the near-threshold fatigue regime; the crack growth took place in the Ni-rich matrix. In the Paris regime; the similar fracture mode was observed; with the exception of PM-SQ; where the tungsten particles fracture contributed significantly. The static failure was exclusively trans-particle in RF WHA; while both PM WHAs failed by a mix of ductile matrix failure and trans-particle cleavage fracture. The fracture toughness of the deposit was significantly lower than the references. These early results indicate that RF-plasma spray is a suitable and efficient manufacturing method for production of WHA materials; however with limited mechanical properties in some aspects.

Fatigue Crack Growth Rate Description of RF-Plasma-Sprayed Refractory Metals and Alloys

“…The high load elbow, where the values asymptotically approach the cyclic fracture toughness A , was only observed for the W-10Ni* material. This fatigue crack growth acceleration was promoted by a brittle fracture of the W particles from an untreated powder [ 15 ]. For other materials, this kind of asymptotic behavior was not observed as it probably happened in the range not captured in our experiments.…”

“…For all these, a recently developed, unified, simple rectangular bar geometry specimen (EDM-cut from the thick deposits) was used [ 18 ], thereby significantly simplifying the sample preparation as well as the testing procedures. The obtained results were complemented with the data published in our earlier studies [ 9 , 15 , 19 ] and numerically characterized using the new fitting method introduced in this paper. This method was designed to provide a simple description of the fracture behavior of a particular material by its fatigue crack growth threshold , fracture toughness , as well as Paris-type coefficient D and exponent p .…”

“…Up to now, only a limited number of papers on the fracture properties of thermally sprayed refractory metals or WHA have been published [ 8 , 9 , 15 ]. In fact, a complex fracture mechanical properties characterization of this important class of materials accompanied with a thorough mechanical properties characterization is still missing.…”