EFFECT OF ALLOY COMPOSITION ON THE MECHANICAL PROPERTIES AND FRACTURE BEHAVIOR OF TUNGSTEN HEAVY ALLOYS

ABSTRACT The objective of this experimental study is to investigate the effect of the alloy composition on the mechanical properties and the fracture behavior of the liquid phase sintered tungsten heavy alloys. Tungsten alloy with three different compositions 93%W-4.9%Ni-2.1%Fe,91%W-6%Ni-3%Co and 90.5%W-5.6%Ni-2.4%Fe-1.5%Co were prepared and used in this investigation. Elemental powders were mixed using planetary mixer for 5 hours to ensure suitable homogeneity. Uni-axial compaction pressure of 200 MPa was applied to obtain standard tensile and impact specimens. Vacuum liquid phase sintering was carried out under different temperatures from 1470ᵒC up to 1530ᵒC for 90 minutes.
The effect of changing the alloy composition particularly, the binder constituents was characterized in terms of density, hardness, impact resistance and tensile properties for samples in the as sintered state. Fracture behavior of the used tensile fracture specimens having different compositions was studied, and the relation between the obtained fracture modes and tensile properties of these alloys was indicated.
The obtained results indicated that hardness increases with increasing the cobalt content, Moreover, the tensile strength increases, in a first stage, by adding cobalt up to 1.5wt.%, due to the strengthening effect of cobalt. Further increase in cobalt content decreases strength. On the contrary, the ductility and impact resistance showed a continuous decrease with increasing the cobalt content. The results, also, clearly showed that the addition of cobalt to tungsten heavy alloy has a great beneficial hardening effect. On the other hand, it causes embrittlement due to the formation of brittle intermetallic compounds, particularly, with tungsten at the tungsten-matrix interface leading to degradation of alloy properties, which makes the dissolution of these intermetallics by a post sintering heat treatment of great importance.
It was also shown that the strength of the matrix or the bonding strength of the interface (between tungsten particles and matrix) was the controlling criterion of thenature of final fracture. Poorer matrix strength or interfacial strength was found to initiate the fracture by separation of tungsten particles either by matrix failure or by interface failure. On the other hand, tensile fracture takes place predominantly by cleavage fracture of tungsten particles, if both the matrix and interface are stronger than the tungsten particles. The tensile fracture surfaces clearly indicated that failure in case of W-Ni-Fe based heavy alloys was due to matrix or interface failure. Whereas, W-Ni-Fe-Co heavy alloys were failed predominantly by cleavage fracture of tungsten particles. While, in case of W-Ni-Co based heavy alloys, the dominant fracture mode was intergranular tungsten separation. This fracture behaviour was in good correlation with the obtained mechanical properties , and indicates that the cobalt content in the binder plays a key role in dictating the failure behaviour.