Ductilizing effect

The rhenium ductilizing effect is more pronounced at higher Re additions (Fig. 6.1) and reaches a maximum at the solid solubility limit (approximately 27 wt% Re). Beyond that, embrittlement occurs due to the formation of a hard and brittle a-phase. The hardness of W-26 Re alloy is 20% larger than that of unalloyed tungsten and the hot tensile strength... 

This ductilization effect of boron depends sensitively on the Al content (Liu et al., 1985 Aoki, 1990). An Al content of 24 at.% corresponds to the Al solubility limit of NijAl and cannot be lowered, i.e. a further reduction only produces the disordered y-Ni-Al phase (see Fig. 20). With higher Al contents the ductilization effect becomes smaller and above the stoichiometric composition of 25 at.% Al, poly-... 

This dramatic ductilization effect of boron has been and still is the subject of elaborate experimental and theoretical studies which, however, have not yet led to an agreement on the physical reasons for the boron effect, i.e. the mechanistic understanding of this ductilization effect is still unclear. This has been the subject of a set of papers recently, and the present state of knowledge and the controversial issues have been overviewed (Liu, 1991b). The important factors are briefly summarized in the following. 

It is known that alloying with, e.g. Fe, can improve the ductility of NiAl (Baker and Munroe, 1990). Indeed substantial increases in the tensile ductility at room temperature of monocrystalline NiAl with a soft orientation have been obtained by alloying with small amounts, i.e. about 0.1 at.% of Mo, Ga. or Fe (Darolia et al., 1992a) 0.1 at.% Fe produces 6 % ductility in contrast to 1 % ductility without Fe whereas 0.5 % and more Fe produces only 2 % ductility. The physical reasons for this ductilization effect are not yet clear (Noe-be and Behbehani, 1992). 

Examination of oven-aged samples has demonstrated that substantial degradation is limited to the outer surface (34), ie, the oxidation process is diffusion limited. Consistent with this conclusion is the observation that oxidation rates are dependent on sample thickness (32). Impact property measurements by high speed puncture tests have shown that the critical thickness of the degraded layer at which surface fracture changes from ductile to brittle is about 0.2 mm. Removal of the degraded layer restores ductiHty (34). Effects of embrittled surface thickness on impact have been studied using ABS coated with styrene—acrylonitrile copolymer (35). 

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