Boron ductilization

The majority of ah these classes, even noneutectic ahoys, have been processed successfuhy by rapid solidification technology. This technology provides a beneficial alternative in the form of a flexible ductile foh when materials that are inherently brittle are used. Examples are the nickel—boron—shicon ahoys and many others, when produced using conventional technology (5). 

It is assumed that the tube is made of a composite material which is composed of stiff duetile fibers arranged in a parallel uniform array in a ductile matrix. The eomposites of metal-metal type are eonsidered, first of all. However, the combination of a ductile matrix with brittle fibers can be easily accounted for, as well. As an example of the latter ease serves the aluminium alloy reinforced with boron fibers. 

It is seen that titanium alloyed with boron and silicon is promising owing to high strength and Young modulus, and has high ductile potential because fails with ductile fracture mode. As to the structural and fractographical features of the alloys studied they are as follows ... 

Most of these are carbon-manganese-molybdenum alloys with small additions of chromium and/or nickel plus vanadium or niobium. Vanadium or niobium acts as a carbide stabilizer and grain refiner, improving both elevated temperature strength and notch ductility. An exception is Fortiweld (MOBO 45 is the same steel), which is a boron-treated 1/2% molybdenum steel. This alloy is the cheapest of the group, but has hardly been used for reactors in the U.K., possible because its impact properties in thick sections are not so attractive as alternative steels. 

Ni3Al is the most studied and best known intermetallic because it has been used as a strengthening phase in the superalloys for a long time, and because its ductility problems can be overcome, i.e. it can be ductilized by microalloying with boron (Aoki and Izumi, 1979 Liu and Koch, 1983 Aoki, 1990). This means that it can be produced and tested without major experimental difficulties, and thus it was selected preferentially in the past for studies on the behavior of intermetallics (see... 

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. 

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