Embrittlement intermetallics

A final method of strengthening Fe->12Ni-0.5Al involved precipitate strengthening. Copper was chosen as the precipitate material, since it has very low solubility in Fe and precipitates as a copper-rich terminal solid solution containing a small amount of iron. Thus, copper would not be expected to form an embrittling intermetallic compound. Copper additions ranging from 0.5 to 3.0at.% were... 

Type 315-This has a composition that provides a similar oxidation resistance to type 309 but has less liability to embrittlement due to sigma formation if used for long periods in the range of 425 to 815°C. (Sigma phase is the hard and brittle intermetallic compound FeCr formed in chromium rich alloys when used for long periods in the temperature range of 650 to 850°.)... 

Chemical reaction This involves the formation of distinct compounds by reaction between the solid metal and the fused metal or salt. If such compounds form an adherent, continuous layer at the interface they tend to inhibit continuation of the reaction. If, however, they are non-adherent or soluble in the molten phase, no protection will be offered. In some instances, the compounds form in the matrix of the alloy, for example as grain-boundary intermetallic compound, and result in harmful liquid metal embrittlement (LME) although no corrosion loss can be observed. 

My own criticism on the theories proposed for superplasticity can be summarized in one word, electrons , or more accurately the lack of electronic consideration. This is similar to the theoretical consideration brought forward in the study of LME (liquid metal embrittlement) described in the previous chapter - no electronic consideration. As shown in Table 2, all superplastic alloys of binary system are found either at eutectic or eutectoid compositions. This is illustrated in Fig. 14 in which a few binary phase diagram involving superplastic alloys are shown. However, the people who made efforts in the formulation of theories did not consider this well-known fact important enough to incorporate into their theory formulation [24], In fact, this observation is so consistent one should ask the question of the special attributes associated with eutectic or eutectoid composition. Or the fact that the intermetallic compounds with superelastic property are all of the peritectic type. It must be emphasized that to this date there is no report of finding superplasticity in congruently-melting compounds. 

The detrimental effect of such intermetallic phases in Ni brazed joints is particularly severe when the microstructures are coarse, and this is true also of Si precipitated from Al-Si brazes that are slow cooled from the brazing temperature. Finally it should be noted that while egress of embrittling species can enhance the mechanical characteristics of the braze joints, their ingress can initially degrade... 

High alloys with little exception suffer some embrittlement if exposed to sustained high-temperature service due to the formation of intermetallic compounds. Conditions and rates of embrittlement vary with the alloy. Check with alloy manufacturers for specific information. High alloys containing enough nickel to ensure an austenitic microstructure are, like austenitic stainless steels, unaffected by low-temperature embrittlement. 

Do not use ferritic stainless steels containing more than 16% chromium in the 750 F to 1,000 F temperature zone. They invariably embrittle, because of precipitation of a chromium-rich constituent. Even the 11 to 13% chromium alloy sometimes seems to embrittle in this way, for reasons not clear. Unfortunately, this is a common temperature zone for many refinery and chemical plant processes. These ferritic stainless steels should not be used above 1,000°F, either. They will embrittle for a somewhat different reason the formation of an iron-chromium intermetallic compound called sigma phase. 

This B segregation affects the segregation of impurities such as O, S, and P which embrittle conventional alloys. However, such impurities seem to be of little importance for NijAl since NijAl, and other intermetallics of high purity with clean grain boundaries are still brittle. 

Environmental embrittlement, which affects the mechanical behavior of various other intermetallics, e.g. the other B2 alu-minide, FeAl, has not been found for NiAl (Liu, 1992 Lahrman etal., 1993b). 

There are no structural alloys which rely on silica formation for protection primarily because Si tends to embrittle most alloys. Some intermetallic compounds (e.g., MoSi2), silicide coatings, and ceramics (e.g., SiC) are silica formers. The following is a brief discussion of the oxidation of model Fe-Si alloys. 

The content of Chapter 9 sheds light on validity and application for different solids of the theory, which has been considered in previous chapters. The calculated values of cohesive energy and bulk modulus are compared with the experimental results. We present data on superconductivity, the embrittlement of metals, the electronic density of states, properties of intermetallic compounds, and the energy of vacancy formation. 

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