Embrittlement theories

Master curves can be used to predict creep resistance, embrittlement, and other property changes over time at a given temperature, or the time it takes for the modulus or some other parameter to reach a critical value. For example, a mbber hose may burst or crack if its modulus exceeds a certain level, or an elastomeric mount may fail if creep is excessive. The time it takes to reach the critical value at a given temperature can be deduced from the master curve. Frequency-based master curves can be used to predict impact behavior or the damping abiUty of materials being considered for sound or vibration deadening. The theory, constmction, and use of master curves have been discussed (145,242,271,277,278,299,300). 

In considering the various theories it is also apparent that many of them may be considered as alternative descriptions of essentially the same physical process, or as descriptions of parallel processes which collaborate in the failure. Thus a complete description of hydrogen embrittlement in a given situation will almost inevitably incorporate aspects of several of the following theories. 

Several theories of the mechanism of embrittlement have been put forward " and further details are given in Section 8.4. 

Among polymers, thermosets are especially difficult to study for many reasons structural complexity, making difficult the chemical analysis, lack of rigorous tools to investigate the macromolecular structure lack of physical theories to interpret the change of properties (e.g., embrittlement) against structural changes. 

However, this problem is especially difficult in the case of thermosets, for the most important practical case of embrittlement. As a matter of fact, there is (to our knowledge), no widely accepted theory capable of predicting the fracture properties as a function of the number of chain scissions or crosslinks created by aging. This research domain is wide open and the problem must be, for the moment, treated empirically. 

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 relationship of brittle fracture to plastic deformation has, of course, been elaborated in various ways with the aid of dislocation theory, e.g. nucleation of microcracks has been discussed in terms of piling-up of dislocations [124]. Davies [145] has shown that embrittlement requires the presence of islands of martensite (about 1 pm in size) and has suggested that cracks are initiated in the martensite or at the martensite-ferrite interface. 

The role of microstructure has been treated in terms of the trap theory of hydrogen embrittlement [65, 133]. The theory assumes that, regardless of the mechanism of embrittlement (see Sect. 2.2.6.3), the concentration of trapped hydrogen... 

Internal Pressure This theory considers hydrogen embrittlement to be caused by the formation of molecular hydrogen and the resulting buildup of pressure at internal voids or other internal surfaces [142, 143]. It provides an explanation for blistering in low-strength steels but does not adequately account for other forms of degradation such as HSC. 

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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