Interface austenite—martensite

Our intention in the remainder of this section is to build up a picture of some of the various interfaces that are present in martensitic systems. Our approach will be to consider microstructural elements of increasing complexity, beginning first with the case of the simple austenite-martensite interface and culminating in the investigation of martensitic wedges within the host austenite. In all of these cases, the primary theoretical engine in our analysis will be the compatibility conditions and their outcome as typified by eqns (10.62) and (10.63). 

Austenite-Martensite Interfaces. As a first exercise in the machinery set fortli above, we pose the question of whether or not a simple interface can exist between a cubic crystal and its tetragonal offspring. Without loss of generality, this situation can be represented via the condition... 

In addition to this constraint, we must also satisfy compatibility across the austenite-martensite interface which in light of the deformation gradients specified... 

Figure 3 (a) [001] HRTEM image of the distorted austenite of figure 4 after (a) 1 min. and (b) 5 min. irradiation with 400 keV electrons inside the microscope. The increase of the modulation amplitude is apparent. The line in (b) indicates an interface between two adjacent martensite variants. 

For concreteness, we assume that a > 1 > yS as is the case in systems such as In-Tl (a = 1.0221 and p = 0.9889) and Ni-Al (a = 1.1302 and = 0.9392). As a result, we note that there is not an ordering of the eigenvalues of the form 1 > 2 = 1 > which immediately lends itself to the conclusion that in the case of a cubic-tetragonal transformation, there is no simple interface between the host austenite and a single variant of the tetragonal martensite except in the unlikely event that the parameter takes the value 1.0. 

From the standpoint of the kinematic arguments introduced earlier, the additional complexity that attends situations such as the wedge microstructure considered here is the fact that we no longer have only a single interface on which to satisfy the conditions of compatibility. Our calculations on the austenite-twiimed-martensite interface served as a warm up exercise for the present case. Each and every interface of the type in fig. 10.35 must itself satisfy conditions such as that of eqn (10.59). As a result, the imposition of compatibility across all of the interfaces takes the form of the collection of equations... 

Like in the simpler case of the interface between an austenite region and twinned martensite, this set of equations can be solved by recourse to a divide and conquer policy. Our main interest in showing these equations is to show the reader what is involved in setting up such a problem and to inspire him or her to read Bhattacharya s beautiful 1991 paper. 

When ferritic-martensitic steels are used for structural components, usually dissimilar welded joints with austenitic stainless steels are used, too. When these are subject to elevated temperatures, creep property evaluation and creep-fatigue evaluation are needed. In the case of dissimilar welds, close attention should be paid to the location of failure. Under certain conditions, failure could occur at the interface between the two materials [27]. 

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