In welding for example there Weld Bead, HAZ, and BM should not have a stark difference in their hardness values. That is why Post Weld Heat treatment is employed (of course, one of the primary reasons is residual stress).
With that analogy in the hindsight, will it be correct to assume that the hardness and E mismatch between the dispersoids/precipitates/inclusions or any other second-phase particles in the Metal alloy matrix will lead to cracking, without giving any external stimulus OR in case of external stimulus the stark mismatch will cause an accelerated failure?
Is there a thumb rule which can say that if there is a 10 percent mismatch or a 40 percent mismatch between the dispersoids/precipitates/inclusions or any other second-phase particles in the Metal alloy matrix, then the crack propagation will happen like this OR tendency to crack increases?
As far as welds go the strength of the weld metal can be lower than base metal referred to as "under matched" , or like wise matched or over matched depending on application.
Mismatch to any degree is crack initiator or reduces ductility - including carbides in steel. The 10% mismatch marginally better that 40% but no were near the required < 1% - on the order of small thermal expansion differences that the base metal can clearly handle without cracking.
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