Understanding the creep fracture behavior of aluminum alloys and aluminum alloy metal matrix composites

In a previous study on the creep fracture behavior of aluminum alloy metal matrix composites and their respective un-reinforced alloys a new form of the phenomenological Monkman–Grant, MG, relationship was proposed as follows: t f ε ˙ min = f ( n ′ ) ( ε ˙ min n ′ − 1 ) ε f tf is time to failure, ε ˙ min the minimum or steady strain rate, f(n′) a scalar function of the Monkman–Grant exponent, n′, and ɛf the strain to failure. This equation is, in fact, very similar to the Dobeš–Milička, DM, one. In the new equation, however, the exponent n′ of the minimum strain rate term, ε ˙ min , is equal to one. In this way, the creep rupture behavior of materials can be deepened by analyzing the physical meaning of a new unit-less parameter, K = f ( n ′ ) / ( ε ˙ min n ′ − 1 ) . It is readily seen that this parameter equates ɛ2/ɛf, where ɛ2 would be the rupture strain accumulated during the secondary creep regime if the entire creep test progressed under the steady state period and ɛf is the strain to failure. So, K quantifies the relative importance of secondary creep strain with respect to that of primary and the tertiary creep stages. For the case of the Al alloys significant variations of K with experimental creep conditions (applied stress) are found when the grain size and/or aspect ratio is large. On the contrary, K is nearly stress independent for fine microstructures. The additional influence of the metal–ceramic interface in metal matrix composites leads to a more complex evolution of this term.