Rheological behaviour of multiphase metallic materials

Why focus on the behavior of multiphase materials?

First of all, real materials are always heterogeneous, with the possible exception of high-purity, undeformed single crystals. In single-phase materials, dislocation density, grain size and orientation are heterogeneously distributed. A fortiori, when several phases are present, the shape, size, orientation and spatial relationships between homogeneous domains are all sources of heterogeneity. Metals in their pure state generally have insufficient mechanical properties (hardness, yield strength), and are therefore most often used in the form of alloys. As early as protohistory, bronzes (alloys of copper and tin) and brasses (alloys of copper and zinc) replaced pure copper. When an element is added to a pure metal in a liquid state, three structures can be observed after the mixture has cooled:

• or a homogeneous solid solution (e.g. for Ni-Cu in any proportion);

• or a matrix containing a second, minority phase, consisting of inclusions solidified before the matrix, or precipitates formed in the solid state (e.g. oxides in copper, carbides in iron);

• or a mixture of two or more phases with distinct chemical, geometric and mechanical characteristics, whose volume fractions are of the same order of magnitude.

Depending on the case, this is referred to as a two-phase or multi-phase alloy. Certain heterogeneous materials obtained by sintering, as well as certain composites, without being alloys in the strict sense of the term, fall into the same category of heterogeneous materials covered in this article.

While multiphase materials are therefore essential, it should also be stressed that the presence of several phases with different properties can be beneficial. The first basic idea is to combine the qualities (and not the defects) of both phases. Thus, mixing a hard but brittle phase (e.g. WC tungsten carbide) with a ductile but soft phase (e.g. cobalt) leads to an interesting compromise of hardness and ductility in the WC-Co composite. However, care must be taken to ensure that the mixture is not both brittle and soft. But the interest in the behavior of two-phase materials doesn't stop there, as the following two examples show. In the case of structural steel plates, the need to increase yield strength by reducing grain size leads manufacturers to end the rolling range in the "warm deformation" range, i.e., depending on the case, in the ferrite + austenite or ferrite + cementite range. Similarly, titanium alloys used in aerospace generally exhibit superplastic behavior in the two-phase α + β range (particularly in the vicinity of equivolumic composition), which can be put to good use...