Modeling two materials in one part

Hi everyone

A colleage and I are trying to analyze a structure that is composed of two materials which are fused together.

I have tried defining a volume region within the part to separate the two, but the only option that we have is to apply a material to specific surfaces.

I tried to apply the material to the surfaces of the volume region (completely enclosed by the other material), but the results of this analysis do not yield the expected result (there is almost no change when compared to the same part analyzed with just one material).

Is there a better way to do this?

Thanks
Mark