Pseudomorphosis
This image is an example of a microstructure that is called ‘pseudomorphosis’ among microstructure researchers. This term denotes a microstructure where the morphology, the physical form and intergrowth relations of the substances come from a different, earlier state of the material, than the strict chemical composition and phase makeup of the material as now observed. It “looks like” it had been at an earlier stage, while it has subsequently been chemically changed into a new state, without changing the morphological microstructure.
In the shown example, we can see a microstructure that is reminiscent of the solidification of a multicomponent melt, a slag from an industrial process in this case. In such a multicomponent melt, some crystals from early, which are then free to grow, so they develop an idiomorphic crystal habitus. Such idiomorphic crystals are easy to spot when the matrix later solidifies, e. g. into a glass, or into a finegrained eutectic intergrowth. In the image shown, you can see the crystals termed “N” (bottom right). They are more or less idiomorphic, and as a package of idiomorphic platelets they surround a central area filled with the substance “C”, which is intergrown in an eutectoid fashion with another minor very bright phase. The structure looks exactly like a eutectic melt pod, even having a central bubble (P), which is a pore, that has a perfectly round outline, as it would have if it were a bubble from late stage volatiles exsolved from a melt.
However, chemical analysis has shown that the substance “C” is in fact CA2, a calcium aluminate phase (CaAl4O7) that solidifies from its relevant slags at a very high temperature (in excess of 1700 C). It is independently known that the industrial process from which this sample is taken, did never reach such temperatures (it operated at circa 1550 C). So this cannot be a solidification microstructure of a melt that crystallized CA2 after the phase N (which is NA11 in this case).
The solution to this riddle is that the composition of the area under observation was changed after the solidification. The original formation was a calcium aluminate slag congruent with the actual process, with much higher Calcium to Alumina ratios, so that other calcium aluminates form (CA CaAl2O4 and C12A7 Ca12Al14O33), which solidify at temperatures consistent with the process. At that time, the idiomorphic phenocryst, eutectic melt pocket, and bubble (pore) structures were formed. After that, diffusion took place, in which calcium diffused out of the local area, and alumina diffused in (such calcium aluminates are fairly easy diffusers). In this stage, preexisting CA was transformed into CA2 by solid state transformation. The CA2 newly formed, passively took over the morphology of the preexisting CA, forming the pseudomorphosis the we can observe in the image. The giveaway that this is what happened, is shown by the white arrow, where the phase C is observed to not smoothly fit to the surrounding crystals N. A melt would not do this (it would wet the preexisting crystals) but a solid-solid phase transformation does create such new, irregularly shaped porosity.
The example shows how important it is to closely observe any microstructure, as the mistaken interpreation that this was a CA2 eutectoid would have implied a major temperature excursion at this point and would have had severe implications for the industrial process.