Observation of Internal Structures of Martensites of Fe-Rh Alloys

There have been few crystallographic studies on the phase transition of Fe rich Fe-Rh alloys and the phase diagram remained obscure in many respects. It was established that the alloys with higher Fe concentration undergo fcc-bcc martensite transformation by quenching from the high-temperature γ phase. However, no transmission electron microscope (TEM) investigation of the martensites has been reported, possibly due to the difficulty in preparing a good TEM specimen of the alloy with extraordinarily excellent corrosion resistance. Recently we have developed a technique to prepare a TEM specimen by electro-polishing and ion-milling, and examined internal structures of the martensites of Fe-5 at%Rh and Fe-25 at%Rh alloys by employing X-ray diffractometry, optical and transmission electron microscopy. The results are summarized as follows: (1) Both Fe-5 at%Rh and Fe-25 at%Rh alloys show surface reliefs, which are due to martensite transformation. (2) The martensites are of a dislocated lath type, whose lattice invariant shear is slip. Burgers vectors of the dislocations are 〈111〉 and 〈001〉 types. (3) In an Fe-25 at%Rh alloy twinning shear is also associated with the lattice invariant shear of the transformation. (4) The Kurdjumov-Sachs orientation relationship is observed between the retained γ and martensite phases of an Fe-25 at%Rh alloy.