How does the Nb-content change the martensite crystallography in Ti-Nb alloys?

What is it about?

In this article we explain the atomic structures of the martensitic phases in the Ti-Nb alloy system, which serves as a model for Ti-alloys with partially or fully stabilized beta-phase. The atomic arrangement, i.e. the crystallography, in the martensites depends strongly on the Nb content (= beta-stabilizer content, in general). Hence, Nb-lean alloys form a hexagonal martensite, Nb-rich alloys do not form martensite at all, and alloys of intermediate Nb-content form an orthorhombic martensite.

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Why is it important?

In the orthorhombic martensite the crystallographic unit cell dimensions vary considerably with chemical composition. This was already well known from the literature. Here, we demonstrate that adding or removing Nb also has a strong impact on the atomic arrangements inside the unit cell (atom positions), too. Hence it is among the first experimental studies that describes the crystallographic changes occurring upon stabilization of the high-temperature Ti bcc beta-phase by alloying in detail.