Martensite Lattice Correspondence and Twinning in Uranium Alloys
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MARTENSITE LATTICE CORRESPONDENCE AND TWINNING IN URANIUM ALLOYS
J.G. SPEER AND D.V. EDMONDS Department of Metallurgy & Science of Materials, University of Oxford, Parks Road, Oxford OXI 3PH England
ABSTRACT A new lattice correspondence has been deduced for the y*a' martensite reaction in dilute uranium alloys. The new correspondence involves a lattice deformation with significantly smaller principal strains than those previously considered, although more atomic shuffles are required. From an analysis of possible transformation twinning modes based on the new correspondence, the further possibility of a deformation twinning mode in these alloys is theoretically redicted. The mode is compound, described by (071)(023) [012] 032], and involves a twinning shear (g = .044) which is smaller than any value previously predicted or observed, although again a larger number of shuffles would be required. The significance of these results is discussed.
INTRODUCTION The crystallography of martensite transformations and of deformation twinning has been a subject of interest for many years. Because of the low symmetry of the P (complex tetragonal) and a (orthorhombic) uranium structures, compared to those of the more commonly studied metals, uranium provides an opportunity for evaluating crystallographic theories. For example,
uranium was
historically
the
first
metal
found
to
exhibit
type
II
(irrational composition plane) deformation twinning, and is known to have several different type I and type II twinning modes [1-3]. The martensitic transformation behaviour of uranium alloys is also complex; two different reactions occur, viz. y+a' and รท+a', depending on the alloy composition. More than one lattice correspondence has been proposed for the P+a' reaction in dilute alloys [4], and different correspondences have also been considered in the case of the yra' reaction [5-7], although little experimental evidence is available for this second reaction. This paper attempts to further the theoretical understanding of the crystallography of these shear processes. LATTICE CORRESPONDENCE The y+a' martensitic reaction will be considered in some detail. The high-temperature y-phase is body-centered cubic and the low temperatue a'martensite is base-centered orthorhombic with two atoms per lattice site. The lattice parameters at room temperature vary slightly with alloy composition and are unknown at the reaction temperature. Alloys which undergo this reaction consist of uranium with small additions of elements such as Cr, Mo, Nb, Ti, Zr. It should also be noted that at somewhat higher alloy contents, the martensitic product becomes slightly monoclinic (4 y * 90*). The procedure of Crocker and Ross [5] has been used to calculate the principal strains and directions for each correspondence. For the purposes of comparison, the lattice parameters of Crocker shall be used for the calculations. For the y-phase ao = 3.465A, and for the a' phase a = 2.854A, b = 5.869A, c = 4.955A and y = 90*
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