DEGRADATION OF SHAPE MEMORY EFFECT

An important parameter for deciding whether or not a SME alloy is suitable for practical applications is the magnitude of the strain reversa1 accompanying martensite reversion. This research is concerned with elucidating metallurgical factors that cause degradation of this heat-activated recovery strain, ER. After explaining what is meant by degradation, two manifestations of degradation recently identified in near-monotectoid uranium-niobium alloys will be described. The first was associated with the onset of plastic deformation of the martensite beyond the reversible strain limit, EL ; a reduction of ER from 5.25% at 8% total strain, i.e. EL, to 2.9% at 12% total strain was observed. A second type of degradation depended strongly on the heating rate during reversion ; the ER for an imposed strain of 6.95% was reduced from a value of 5.25% to 1.3% when the heating rate was decreased from 40 deg/sec to 0.05 deg/sec. Degradation was attributed to a change in the transformation path and the interjection of time-dependent, low temperature aging reactions

Recent observations of phase transformations in a U--Nb--Zr alloy

Dilatometry, x-ray diffraction, hardness, and optical microscopy were used to characterize polycrystalline U--7.5% Nb--2.5% Zr. Upon quenching this alloy from the bcc phase at 800 deg C, a metastable modification corresponding to a slight tetragonal distortion of the bcc lattice along with a short-range atom displacement ordering was revealed by x-ray diffraction. Minor compositional variations, differing quenching rates, and low temperature aging cause important alterations in this metastable structural state. Slight depletion of the alloying elements increases the degree of tetragonality. Aging the quenched alloy initiates transformations to other anisotropic structural states. Altogether four transformation stages have been detected by dilatometry. At temperatures as low as 150 deg C, two stages (one beginning after a few minutes and the otaer after a few hours) produce a volume expansion of the alloy and a dramatic increase in hardness. The first stage is attributed to solute clustering. The second stage corresponds to an isothermal martensitic shear transformation, resulting in a second phase which may be described as a monoclinic modification of the alpha -uranium orthorhombic structure. Cellular or discontinuous predipitation is observed at temperatures between 500 and 620 deg C. Light rolling deformation of the quenched alloy causes an immediate crystal reorientation to take place in certain grains. Metallographic and x-ray evidence hints that this reorientation is accomplished by mechanical twinning at low strains. (22 figures, 37 references) (auth

AN X-RAY DIFFRACTION STUDY OF A MARTENSITIC TRANSFORMATION IN URANIUM ALLOYS

The martensitic bcc (γ) to orthorhombic (α') crystal structure change in uranium alloys is similar to the bcc to hcp transformation in Ti and Zr. One characteristic is that the change can be understood in terms of a Burger's shear mechanism. It has been proposed theoretically and with some experimental support that by progressively restricting the magnitude of the shear (by alloying, for example) a continuous series of displacively-produced transition states could be developed as follows : γ → γ° → α" → α' where γ° and α'' are tetragonal and monoclinic distortions of the cubic and orthorhombic phases, respectively. Previous studies have not been clear as to whether the intermediate transformation is continuous or discontinuous. In this work an alloy containing 6.4 wt% niobium in uranium was prepared and heat treated so that the γ° to α'' transformation could be induced at room temperature by an applied stress. The structure change was detected by insitu x-ray diffraction analysis. Lattice parameter measurements indicated that the transformation was martensitic involving a significant deviatoric component of the lattice distortion, a volume change, and a discontinuous rather than continuous change in structure

Development of texture and its role in shape memory behavior in a uranium alloy

The development of preferred orientations in alloys quenched to produce metastable, crystalline phases, e.g., alloys capable of undergoing phase transformation with the application of stress and alloys already martensitic which deform by processes that are not the customary irreversible slip mechanisms, was studied. Texture development in a uranium alloy containing nominally 7.5 wt.% Nb + 2.5 wt. % Zr is reported. (GHT