Browsing by Author "Bhatia, VK"

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    AuCuAl shape memory alloys for use in nano-actuators
    (Australian Institute of Physics, 2010-02-02) Bhatia, VK; Kealley, CS; Thorogood, GJ; Dowd, A; Cortie, MB
    Although Al, Au and Cu each has the face centered cubic structure in elemental solid form, they do not readily alloy with one another, and instead form a series of binary and ternary intermetallic compounds. The ternary system is very interesting and contains, amongst other features, an 18-carat shape memory electron compound sometimes called ‘Spangold’ [1] ( Figure 1). Shape memory alloys are remarkable in that they have the ability to return to their initial state and shape after being deformed. This ability arises from a martensitic (displacive) phase transformation (see Figure 2), which is strongly influenced by temperature, crystal structure and degree of ordering. It is already known that the parent phase of ‘Spangold’ must contain at least a minimum degree of ternary ordering before the reversible displacive transformation needed for the shape memory effect can take place [3]. Furthermore, the parent phase has been found to have the L21 ordered body-centered cubic packing arrangement [4], while it has been reported that the martensite can be described using a monoclinic unit cell [5]. Here we examine the possibility of using this compound as a nano-actuator. Magnetron sputtering was used to deposit the Au, Cu and Al. The films were then characterized using x-ray reflectometry (see figure 3), grazing incidence X-ray diffraction, scanning electron microscopy and atomic force microscopy. The properties of these films are compared with that of bulk samples.
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    Determination of martensite structures of the Au7Cu5Al4 and Au7Cu5.7Al3.3 shape-memory alloys
    (Elsevier, 2014-10-15) Elcombe, MM; Kealley, CS; Bhatia, VK; Thorogood, GJ; Carter, DJ; Avdeev, M; Cortie, MB
    The β-phase of Au7Cu5Al4 undergoes a reversible shape-memory phase transformation for which several conflicting martensite phases have been reported. Here we show the significance of the cooling temperature used to obtain the martensite. If Au7Cu5Al4 is cooled from the parent phase condition to cryogenic temperatures, e.g. below 200 K, the martensitic phase is orthorhombic (space group Pcmn, a = 4.4841 Å, b = 5.8996 Å, c = 17.8130 Å); however, when this composition is cooled to only ∼260 K it will in general consist of a mixture of orthorhombic and monoclinic phase (the latter has space group P21/m, a = 4.4742 Å, b = 5.9265 Å, c = 13.3370 Å, β = 91.425°). In contrast, a sample with decreased Al content (Au7Cu5.7Al3.3) transforms fully to monoclinic phase if cooled to ∼260 K. © 2014 Acta Materialia Inc.
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    High temperature transformations of the Au7Cu5Al4 shape-memory alloy
    (Elsevier, 2011-02-24) Cortie, MB; Kealley, CS; Bhatia, VK; Thorogood, GJ; Elcombe, MM; Avdeev, M
    The β-phase of Au7Cu5Al4 undergoes a reversible shape-memory phase transformation, however there has been some uncertainty regarding the crystal structure or structures of the parent phase. Here we show that, under equilibrium conditions, the parent phase possesses the L21 structure between its Ap (about 79°C) and 630°C, and the B2 primitive cubic structure between 630°C and its melting point. It melts directly from B2 into the liquid state and hence never achieves the random bcc A2 structure that has been previously mooted. Splat-cast samples of the alloy are martensitic, proving that development of equilibrium order and defect concentration are not pre-requisites for the A → M transformation to occur. © 2011, Elsevier Ltd.
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    Martensite destabilization in Au7Cu5Al4 shape-memory alloy
    (Elsevier, 2011-03) Bhatia, VK; Kealley, CS; Prior, MJ; Cortie, MB
    Aging-induced changes in the austenite peak (Aρ) temperature of Au7Cu5Al4 shape-memory alloy are investigated. Whereas heat treating the parent phase at temperatures >140°C or aging the martensite for long times at room temperature both stabilized the Aρ to ~80°C, low-temperature excursions into the parent phase caused the subsequent Aρ to drop to ~60°C and the transformation hysteresis to decrease. The evidence indicates that this destabilization of the martensite is caused by time-dependent relaxation of elastic constraint due to parent-phase lath migration during the preceding low-temperature austenitizing treatment. This mechanism of aging is different from that of the better-known symmetry-conforming short-range order phenomenon. © 2011, Elsevier Ltd.
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    Structure determination of gold-based shape memory alloy
    (Australian Institute of Physics, 2009-02-04) Kealley, CS; Bhatia, VK; Thorogood, GJ; Elcombe, MM; Cortie, MB
    Not available
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    Thin films of AuCuAl shape memory alloy for use in plasmonic nano-actuators
    (Cambridge University Press/Springer Nature, 2011-02-04) Bhatia, VK; Thorogood, GJ; Dowd, A; Cortie, MB
    We describe the fabrication and structure of nanoscale thin films of β phase shape memory alloys with the nominal atomic stoichiometry Au7Cu5Al4 (corresponding to 5.8 wt% Al). These alloys possess properties that suggest they could be used in nanoscale actuators. The films described here are between 20 and 50 nm thick which is below the thickness at which some other shape memory alloys cease to transform. However, microstructural and X-ray studies confirm that the coatings still exhibit the displacive transformations that are a prerequisite for the shape memory effect. © Materials Research Society 2011