Shape memory alloy (SMA) wires are becoming increasingly popular as actuators in automotive applications due to properties such as large recovery strain, low weight, and silent actuation. The length change and thus actuation in SMA wires occur when the wire is heated, usually by passing a direct current through them. One of the difficulties in controlling electrically-heated SMAs occurs in monitoring their temperature, which is done to control the transformation and hence, actuation and avoid possibly permanent damage due to overheating. The temperature of a SMA wire is usually calculated theoretically based on the wire???s natural convection heat transfer coefficient(h).First-order convective heating models are typically used to calculate the natural convection heat transfer coefficient for SMA wires, but there is often significant uncertainty in these calculations due to a lack of existing correlations for thin cylinders, where curvature effects are significant. The purpose of this investigation is to develop models for SMA wires that may be used to predict the temperature of a current-carrying SMA wire without using direct temperature measurement methods. The models were developed based on experimental results for 0.5 mm diameter NiTi SMA wire. First the effect of various parameters such as wire inclination angle, wire length, ambient pressure, phase transformation time rate and applied external stress were investigated on the SMA wire???s electrical resistance. The electrical resistance of the SMA wire was monitored during one complete heating and cooling cycle. Later, based on the experimental results, a resistance model was developed for the current-carrying SMA wires that can be used to predict the wires??? temperature based on electrical resistance. Second, a natural convection heat transfer correlation was developed for NiTi SMA wire, in the range 2.6E-8??? RaD ??? 6E-1, which is appropriate for modeling natural convection in most practical applications at ambient conditions. A pressure variation method was used to obtain a range of Rayleigh number for a heated SMA wire. The ambient pressure was controlled within a vacuum chamber, from 1 atm to 2E-4 atm (0.1 MPa to 2E-5 MPa). Data were collected for the wire at various angles under both 100 MPa and stress-free conditions between horizontal to vertical at each set pressure. The new correlation can be used to determine the convective heat transfer coefficient of an SMA wire of known diameter and inclination angle. The convection coefficient (h) is determined using the correlation along with the Prandtl number (Pr), air dynamic viscosity (??), air compressibility factor (Z), air thermal conductivity (k), and gas constant (Rc). The wire temperature can then be determined by substituting this coefficient into the convective heat transfer equation.

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Thesis (Ph. D.)--University of Wisconsin--Madison, 1942. / Typescript. Includes abstract and vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 125-126).

Thesis (Ph. D.)--University of Hong Kong, 1999. / Includes bibliographical references.

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The development of aluminium alloys that can be processed by Rapid Solidification (RS) techniques for use in high temperature applications has recently been an area of intense study. One of the alloy systems of interest is Al-Cr-Zr-Mn. This work comprises a study of the microstructure and tensile properties of alloys of this system processed by melt spinning, high pressure gas atomisation (HPGA) and chill casting. The RS microstructures of Al-Cr and Al-Zr binary alloys were also compared with those of the quaternary alloys. The variety of microstructures observed in the powders of the quaternary alloys was consistent with the different cooling rates and nucleation temperatures experienced by droplets of different sizes, A cubic phase not previously reported was observed in the finer powder. The transition from a partitionless to a cellular microstructure occurred at estimated solidification front velocities similar to those predicted by morphological stability theory. The distribution of discrete Al[13]Cr[2] intermetallic particles within Al-Cr gas atomised powders of different sizes was found to be consistent with a probabilistic model of nucleants distributed in the volume of the alloy melt. Based on these results the original Al-5.2Cr-1.4Zr-1.3Mn (wt%) alloy was diluted to give an Al-3.3Cr-0.7Zr-0.7Mn (wt%) alloy so that the bulk of the powder (the sub-45mum size fraction) did not contain coarse intermetallic particles but exhibited a mainly cellular microstructure. A relationship has been determined between the thickness of wedge shaped chill castings and powder diameters for. similar microstructures. Prediction of alloy compositions designed to give a particular microstructure in a specified powder size can therefore be tested by a simple casting technique. The mechanical properties of the original and optimised quaternary alloy powders consolidated by Conform and extrusion have been determined and related to the as-consolidated and aged microstructures. The extruded powders of both alloys exhibited better properties than the Conformed powder. A large contribution to the strength of the extruded materials is made by their stabilised fine grain size. The optimised alloy had a consistently better ductility. Neither of the alloys retained its strength after prolonged treatment at 400°C, but the results suggest that a service temperature of 300°C may be possible.

It was proposed that amorphous alloys may be more resistant to radiation damage than crystalline metals. In crystalline metals neutron induced transmutations lead to the formation of inert gas bubbles. These preferentially nucleate near line defects and result in embrittlement. Amorphous alloys do not contain sites where nucleation can occur preferentially. In this work the growth of argon bubbles in amorphous Cu[50]Zr[50] has been induced by implanting thin specimens with 80keV argon ions at room temperature. The bubble size distribution was obtained over the dose range 5x10[16] to 3x10[17] Ar[+] cm[-2]. Larger bubbles grew in the amorphous alloy than would have been expected to grow in a crystalline metal implanted under the same conditions. It was found that ion bombardment caused surface atoms to be sputtered away from the specimens at a rate of 2.3at.ion[-1]. The sputtering process led to saturation in the amount of argon retained by the material and caused the formation of copper rich near-surface layer. This layer also contained significant amounts of oxygen. Blister formation was induced at the surface of the amorphous alloy by implanting it with 100keV helium ions. At a critical dose of 3x10[17] He[+]cm[-2] a population of very small blisters was formed. These were the result of large bubbles forming just below the specimen surface. As higher doses were used the features joined up to produce large, thin-lidded blisters at a dose of 10[18] He[+] cm[-2]. These observations could not be completely explained in terms of the two popular models of blister formation, where interbubble fracture or lateral stress result in surface deformation.