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Motemani Y.,Institute For Werkstofferuhr University Bochum44801Bochumgermany | Rynko R.,Institute For Werkstofferuhr University Bochum44801Bochumgermany | Somsen C.,Institute For Werkstofferuhr University Bochum44801Bochumgermany | Paulsen A.,Institute For Werkstofferuhr University Bochum44801Bochumgermany | And 3 more authors.
Advanced Engineering Materials | Year: 2015

Ti-Ta based alloys are an interesting class of high-temperature shape memory materials. When fabricated as thin films, they can be used as high-temperature micro-actuators with operation temperatures exceeding 100°C. In this study, microstructure, shape memory effect and thermal cycling stability of room-temperature sputter deposited Ti67Ta33 thin films are investigated. A disordered α″ martensite (orthorhombic) phase is formed in the as-deposited Ti67Ta33 films. The films show a columnar morphology with the columns being oriented perpendicular to the substrate surface. They are approximately 200nm in width. XRD texture analysis reveals a martensite fiber texture with {120} and {102} fiber axes. The XRD results are confirmed by TEM analysis, which also shows columnar grains with long axes perpendicular to the {120} and {102} planes of α″ martensite. The shape memory effect is analyzed in the temperature range of -10 to 240°C using the cantilever deflection method, with special emphasis placed on cyclic stability. Ti67Ta33 thin films undergo a forward martensitic transformation at Ms ≈ 165°C, with a stress relaxation of approximately 33MPa during the transformation. The actuation response of the film actuators degrades significantly during thermal cycling. TEM analysis shows that this degradation is related to the formation of nanoscale ω precipitates (5-13nm) which form above the austenite finish temperature. These precipitates suppress the martensitic transformation, as they act as obstacles for the growth of martensite variants. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Mujica Roncery L.,Institute For Werkstofferuhr University Bochum44801Bochumgermany | Lopez-Galilea I.,Institute For Werkstofferuhr University Bochum44801Bochumgermany | Ruttert B.,Institute For Werkstofferuhr University Bochum44801Bochumgermany | Burger D.,Institute For Werkstofferuhr University Bochum44801Bochumgermany | And 3 more authors.
Advanced Engineering Materials | Year: 2016

The creep behavior of a single-crystal Ni-base superalloy in two microstructural states is compared. One is obtained by casting followed by a conventional heat treatment. The other results from the same nominal heat treatment integrated into a hot isostatic pressing process. The microstructure after HIP differed from that in the conventional route in two respects. First, the γ′ particles are smaller and the γ channels are narrower. Second, after HIP, the number density of pores is lower and the pore sizes are smaller. The HIP microstructure improves creep in two respects: the finer γ/γ′-microstructure results in lower minimum creep rates. Moreover, the shrinkage of cast porosity during HIP delays the nucleation and growth of micro cracks and results in higher rupture strains in the low-temperature high stress regime. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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