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Evirgen A.,Texas A&M University | Ma J.,Texas A&M University | Karaman I.,Texas A&M University | Luo Z.P.,Texas A&M University | Chumlyakov Y.I.,Siberian Physical Technical Institute
Scripta Materialia | Year: 2012

This study reports the effect of selected aging treatments on the superelastic response of 〈1 0 0〉-oriented Fe-28% Ni-17% Co-11.5% Al-2.5% Ta (at.%) single crystals in tension. A smaller precipitate size and lower precipitate fraction of the samples aged at 700 °C for 7 h leads to lower transformation temperatures, a wide temperature range for superelasticity and higher transformation stress levels as compared to the samples aged at 600 °C for 90 h. Both aging conditions results in fully recoverable transformation strain of 3.4% in tension. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

Karaca H.E.,University of Kentucky | Saghaian S.M.,University of Kentucky | Basaran B.,University of Kentucky | Bigelow G.S.,NASA | And 2 more authors.
Scripta Materialia | Year: 2011

The shape memory properties of Ni-rich NiTiHf single crystals along the [1 1 1] orientation were investigated by thermal cycling under constant stress and stress cycling at constant temperature in compression. The single crystals demonstrate stable shape memory behavior under ultrahigh stress levels (1000 MPa) with about 3% transformation strain, a maximum work output of 30 J cm -3 and nearly perfect pseudoelasticity at high temperatures (180-200 °C). © 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. Source

Dogan E.,Texas A&M University | Karaman I.,Texas A&M University | Chumlyakov Y.I.,Siberian Physical Technical Institute | Luo Z.P.,Texas A&M University
Acta Materialia | Year: 2011

Microstructural evolution and martensitic transformation characteristics of Co46Ni27Ga27 and Co44Ni 26Ga30 high temperature shape memory alloys were investigated in as-cast and hot-rolled conditions as a function of heat treatment. Heat treatments were selected to introduce single-, two-, and three-phase structures, where the precipitate phases do not martensitically transform. The effects of these precipitates, and associated compositional changes, on transformation temperatures, thermal hysteresis, and microstructural evolution during thermal cycling, were revealed. It was found that martensite start temperature linearly depends on the valence electron concentration (e/a) of the matrix, if the Ga content is constant. For a given e/a, the higher the Ga content is, the higher the transformation temperatures become. The presence of γ′ precipitates and the volume fraction of γ phase were shown to have strong influence on transformation thermal hysteresis. The most cyclically stable compositions with narrow hysteresis (<40 °C) were identified. In these compositions, a room-temperature aging phenomenon, possibly mediated by point defects, was discovered, which recovers the transformation temperature changes upon thermal cycling. They also demonstrate reversible martensitic transformation in constant-stress thermal cycling experiments. However, their crystallographic texture should be engineered to increase the transformation strain, and ductile γ-phase content should be reduced to improve cyclic reversibility. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

Ozdemir N.,Texas A&M University | Karaman I.,Texas A&M University | Mara N.A.,Los Alamos National Laboratory | Chumlyakov Y.I.,Siberian Physical Technical Institute | Karaca H.E.,University of Kentucky
Acta Materialia | Year: 2012

The superelastic behavior of Ni 54Fe 19Ga 27 shape memory alloy (SMA) single crystalline pillars was studied under compression as a function of pillar diameter. Multiple pillars with diameters between 10 μm and 200 nm were cut on a single crystalline bulk sample oriented along the [1 1 0] direction as the compression axis and that had undergone fully reversible two stage martensitic transformation, i.e. L2 1 austenite to 10M/14M modulated martensite and then to L1 o martensite. The results revealed an increase in the critical stress for stress-induced martensitic transformation and the yield strength of martensite with decreasing pillar size. The stress hysteresis also increased with the reduction in pillar size and the superelastic response started to diminish below 500 nm pillar diameter. Two-stage martensitic transformation was suppressed for pillar sizes of 1 μm and below, which were shown to exhibit a direct austenite to L1 o transformation. Such a change in the transformation pathway, i.e. from a two stage to one stage transformation, was also observed in bulk single crystals with increasing temperature. We demonstrated the absence of two stage transformation in bulk at high temperatures. This finding suggests that decreasing the sample size and increasing the temperature have similar effects on the superelastic response of NiFeGa SMAs that had undergone two-stage transformation and indicates that a reduction in pillar diameter decreases the transformation temperature due to the difficulty of martensite nucleation on small scales. The damping coefficients of the pillars were also calculated and the results highlighted that damping capacities higher than those of bulk metallic alloys can be achieved using submicron sized pillars. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

Ma J.,Texas A&M University | Kockar B.,Texas A&M University | Kockar B.,Hacettepe University | Evirgen A.,Texas A&M University | And 3 more authors.
Acta Materialia | Year: 2012

A 〈1 0 0〉 textured polycrystalline FeNiCoAlTa shape memory alloy was recently shown to possess large superelastic strain and stress levels. In this study, the shape memory behavior of a Fe-28Ni-17Co-11.5Al-2.5Ta (at.%) single-crystalline material oriented along the 〈1 0 0〉 direction was studied, for the first time, by thermal cycling under constant stress levels in both tension and compression. When γ′ precipitates with an average size of 5 nm are introduced by an aging heat treatment, the single crystals show fully recoverable transformation strains up to 3.75% in tension and 2% in compression. The change in transformation temperatures for a unit change in applied stress level was higher in compression than in tension, in accord with the lower transformation strains in compression obtained both from theoretical calculations and experimental observations. However, in all specimens, the observed transformation strain levels were lower than theoretically predicted, possibly owing to significant volume fraction of non-transforming precipitates, incomplete martensite reorientation due to martensite variant interactions, and a slightly higher-than-expected martensite c/a ratio in the samples used in this study. The ramifications of relevant structural parameters and microstructural features on reaching theoretical transformation strain and high strength levels are also discussed. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

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