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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.

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.

Atli K.C.,Texas A&M University | Karaman I.,Texas A&M University | Noebe R.D.,NASA | Garg A.,NASA | And 2 more authors.
Acta Materialia | Year: 2011

A Ti49.5Ni25Pd25Sc0.5 high-temperature shape memory alloy is thermomechanically processed to obtain enhanced shape-memory characteristics: in particular, dimensional stability upon repeated thermal cycles under constant loads. This is accomplished using severe plastic deformation via equal channel angular extrusion (ECAE) and post-processing annealing heat treatments. The results of the thermomechanical experiments reveal that the processed materials display enhanced shape memory response, exhibiting higher recoverable transformation and reduced irrecoverable strain levels upon thermal cycling compared with the unprocessed material. This improvement is attributed to the increased strength and resistance of the material against defect generation upon phase transformation as a result of the microstructural refinement due to the ECAE process, as supported by the electron microscopy observations. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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.

Atli K.C.,Texas A&M University | Karaman I.,Texas A&M University | Noebe R.D.,NASA | Garg A.,NASA | And 2 more authors.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2010

A Ti50.5Ni24.5Pd25 high-temperature shape memory alloy (HTSMA) is microalloyed with 0.5 at. pct scandium (Sc) to enhance its shape-memory characteristics, in particular, dimensional stability under repeated thermomechanical cycles. For both Ti50.5Ni 24.5Pd25 and the Sc-alloyed material, differential scanning calorimetry is conducted for multiple cycles to characterize cyclic stability of the transformation temperatures. The microstructure is evaluated using electron microscopy, X-ray diffractometry, and wavelength dispersive spectroscopy. Isobaric thermal cycling experiments are used to determine transformation temperatures, dimensional stability, and work output as a function of stress. The Sc-doped alloy displays more stable shape memory response with smaller irrecoverable strain and narrower thermal hysteresis than the baseline ternary alloy. This improvement in performance is attributed to the solid solution hardening effect of Sc. © The Minerals, Metals & Materials Society and ASM International 2010.

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.

Ma J.,Texas A&M University | Hornbuckle B.C.,University of Alabama | Karaman I.,Texas A&M University | Thompson G.B.,University of Alabama | And 2 more authors.
Acta Materialia | Year: 2013

FeNiCoAlTa shape memory alloys were recently discovered to possess a combination of high transformation strain and high resistance to plastic deformation. However, the transformation strain observed from single crystals is much smaller than theoretically predicted, which could be related to γ′ (L12) precipitates. Therefore, we examined the effect of nanosized γ′ precipitates on various superelastic properties of Fe-28%Ni-17%Co-11.5%Al-2.5%Ta single crystals, and found that they have profound influence on the superelastic stress hysteresis, transformation temperatures, stress-temperature phase diagram, and the characteristics of the stress-strain response along the 〈1 0 0〉 orientations. The size and volume fractions of precipitates were determined quantitatively with 3-D atom probe tomography, and the composition of these precipitates was determined for the first time. Aging at 600 °C for 7 h and above resulted in little or no change in the volume fraction of the precipitates, but coarsening of the precipitates accompanied by modest changes in their chemical compositions was observed with increasing aging time, which resulted in an increase in the transformation temperatures. Furthermore, the change in the precipitate size affected tensile and compressive superelastic behavior differently. An increasing size of the precipitates, and thus decreasing number density, caused tension-compression asymmetry in the superelastic characteristics, such as recoverability, stress hysteresis, and intensified the critical stress vs. temperature slope. We explain this observation based on the inherent differences in the morphology and variant structures of the martensite formed during tension and compression superelastic experiments. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Ma J.,Texas A&M University | Karaman I.,Texas A&M University | Maier H.J.,University of Paderborn | Chumlyakov Y.I.,Siberian Physical Technical Institute
Acta Materialia | Year: 2010

The superelastic cyclic response of Ti74Nb26 shape memory alloy (SMA), and the nature of cyclic evolution of its superelastic properties and their unexpected static recovery process after cycling, were investigated at room temperature. The critical stress for stress-induced martensitic transformation (σSIM) and stress hysteresis (Δσ) continuously decrease with increasing number of superelastic cycles. However, cumulative irrecoverable strain during cycling in samples of particular processing conditions increases only up to a certain number of cycles before decreasing with further cycling. Stress-free aging at room temperature after cycling was shown to increase σSIM and Δσ. The unexpected room temperature recovery is attributed to the recovery of retained martensite and point defects. Similar experiments conducted on conventional Ni-rich Ni-Ti SMAs also show static recovery at room temperature, indicating that the recovery process is not unique to Ti-Nb SMAs. © 2009 Acta Materialia Inc.

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.

Weighardt S.C.,University of Paderborn | Maier H.J.,University of Paderborn | Chumlyakov Y.I.,Siberian Physical Technical Institute
Journal of Alloys and Compounds | Year: 2013

Most shape memory applications require anisotropic properties; hence the materials used for sensors and actuators display properties like shape memory or superelastic strains that are usually optimized along a certain direction. It is well known that stress-free aging of near equiatomic NiTi single crystals improves microstructural stability under cyclic loading conditions. The present study shows that aging under compressive stress applied along the [1 1 1] orientation results in the formation of precipitates perpendicular to the loading axis, which further improves fatigue resistance. So far, the dependence of the functional degradation on the crystallographic orientation has only been studied after stress-free aging or after aging under compression followed by cyclic loading along the same crystallographic direction. Therefore, the scope of this study was to investigate the dependence of superelasticity on crystallographic orientation for different angles between the normal to the habit plane of the precipitates and the loading axis. It is shown that the functional properties can be controlled by aging of the single crystals in [1 1 1] orientation under stress. It was also observed that after aging under compression, functional degradation resistance is strongly dependent on the crystallographic orientation during cyclic loading. The effect of different superimposed stress modes and the influence of crystallographic orientation on the microstructural mechanisms that govern functional degradation as well as the ramifications on the design of NiTi nanocomposites with an optimized microstructure are discussed. © 2012 Elsevier B.V.

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