One-way shape memory effect is achieved utilizing stress-assisted field-induced martensitic phase... more One-way shape memory effect is achieved utilizing stress-assisted field-induced martensitic phase transformation in Ni2MnGa magnetic shape memory alloy single crystals. Depending on temperature, one-way and reversible magnetic field-induced strains of up to 3.1% and ...
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2008
As-grown Co49Ni21Ga30 [001]- and [123]-oriented single crystals were subjected to cyclic compress... more As-grown Co49Ni21Ga30 [001]- and [123]-oriented single crystals were subjected to cyclic compression loading at room temperature above the austenite finish temperature of 15 °C. Strain-controlled experiments were performed using both incremental strain steps and constant strain amplitudes. Cyclic deformation with a maximum strain amplitude of 2.5 pct resulted in rapid accumulation of irrecoverable strains in the [123]-oriented crystals. However, after a few cycles, the samples demonstrated cyclic stability with fully recoverable transformation. By contrast, the [001]-oriented crystals displayed excellent cyclic stability with hardly any change in stress-strain characteristics. In-situ optical microscopy and electron backscattered diffraction analysis were employed to clarify the events that take place at different stages of a typical loading-unloading history. The in-situ observations also revealed that the initiation and growth characteristics of stress-induced martensite (SIM) are heterogeneous on the microscopic scale in CoNiGa alloys. In addition, theoretical transformation and detwinning strains, and resolved shear stress factors (RSSFs), were calculated based on the energy minimization theory and are compared to the experimentally obtained orientation-dependent transformation stress and strain levels. It is shown that the selection of an appropriate orientation is one of the key criteria to optimize the pseudoelastic (PE) response and cyclic stability of CoNiGa alloys.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2003
A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-chan... more A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-channel angular extrusion (ECAE). Three deformation temperatures—room temperature (below the martensite finish temperature), 50 °C (below the austenite start temperature), and 150 °C (above the austenite finish temperature)—were selected such that the initial deforming phase (B2 austenite or B19’ martensite) and the initial governing deformation mechanism (martensite reorientation, stress-induced martensitic transformation, or dislocation slip in martensite) would be different. The X-ray analysis results revealed that all processed samples mostly contained a deformed martensitic phase, regardless of the initial deforming phase and the deformation mechanism. Although the martensite start temperature did not change, the austenite start temperature decreased significantly in all deformation conditions, probably because of the effect of the internal stress field caused by the deformed microstructure. All deformation conditions led to an increase in the strength levels and some deterioration of shape-memory characteristics. However, a subsequent low-temperature annealing treatment significantly improved pseudoelastic strain levels while preserving the ultrahigh strength levels. The sample deformed at room temperature followed by the low-temperature annealing resulted in the most promising strength and shape-memory characteristics under compression, such that a 5.3 pct shape-memory strain at a 2200 MPa strength level and a 3.3 pct pseudoelastic strain at a 1900 MPa strength level were achieved. The differences between the strength levels and the shape-memory characteristics after severe deformation at different temperatures were attributed to the different amounts of plastic deformation and the resulting deformation textures, since at each deformation temperature the deformation mechanism was different. It is concluded that the severe marforming using ECAE could easily improve strength levels of NiTi alloys while preserving the shape-memory and pseudoelasticity (PE) characteristics and, thus, improve the thermomechanical fatigue behavior. However, lower deformation temperatures are necessary to hinder formation of macroshear bands, and ECAE angles larger than 90 deg should be used to reduce the amount of strain applied in one pass.
ABSTRACT Reversible magnetic field-induced martensitic phase transformation is demonstrated for t... more ABSTRACT Reversible magnetic field-induced martensitic phase transformation is demonstrated for the first time in Ni2MnGa magnetic shape memory alloy single crystals under constant stress levels and low field magnitudes. Actuation stress levels larger than 25 MPa were achieved providing an order of magnitude increase with respect to the values obtained from field-induced martensite reorientation. The necessary magneto-thermo-mechanical conditions for the field-induced phase transformation are discussed. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
ABSTRACT Pseudoelasticity as a function of temperature of as-grown Co49Ni21Ga30 [0 0 1] oriented ... more ABSTRACT Pseudoelasticity as a function of temperature of as-grown Co49Ni21Ga30 [0 0 1] oriented single crystals was studied under compression. These alloys demonstrate a large pseudoelastic temperature range, greater than 385 °C, with a maximum recoverable pseudoelastic strain of about 4%. We provide an insight into the evolution of stress–strain behavior and corresponding stress–hysteresis with test temperatures, and the ramifications on the potential use of CoNiGa alloys as high temperature pseudoelastic alloys are presented.
The stress-induced martensitic transformation characteristics of a new CoNiAl alloy were investig... more The stress-induced martensitic transformation characteristics of a new CoNiAl alloy were investigated under compression. Pseudoelasticity, stages of transformation, and thermal cycling under constant stress were revealed. The present CoNiAl alloy is a candidate material not only for magnetic but also for conventional and high-temperature shape memory alloy applications.
Magnetic shape memory properties of a single crystal Ni2MnGa alloy were characterized through mon... more Magnetic shape memory properties of a single crystal Ni2MnGa alloy were characterized through monitoring magnetic field induced strain (MFIS) as a function of compressive stress, and applied stress induced strain as a function of magnetic field. Compressive stress and ...
One-way shape memory effect is achieved utilizing stress-assisted field-induced martensitic phase... more One-way shape memory effect is achieved utilizing stress-assisted field-induced martensitic phase transformation in Ni2MnGa magnetic shape memory alloy single crystals. Depending on temperature, one-way and reversible magnetic field-induced strains of up to 3.1% and ...
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2008
As-grown Co49Ni21Ga30 [001]- and [123]-oriented single crystals were subjected to cyclic compress... more As-grown Co49Ni21Ga30 [001]- and [123]-oriented single crystals were subjected to cyclic compression loading at room temperature above the austenite finish temperature of 15 °C. Strain-controlled experiments were performed using both incremental strain steps and constant strain amplitudes. Cyclic deformation with a maximum strain amplitude of 2.5 pct resulted in rapid accumulation of irrecoverable strains in the [123]-oriented crystals. However, after a few cycles, the samples demonstrated cyclic stability with fully recoverable transformation. By contrast, the [001]-oriented crystals displayed excellent cyclic stability with hardly any change in stress-strain characteristics. In-situ optical microscopy and electron backscattered diffraction analysis were employed to clarify the events that take place at different stages of a typical loading-unloading history. The in-situ observations also revealed that the initiation and growth characteristics of stress-induced martensite (SIM) are heterogeneous on the microscopic scale in CoNiGa alloys. In addition, theoretical transformation and detwinning strains, and resolved shear stress factors (RSSFs), were calculated based on the energy minimization theory and are compared to the experimentally obtained orientation-dependent transformation stress and strain levels. It is shown that the selection of an appropriate orientation is one of the key criteria to optimize the pseudoelastic (PE) response and cyclic stability of CoNiGa alloys.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2003
A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-chan... more A Ti-49.8 at. pct Ni alloy was severely deformed at three different temperatures using equal-channel angular extrusion (ECAE). Three deformation temperatures—room temperature (below the martensite finish temperature), 50 °C (below the austenite start temperature), and 150 °C (above the austenite finish temperature)—were selected such that the initial deforming phase (B2 austenite or B19’ martensite) and the initial governing deformation mechanism (martensite reorientation, stress-induced martensitic transformation, or dislocation slip in martensite) would be different. The X-ray analysis results revealed that all processed samples mostly contained a deformed martensitic phase, regardless of the initial deforming phase and the deformation mechanism. Although the martensite start temperature did not change, the austenite start temperature decreased significantly in all deformation conditions, probably because of the effect of the internal stress field caused by the deformed microstructure. All deformation conditions led to an increase in the strength levels and some deterioration of shape-memory characteristics. However, a subsequent low-temperature annealing treatment significantly improved pseudoelastic strain levels while preserving the ultrahigh strength levels. The sample deformed at room temperature followed by the low-temperature annealing resulted in the most promising strength and shape-memory characteristics under compression, such that a 5.3 pct shape-memory strain at a 2200 MPa strength level and a 3.3 pct pseudoelastic strain at a 1900 MPa strength level were achieved. The differences between the strength levels and the shape-memory characteristics after severe deformation at different temperatures were attributed to the different amounts of plastic deformation and the resulting deformation textures, since at each deformation temperature the deformation mechanism was different. It is concluded that the severe marforming using ECAE could easily improve strength levels of NiTi alloys while preserving the shape-memory and pseudoelasticity (PE) characteristics and, thus, improve the thermomechanical fatigue behavior. However, lower deformation temperatures are necessary to hinder formation of macroshear bands, and ECAE angles larger than 90 deg should be used to reduce the amount of strain applied in one pass.
ABSTRACT Reversible magnetic field-induced martensitic phase transformation is demonstrated for t... more ABSTRACT Reversible magnetic field-induced martensitic phase transformation is demonstrated for the first time in Ni2MnGa magnetic shape memory alloy single crystals under constant stress levels and low field magnitudes. Actuation stress levels larger than 25 MPa were achieved providing an order of magnitude increase with respect to the values obtained from field-induced martensite reorientation. The necessary magneto-thermo-mechanical conditions for the field-induced phase transformation are discussed. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
ABSTRACT Pseudoelasticity as a function of temperature of as-grown Co49Ni21Ga30 [0 0 1] oriented ... more ABSTRACT Pseudoelasticity as a function of temperature of as-grown Co49Ni21Ga30 [0 0 1] oriented single crystals was studied under compression. These alloys demonstrate a large pseudoelastic temperature range, greater than 385 °C, with a maximum recoverable pseudoelastic strain of about 4%. We provide an insight into the evolution of stress–strain behavior and corresponding stress–hysteresis with test temperatures, and the ramifications on the potential use of CoNiGa alloys as high temperature pseudoelastic alloys are presented.
The stress-induced martensitic transformation characteristics of a new CoNiAl alloy were investig... more The stress-induced martensitic transformation characteristics of a new CoNiAl alloy were investigated under compression. Pseudoelasticity, stages of transformation, and thermal cycling under constant stress were revealed. The present CoNiAl alloy is a candidate material not only for magnetic but also for conventional and high-temperature shape memory alloy applications.
Magnetic shape memory properties of a single crystal Ni2MnGa alloy were characterized through mon... more Magnetic shape memory properties of a single crystal Ni2MnGa alloy were characterized through monitoring magnetic field induced strain (MFIS) as a function of compressive stress, and applied stress induced strain as a function of magnetic field. Compressive stress and ...
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Papers by Ersin Karaca