H.E. Karaca

Energy harvesting using martensite variant reorientation mechanism in a NiMnGa magnetic shape memory alloy

Magnetic shape memory alloys demonstrate significant potential for harvesting waste mechanical energy utilizing the Villari effect. In this study, a few milliwatts of power output are achieved taking advantage of martensite variant reorientation mechanism in Ni51.1Mn24Ga24.9 single crystals under slowly fluctuating loads (10Hz) without optimization in the power conversion unit. Effects of applied strain range, bias magnetic field, and loading frequency on the voltage output are revealed. Anticipated power outputs under moderate frequencies are predicted showing that the power outputs higher than 1W are feasible.

Recoverable stress-induced martensitic transformation in a ferromagnetic CoNiAl alloy

Abstract 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.

Large inverse magnetic entropy changes and magnetoresistance in the vicinity of a field-induced martensitic transformation in Ni50−xCoxMn32−yFeyGa18

Significantly large inverse magnetic entropy changes (ΔSM) and magnetoresistance (MR) were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys: Ni50−xCoxMn32−yFeyGa18. The crystal structures of alloys were tetragonal at 300 K and the phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion. The maximum peak values of the ΔSM and MR at H=5u2002T were determined as ≈(+)31u2002Ju2009Kg−1u2009K−1 and ≈−21%, respectively, for x=8 and y=2. The relatively small hysteretic loss and large refrigeration capacity observed in this system make these compounds promising materials for applications.

Stress-strain-temperature behaviour of [001] single crystals of Co49Ni21Ga30 ferromagnetic shape memory alloy under compression

The conventional shape memory effect (SME) and pseudoelasticity (PE) in as-grown [100] single crystals of Co49Ni21Ga30 alloy under compression are reported. The parent single crystals exhibit about 5% transformation strain at compressive stress levels as low as 4u2009MPa, and a pseudoelastic strain of 4.5%. Complete PE was observed in the temperature range from 35 to 285°C, along with increasing stress hysteresis with temperature. The latter is attributed to increasing number of variants and the corresponding variant–variant interactions. We demonstrate that the current material can be utilized in applications that demand high strength at elevated temperatures. Moreover, the current results also indicate the potential of this material to exhibit magnetic shape memory effect, which could broaden the scope of utility of this material upon further research.

On the role of the cooling rate and crystallographic orientation on the shape memory properties of CoNiAl single crystals under compression

Crystallographic orientation and cooling rate dependences of the shape memory effect and pseudoelasticity are reported for CoNiAl alloy single crystals oriented along the [001], [110] and [123] directions. Phase transformation response, pseudoelasticity and two-way shape memory effect were investigated through thermo-mechanical experiments. The experimentally determined transformation strains exceeding the theoretical values in the [110] orientation single crystals are attributed to the additional contribution from the detwinning strain. High cooling rates (oil and water quenching) were found to lead to a lower γ-phase volume fraction that resulted in higher shape memory strains under low external stresses. Moreover, the [001] oriented single crystals demonstrated perfect pseudoelasticity (up to 4.3% strain), a large pseudoelastic window (>140u2009°C), high resistance to dislocation slip and small thermal hysteresis. The existence of detwinning, easy reorientation of favorable variants under low external stresses and significant fully recoverable strains have important implications regarding the magnetic field-induced shape changes in this class of alloys.

Stress-induced martensite to austenite phase transformation in Ni2MnGa magnetic shape memory alloys

The effects of temperature on the stress-induced variant reorientation process and the possibility to trigger stress-induced martensite to austenite phase transformation in Ni2MnGa single crystals were investigated under compression. It is revealed that, as the temperature decreases, both the critical stress (under 16?kG magnetic field) and the critical magnetic field levels (under ?1?MPa) for martensite variant reorientation increase. Magnetic-field-induced strain is also found to increase with temperature. Near the austenite start temperature, it is possible to attain stress-induced martensite to austenite transformation under constant magnetic field. A thermodynamical guideline is introduced to explain the conditions for ?stress-induced martensite to austenite transformation? in Ni2MnGa alloys. It is concluded that, when magnetocrystalline anisotropy energy is large enough, stress-induced martensite to austenite transformation can be achieved within a narrow temperature range below austenite start temperature.