Key Properties of NiMnGa Based Single Crystals Grown with the SLARE Technique

Abstract

Magnetic shape memory alloys (MSMAs), exhibit large strains and hence are materials, which could substitute giant magnetostrictive and piezoelectrical materials in actuating devices. The actuation stress needed to induce the strain is much lower than in other actuator materials. Since the strain can be induced without phase transformation by a magnetic field, the development of actuators with high working frequencies is possible. However, for reasonable applications, large strains have to be induced with small magnetic fields. Up to now repeatable magnetically induced strains of 5–10% in magnetic fields of less than 500 mT have been achieved only in single crystals. The production of NiMnGa based single crystals is difficult and time consuming. The crystal quality is affected by porosity and impurities. With the Bridgeman based method called Slag Remelting and Encapsulation (SLARE) single crystalline ingots of NiMnGa, NiMnGaFe, and NiMnGaCo of high quality were grown and characterized. The results show that MSMA properties depend on the position within the single crystalline rods due to a composition gradient. The influence of surface treatment demonstrates that the decrease of surface roughness leads to a decrease of twinning stress. MSMAs with twinning stresses above 1 MPa show a magnetic field induced strain (MFIS) when tilting is not restricted by constraints. Softer samples can adapt to constraints much better and show large MFIS. Substituting Ni by Fe and Co, shifted the phase transitions successfully to higher temperatures. NiMnGa alloyed with up to 6 at.% Co showed three different martensite structures: a non‐modulated tetragonal structure, a modulated tetragonal structure, showing the same behavior as NiMnGa with identical structures and a non‐modulated orthorhombic structure with a stress–strain‐behavior explainable by the double twinning mechanism.

Vibrational properties of Ni–Mn–Ga shape memory alloy in the martensite phases

“…Single crystals of different composition of Ni-Mn-Ga have been grown by a modified Bridgman technique (Rolfs et al 2012) by Rolfs and Schneider from Helmholtz-Zentrum Berlin. Rectangular samples with typical dimensions of 8 × 5 × 3 mm 3 for 5M, 7 × 5 × 4 mm 3 for 7M and 6 × 4 × 3 mm 3 for NM have been cut out of the crystal bars and have then been transformed to martensite single crystals by mechanical training (Rolfs et al 2012). The transformation under uniaxial compressive stress selects the c-axis.…”

“…Note that cooling the austenite phase down into the martensite phase without precaution would fail due to the emergence of multiple martensite variants. For the work at hand, austenite crystals of different compositions have been successfully transformed to single variant non-modulated (NM) martensite and double variant 5M and 7M modulated martensite crystals by mechanical training (Rolfs et al 2012). By means of those the complete 5M martensite phonon dispersion has been measured at room temperature and compared to those previously measured in austenite (Ener et al 2012).…”

“…Composition and structure in the martensite phase. All samples have been mechanically trained(Rolfs et al 2012). The sample with asterisk has been used byEner et al (2012) for austenite measurements.…”

Fatigue life and fracture mechanics of unconstrained Ni–Mn–Ga single crystals in a rotating magnetic field

“…The effective twinning stress was determined from models by Chernenko and L'Vov with consideration of the demagnetizing field [22,23]. While the applied magnetic field was increased monotonically, the effective field in the sample decreases when twin boundaries move rapidly.…”

A Giant Magneto‐Superelasticity of 5% Enabled by Introducing Ordered Dislocations in Ni34Co8Cu8Mn36Ga14 Single Crystal

“…Several percent of strain has been obtained in NiMnGa magnetic shape memory alloys through magnetic field‐induced twin boundary motion. [ 42 , 43 , 44 , 45 , 46 , 47 , 48 ] For example the strain of 5%, 9.5% and 12% has been achieved in in Ni 48 Mn 31 Ga 21 , [ 49 ] Ni 48.8 Mn 29.7 Ga 21.5 [ 50 ] and Ni 46 Mn 24 Ga 22 Co 4 Cu 4 [ 51 ] alloys, respectively. However, the strain is unrecoverable with very small output stress of 5 MPa.…”