Novel Ni-Mn-Ga alloys and their magnetic shape memory behaviour

Abstract

The ternary Ni-Mn-Ga system has already been studied for several decades. Although the present work concentrates on the alloys with the 7-layered near-orthorhombic martensite structure (7M) and the non-modulated tetragonal martensite structure (T), it introduces 37 alloys altogether, including materials that have the 5-layered tetragonal martensite (5M) at ambient temperature. The dissertation shows briefly how the different compositions of ternary Ni-Mn-Ga alloys affect the phase transformations and the crystallographic structures of various phases. This thesis presents new alloys with the 7M structure and, for the first time, the Curie point of such a martensitic structure (TC7M). This TC7M is mainly detected in the cooling of the alloys, showing the co-occurrence of the magnetic transition and the double-step reverse transformation where the hysteresis of the Curie point is connected to the two martensitic phases.

The service temperature region of the MSM alloy depends on the existence of the proper ferromagnetic twinned martensite. Consequently, the transformation sequence of the studied 7M and T alloys is investigated in more detail. According to the magnetic and crystalline transformation, the alloys are divided into six groups: A (7M ambient), B (7M above), C (7M co-transition), D (T high), E (T low) and F (T co-transition). In the groups A, B, D and E, the magnetic and crystalline transitions are separated, while in the groups C and F they co-occur.

The pre-straining processes for obtaining the single-variant state in the 7M and the T martensite are presented. In the compression of the 7M alloys, three deformations to two crystallographic directions are needed, while the T alloys require three deformations to three crystallographic directions. However, it is also shown that with the tensile/compressive cycling of the T phase the full single-variant state has been obtained already, during the second cycle. The twinning stresses (σtw) needed for the martensite variant reorientation could be lowered by pre-straining close to 1 MPa in the 7M phase, while in the T phase only the level 6 MPa was reached, even at elevated temperatures. The magneto-mechanical tests confirmed that the magnetically induced stress (Δσmag) in the 7M structure is 1.5 MPa and in the T structure approximately 1 MPa. By applying the criteria of Δσmag ≥ σtw for the magnetic shape memory effect (MSME), it is obvious that MSME is possible in the 7M structure, but it can not be obtained in the T structure.