Microstructure and magnetic properties of as-cast Ni2MnGa rods and tubes solidified by suction casting

A.M. CONDÓ; E. WINKLER; G. POZO LÓPEZ; R.H. MUTAL; L.M. FABIETTI; S.P. LIMANDRI; S.E. URRETA; A.M. CONDÓ; E. WINKLER; G. POZO LÓPEZ; R.H. MUTAL; L.M. FABIETTI; S.P. LIMANDRI; S.E. URRETA

MATERIALS CHARACTERIZATION

ELSEVIER SCIENCE INC

Lugar: Amsterdam; Año: 2019

1044-5803

Ni2MnGa cylinders and tubes are solidified in water chilled copper molds, a few millimeters in external diameterand 5 cm long, by the suction casting technique. At room temperature, all samples are in cubic austenitic phase.Microstructure and crystallographic texture of the as-cast rods and tubes are characterized by XRD, SEM, EBSDand TEM. Because of the heat extraction geometry samples exhibit a strong texture, with the [100] directionpreferentially oriented in the radial direction, together with a random distribution on the long axis. This textureis more marked in the tubes. XRD and TEM results indicate that the major austenitic phase is fcc, with L21 order.A minority volume of the equilibrium B2′ disordered phase is detected by the presence of two close Curietemperatures in cylinders and tubes 2mm in external diameter, but not when this diameter is near 1 mm.Precipitates of the stable compounds α-Mn(S,Se), with a NaCl-type structure, and monoclinic P4S5 are observedin all the samples. Cylinders and tubes in austenitic phase are magnetically soft. Hysteresis loops in martensiticphase exhibit local steps associated to a magnetization mechanism involving twin boundary displacement, indicatingthat a field-induced variant reordering takes place. The switching field Hsw, corresponding to themagnetization step observed, is identified as the field at which twin boundaries become mobile. The measuredvalues of 0.37 T?0.49 T are consistent with those corresponding to the onset of Type I twin boundaries displacementin 5M martensite, with an equivalent threshold stress of 1 MP. Ni2MnGaFerromagnetic shape?memory alloysSuction castingMicrostructureMartensitic transformationMagnetic properties