Papers by Keyword: Melt Spun Ribbons

Abstract: Four alloys with nominal compositions Ni₄₆Mn_41.5-xFe_xSn_12.5 (x=0, 2, 4, 6 at.%) were cast in an induction vacuum furnace and homogenized. Then they were melted in quartz tubes and ejected onto a rotating copper wheel to produce ribbons. The X-Ray phase analyses of as melt spun ribbons have shown that in both, the ternary as well as in the quaternary alloys a single phase of the Heusler L2₁ type ordered structure was found. The characteristic temperatures of magnetic (T_C) and martensitic (M_s) transformations were determined by a vibrating sample magnetometer (VSM). Both the M_s and T_C increase with the increase of Fe content in all alloys, which is in accordance with the theory of valence electron concentration (e/a) influence on M_s. The phase structures, chemical compositions, grains sizes and type of microsegregation were characterized by transmission electron microscope (TEM). The equi-axed grains of size from 0.95 to 1.7 μm were observed in all ribbons. The grains posses the L2₁ structure at room temperature, however in the alloys with higher Fe content the different type of martensite was observed at the grain boundaries of L2₁ phase. Appearance of this martensite was explained in relation to microsegregation of particular elements during melt spinning process and simultaneous change in the e/a ratio.

Abstract: The influence of Al substitution for Sn in Ni₄₄Mn_43.5Al_xSn_12.5-x (x= 0, 1, 2, 3) ferromagnetic shape memory alloy ribbons on phase transformation and microstructure evolution is outlined in this paper. Ribbons produced by melt spinning technique showed fully crystalline structure, however non uniform. Energy dispersive spectroscopy microanalysis (EDS) confirmed the average composition of ribbons in accord with the initial alloys. The higher symmetry parent phase was identified with the aid of X-ray diffraction (XRD) as bcc L2₁ Heusler type structure. The unit cell parameters were determined applying the XRD profile fitting method. It was observed that with increase of Al content unit cell parameters and in turn unit cell volume decrease. This may be attributed to the fact that Al has a smaller radius compared to Sn, which it was substituted for. Differential scanning calorimetry (DSC) measurements did not allow to detect the martensitic transformation above -150°C.

Abstract: Microstructure of Ni_50-xCo_xMn_35.5In_14.5 (x=0, 3, 5) melt-spun ribbons was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The typical layered microstructure consisting of oriented columnar grains and colonies of fine equi-axed grains was observed in the cross section of ribbons. The crystallographic structure of ribbons varied with the content of Co in alloys what affected of their M_s temperature. For the x=0 the single phase of monoclinic 14M modulated martensite was observed, but for x=3 and 5, a two phase structure of L2₁ austenite and monoclinic 14M or orthorhombic 10M modulated martensite were identified. Different temperature range of martensitic transformations were explained basing on valence electron concentration per atom e/a versus M_s relationship.

Abstract: Rapid solidification route by melt spinning has been adopted for preparation of a Ni52.5Mn24.5Ga23 (at %) ferromagnetic shape memory alloy in the form of ribbons. In the as-spun state, the ribbon revealed a predominant austenitic L21 structure in combination with martensitic feature as observed from x-ray diffraction studies. Transmission electron microscopic (TEM) evaluation showed these features in the form of martensitic plates. At low temperature, martensite to austenite transformation was exhibited by an increase in magnetization during heating cycle. The reverse effect was observed during cooling cycle. Annealing temperature and magnetising field was also found to effect this transformation.