Influence of annealing and phase decomposition on the magnetostructural transitions in Ni50Mn39Sn11

Abstract

Magnetic and structural transitions in the Ni 50 Mn 50−x Sn x (x = 10-25) ferromagnetic shape memory alloys are currently of interest. As in Ni-Mn-Ga, these alloys feature high-temperature austenite and lowtemperature martensite phases, where the magnetic state is strongly composition dependent. To study the role of chemical ordering in fine-tuning their magnetostructural properties, they were first annealed for 4 weeks/ 1223 K to achieve structural and compositional homogeneity, and were then further annealed for 1 week ( ∼ 150 K below the reported B2 to L2 1 transition) at 773 K to increase the degree of chemical ordering. For x = 11, this anneal resulted in a dramatic change in the magnetic ordering temperature. Following the 1223 K anneal, the sample exhibited ferromagnetic ordering at 140 K. After the 773 K anneal, the ferromagnetic transition is at 350 K, a characteristic of the ferromagnetic austenite phase with 15<<em>x<25. Consistent with the magnetization data, transmission electron microscopy examination confirms that the alloy decomposed into two phases with x = 20 and 1. From this result one can conclude that the martensitic transformation occurs only in those compositions where the single phase L2 1 has been retained in a metastable state on cooling. Magnetic and structural transitions in the Ni 50 Mn 50−x Sn x ͑x =10-25͒ ferromagnetic shape memory alloys are currently of interest. As in Ni-Mn-Ga, these alloys feature high-temperature austenite and low-temperature martensite phases, where the magnetic state is strongly composition dependent. To study the role of chemical ordering in fine-tuning their magnetostructural properties, they were first annealed for 4 weeks/1223 K to achieve structural and compositional homogeneity, and were then further annealed for 1 week ͑ϳ150 K below the reported B2 to L2 1 transition͒ at 773 K to increase the degree of chemical ordering. For x = 11, this anneal resulted in a dramatic change in the magnetic ordering temperature. Following the 1223 K anneal, the sample exhibited ferromagnetic ordering at 140 K. After the 773 K anneal, the ferromagnetic transition is at 350 K, a characteristic of the ferromagnetic austenite phase with 15Ͻ x Ͻ 25. Consistent with the magnetization data, transmission electron microscopy examination confirms that the alloy decomposed into two phases with x = 20 and 1. From this result one can conclude that the martensitic transformation occurs only in those compositions where the single phase L2 1 has been retained in a metastable state on cooling.

Metastability of ferromagnetic Ni–Mn–Sn Heusler alloys

“…4) features a eutectoid-like microstructure. These regions did not display any structural transformation between 109 and 443 K-consistent with the type of behavior observed in Ni 50 Mn 39 Sn 11 eutectoid structures [27].…”

“…Phase decomposition into two phases was confirmed by TEM analysis of Ni 50 Mn 39 Sn 11 , following the 1-week anneal at 773 K [27]. Detailed analysis of the TEM micrograph (Fig.…”

“…In the case of Ni 50 Mn 50−x Sn x (x = 11, 13, and 15) alloys, a 4-week anneal at 1223 K was determined necessary to achieve the required chemical and structural homogeneity [26]. As part of these studies, a low temperature anneal study (∼150 K below the reported B2 to L2 1 transition temperature) was undertaken to gain a better understanding of how atomic ordering alters the properties of the Ni-Mn-Sn alloys [27]. As a result of this study, it was found that the alloy Ni 50 Mn 39 Sn 11 (following the 4-week 1223 K homogenization anneal) is metastable at 773 K and decomposes after 1 week at 773 K into Ni 54 Mn 45 Sn 1 and Ni 50 Mn 30 Sn 20 .…”

Effect of Co substitution on the martensitic transformation and magnetocaloric properties of Ni50Mn35−xCoxSn15

“…The chemical formulas are Ni 50.1 Mn 30.5 Sn 17.3 Co 2.1 and Ni 59 Mn 32.5 Sn 2.5 Co 6 , respectively. It is in good agreement with the reported decomposition of Ni 50 Mn 50−x Sn x into Ni 50 Mn 30 Sn 20 and Ni 54 Mn 45 Sn after 4 weeks annealing at 1223 K (Yuhasz et al, 2009). As the chemical composition of Ni 59 Mn 32.5 Sn 2.5 Co 6 is closer to that of NiMn, the alloy should be crystallized in the NiMn-type structure and Ni 50.1 Mn 30.5 Sn 17.3 Co 2.1 as the L2 1 austenite phase.…”

Grain growth in Ni–Mn–Ga alloys

“…Annealing times of 1 h or 2 h did not suffice to dissolve the netlike segregations, but in contrast an annealing time of 72 h did. Also Yuhasz et al showed that an extended annealing (about 4 weeks) at 1223 K is needed to gain a chemical and structural homogenization for Ni 50 Mn 50−x Sn x (x = 11-15) [14].…”