Correlation between solid-state transformations and solidification in Ni-Mn-Ga alloys
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sitic materials ranging from single-element metals and alloys to ceramics have formed a unique group of functional materials.[1,2] Novel physical functions have been obtained in martensitic materials as a result of their solid-state transformations, i.e., martensitic transformation (MT) and premartensitic transformation (PMT).[3,4] Effectively controlling the MT and PMT is essential for realizing the expected physic functions. Previous studies of martensitic materials mainly focus on the MT and PMT behaviors, including the effects of chemical components and bias fields (stress, magnetic, electrical, etc).[5–8] Prior to the PMT and MT, another typical first-order phase transformation, solidification, usually takes place. It has been well known that solidification drastically influences the properties of materials by controlling the morphologies, including planar, cellular, and dendrite.[9] The solidification morphology is a major issue for solidification studies when improving the mechanical or physical properties.[10] Up to present, both types of first-order phase transformations, the solid-state transformations and the solidification, were mainly studied in their respective separate fields. However, the relationship JUNYI LIU, PhD candidate, JINGMIN WANG, Research Scholar, CHENGBAO JIANG, and HUIBIN XU, Professors, are with the Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, P.R. China. Contact e-mail: [email protected] Manuscript submitted June 15, 2011. Article published online September 21, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
between the solid-state transformation and the solidification has been ignored in the long term. The ferromagnetic shape memory alloys (FSMAs) Ni-Mn-Ga, an attractive martensitic material developed since 1996,[11] have drawn lots of attention. Previously we have intensively studied the martensitic transformation in Ni-Mn-Ga alloys.[12–14] We have also studied the solidification behaviors of Ni-Mn-Ga alloys.[15,16] For this article, we investigated the solid-state transformations of Ni-Mn-Ga in the planar and cellular solidification morphologies, respectively. The correlation between the two types of first-order phase transformations was revealed. This is of great significance for understanding the nature and functionalities of martensitic materials, and for adjusting the transformations to get optimized or unexpected functions. High purity elements Ni (99.98 pct), Mn (99.9 pct), and Ga (99.99 pct) were arc melted into buttons for four times under argon atmosphere, and then they were dropped cast into a chilled copper mold to obtain nominal composition Ni50.5Mn25Ga24.5 (at. pct) master rods of B 7 mm 9 100 mm. Directional solidification was performed for the master rods in the zone melting crystal growth furnace with the pulling rates of 10 mm/ hour and 400 mm/hour. The solid/liquid interfaces were quenched at the stable stage of the directional solidification. Parts of the solidified ro
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