Ni-Fe-Ga-Co磁性形状记忆合金定向凝固稳定生长区的组织及择优取向

MICROSTRUCTURE AND CRYSTAL ORIENTATION OF THE STEADY GROWTH ZONE IN THE DIRECTIONALLY SOLIDIFIED Ni-Fe-Ga-Co MAGNETIC SHAPE MEMORY ALLOYS

采用区域熔炼液态金属冷却定向凝固法制备具有择优取向的Ni_(52)Fe_(17)Ga_(27)Co_4磁性形状记忆合金,借助OM,SEM和EBSD研究了稳定生长区的组织特征和择优取向.结果表明,在低温度梯度和高凝固速率下,晶体沿〈100〉方向生长,但生长不稳定,存在初生γ相;当提高温度梯度和降低凝固速率后,晶体沿〈110〉方向生长,柱状晶大小均匀,无γ相,室温时为沿(222)面取向的马氏体单相组织.马氏体孪晶孪生面为(110)和(110),内部无微孪晶;在马氏体组织中,观察到了条状分布的磁畴.理论分析结果表明,晶体稳定生长的条件为高的温度梯度和低的凝固速率.在该条件下获得了大体积稳定马氏体单变体区.

Ni-Fe-Ga-Co is a new ferromagnetic shape memory alloy system, which is promising for magnetically controlled actuators and refrigerators. In order to improve its performance, single crystal or polycrystals with preferred orientation are required. To prepare these crystals, directional solidification is the most effective way. In this work, directionally solidified Ni52FelTGa27Co4 fer- romagnetic shape memory alloys were prepared by zone melting liquid metal cooling （ZMLMC） method. The preferred crystal orientation and microstructure in the steady growth zone under three different conditions were studied. It is indicated that under low temperature gradient and high growth velocity, the solidified crystals grew along （100/ direction and primary ~/ phase was formed during the unstable growth; under high temperature gradient and low growth velocities, the coarse columnar crystals had uniform sizes, single martensitic phase and well-developed pre- ferred orientation, i.e. along （222） planes of the martensite. It is deduced that the condition for stable crystal growth is high temperature gradient and low growth velocity. Under this condition,giant martensitic single variant sets in the oriented columnar crystals were revealed, which is mean- ingful for potential large magnetic field induced strain. EBSD results revealed that there was no micro twin inside the martensitic twins and the twinning plane turned out to be （110） and Additionally, magnetic domains intersecting with martensitic lamellae were observed.