Crystal structures and textures in the hot-forged Ni-Mn-Ga shape memory alloys
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hape-memory alloys (FSMAs) have attracted great interest during the past several years due to their potential applications as sensors and actuators. Among those FSMAs, Ni-Mn-Ga alloys with chemical compositions close to the stoichiometric intermetallic compound Ni2MnGa have been studied extensively[1–9] as they exhibit a giant shape-memory effect (SME) under applied magnetic fields. However, many fundamental issues remain unclear, such as crystal structures and crystallographic textures. Some controversial results[7,8,9] on the crystal structure, which is highly sensitive to the chemical compositions of the alloys, have been obtained from some earlier investigations by X-ray diffraction due to similar X-ray scattering factors of Ni, Mn, and Ga atoms. The neutron diffraction technique provides accurate information on the crystal structures and shows advantages in texture measurements for the Ni-Mn-Ga alloys, attributed to the large difference in the neutron diffraction factors for the Ni, Mn, and Ga atoms. The magnetic SME in Ni-Mn-Ga alloys is caused by the rearrangements of the martensitic variants and/or the growth of favorably oriented variants under applied magnetic fields.[2,4,6] Meanwhile, the temperature- or stressinduced SME is also attributed to some preferential arrangements of martensitic variants during the phase transformation.[10] The preferred orientation distribution (crystallographic texture) in the parent phase and the martensite clearly improves the SME in those alloys. Shu et al.[11] and Thamburaja et al.[12] studied the influence of D.Y. CONG, Graduate Student, and Y.D. WANG, X. ZHAO, and L. ZUO, Professors, are with the School of Materials and Metallurgy, Northeastern University, Shenyang 110004, People’s Republic of China. Contact e-mail: [email protected] R. LIN PENG, Lecturer, is with the Department of Mechanical Engineering, Linko¨ping University, S-58183 ¨ M, Researcher, is with the Studsvik Linko¨ping, Sweden. P. ZETTERSTRO Neutron Research Laboratory (NFL), Uppsala University, S-61182 Nyko¨ping, Sweden. P.K. LIAW, Professor, is with the Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, U.S.A. Manuscript submitted August 29, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
texture on the SME in the polycrystalline shape memory alloys, which verified that the texture plays an important role in improving the SME. Therefore, it is of vital importance to explore some novel fabrication processes to enhance the crystallographic texture in the polycrystalline FSMAs. Despite the brittleness of Ni-Mn-Ga alloys at room temperature, the hot-forging process, a standard procedure for fabricating some structural intermetallic alloys, provides an effective way to modify textures in these alloys. It is well known that a rearrangement of martensitic variants occurs during mechanical loading, which may change the textures in the Ni-Mn-Ga alloys. However, many earlier reports[5,13,14] concentrated mainly on the deformation behavior during loading, and the stre
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