X-ray study of phase transformations in martensitic Ni-Al alloys
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TRODUCTION
AT the Ni-rich side of the Ni-Al phase diagram, several ordered phases are known to exist,[1] as shown in Figure 1. The ordered B2 (CsCl-type) structure shown in Figure 2(a) is stable at high temperatures over a wide composition range of 40 to 68 at. pct Ni. For Ni-rich compositions, excess Ni atoms distribute themselves randomly on the Al sublattice.[2] Above 63 at. pct Ni, the B2 phase undergoes a martensitic transformation up on quenching to the tetragonal L1o ordered structure (Fig. 2b),[3,4] while at lower Ni contents, the B2 transforms into the ordered 14M martensitic structure[5,6] (formerly, 7R or 7M notation[7]). The latter is a long period structure that can be described as L1o with a complex arrangement of periodic stacking faults.[6] The martensitic transformation was reported to be thermoelastic with a small hysteresis much like that in Ni-Ti, Cu-Zn-Al, and other shape memory alloys.[4–9] The Ms temperature for the transformation in Ni-Al varies sharply with Ni content, from 2280 7C to 500 7C over the composition range of 60 to 68 at. pct Ni.[9,10] These alloys are therefore considered as promising shape memory alloys for operation at elevated temperatures.[11,12] However, the stability of the martensitic transformation, and hence the shape memory effect, has been shown to be dependent on the presence of the Ni5Al3 and Ni2Al phases.[13,14] The Ni5Al3 (hereafter, abbreviated as 5:3 following the notation in Reference 15) structure was first observed in Ni63.8Al35.2Co1 by Enami and Nenno.[16] They suggested an ordered orthorhombic unit cell of the Pt5Ga3 type, schematically presented in Figure 2(c). This unit cell was later confirmed in binary Ni65.3Al34.7 by Khadkikar and Vedula, who determined the lattice parameters to be a 5 0.7475 nm, b 5 0.3732 nm, and c 5 0.6727 nm.[17] From these parameters, a basic nearly tetragonal lattice is apparent (a ' 2b), and the small deviations from tetragonality are neP.L. POTAPOV, Researcher, and V.A. UDOVENKO, Professor, are with the Institute of Physical Metallurgy, 107005 Moscow, Russia. S.Y. SONG, Postgraduate Student, and S.D. PROKOSHKIN, Professor, are with the Moscow Institute of Steel and Alloys, 117936 Moscow, Russia. Manuscript submitted August 20, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
glected in most cases.[18,19,20] The 5:3 phase does not seem to be exactly stoichiometric, being stable over a composition range of 62.5 to 67.5 at. pct Ni up to approximately 700 7C.[20] The locations of Ni5Al3 boundaries included in Figure 1 are taken from the work of Khadkikar et al.[18] Microstructural studies suggested that the 5:3 phase can form both from the L1o and B2 phases on aging at 300 7C to 700 7C. In the former case, a simple reordering occurs in L1o martensite at temperatures as low as 300 7C, while the main morphology of the martensite plates and their internal twinning remains unchanged.[21,22,23] In the latter case, fine highly symmetric 5:3 precipitates appear in the B2 matrix and grow into large plates, also resembling the L1o martensi
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