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Characterization of the Phase Transformations in the Shape Memory Alloy Ni-36 at.% Al

Published online by Cambridge University Press:  25 February 2011

J.A. Horton ,
E.P. George ,
C.J. Sparks ,
M.Y. Kao ,
O.B. Cavin  and
P. Thoma
J.A. Horton
Affiliation:
Metals and Ceramics Div., Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6115
E.P. George
Affiliation:
Metals and Ceramics Div., Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6115
C.J. Sparks
Affiliation:
Metals and Ceramics Div., Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6115
M.Y. Kao
Affiliation:
Johnson Controls, Inc., Milwaukee, W1 53201-0591
O.B. Cavin
Affiliation:
Metals and Ceramics Div., Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6115
P. Thoma
Affiliation:
Johnson Controls, Inc., Milwaukee, W1 53201-0591
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Abstract

A survey by differential scanning calorimetry (DSC) and recovery during heating of indentations on a series of nickel-aluminum alloys showed that the Ni-36 at.% Al composition has the best potential for a recoverable shape memory effect at temperatures above 100°C. The phase transformations were studied by high temperature transmission electron microscopy (TEM) and by high temperature x-ray diffraction (HTXRD). Quenching from 1200°C resulted in a single phase, fully martensitic structure. The initial quenched-in martensites were found by both TEM and X-ray diffraction to consist of primarily a body centered tetragonal (bct) phase with some body centered orthorhombic (bco) phase present. On the first heating cycle, DSC showed an endothermic peak at 121°C and an exothermic peak at 289°C, and upon cooling a martensite exothermic peak at 115° C. Upon subsequent cycles the 289°C peak disappeared. High temperature X-ray diffraction, with a heating rate of 2°C/min, showed the expected transformation of bct phase to B2 between 100 and 200°C, however the bco phase remained intact. At 400 to 450°C the B2 phase transformed to Ni2Al and Ni5Al3. During TEM heating experiments a dislocation-free martensite transformed reversibly to B2 at temperatures less than 150°C. At higher temperatures (nearly 600°C) 1/3, 1/3, 1/3 reflections from an ω-like phase formed. Upon cooling, the 1/3, 1/3, 1/3 reflections disappeared and a more complicated martensite resulted. Boron additions suppressed intergranular fracture and, as expected, resulted in no ductility improvements. Boron additions and/or hot extrusion encouraged the formation of a superordered bct structure with 1/2, 1/2, 0 reflections.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 246: Symposium M – Shape-Memory Materials and Phenomena--Fundamental Aspects and Applications , 1991 , 61
Copyright
Copyright © Materials Research Society 1992

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