The Effect of Quenching Variables on the Transformation Characteristics of Some RSP Shape-Memory Alloys
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-THE EFFECT OF QUENCHING VARIABLES ON THE TRANSFORMATION CHARACTERISTICS OF SOME RSP SHAPE-MEMORY ALLOYS
J.V. Wood Department of Materials Science, The Open University, Walton Hall, Milton Keynes MK7 6AA, England.
ABSTRACT Ribbons of a Cu-Al-Ni shape memory alloy have been rapidly solidified by planar flow casting. Attempts have been made to vary casting and secondary quench conditions in order to identify the predominant process variable in transformation temperdetermining the subsequent Rk-+ y atures. By varying wheel speed, wheel material and secondary quenching rate it is concluded that the large excess vacancy concentration has most influence on the as cast ribbon, but that after subsequent heat treatment, the grain size dominates.
INTRODUCTION The interest in rapidly solidified shape memory alloys has increased The impetus report by Stobbs and Wood (1). significantly since the first behind this work has been to explore the possibility of creating a fine grained shape memory device which would display adequate reversible strain while showing significant improvements in thermal fatigue behaviour compared Most of this work has with material processed by conventional techniques. concentrated on copper-base alloys which are relatively easy to melt spin without a controlled atmosphere (e.g. 2-4), although problems of dezincification have been reported for Cu-Zn-Al alloys by all workers and this is partly responsible for the apparently random trends observed in that system (5). For this reason the Cu-Sn (31 and Cu-Ni-Al (6, 7) are more attractive systems for determining the relationship between rapid solidification processing and transformation characteristics. The phase transformation of interest is the 8."-- y{ reversible reaction. However, for convenience the characteristics of all the pertinent phases are listed below:
S
-
high temperature b.c.c. phase
ý1
-
ordered 8 (b.c.c. with DO3 ordering) intermediate phase
yi
-
ordered martensite - 2H structure
Sj and a"
-
ordered martensites found on deforming 81 andy 1 Both have an 18 R structure (8). respectively.
a and
-
equilibrium phases formed by eutectoid decomposition of 8 or 81 phases.
.
and y' are metastable and are formed during solid state In general, both 5 occurs above 5-) + 2 under certain quenching (It is possible that -'$ Although there is considerable thermal and kinetic data for circumstances.) similar reactions in binary Cu-Al alloys, the effect of adding nickel is not In general terms, the adding of nickel reduces the transforwell reported. mation temperature of aI--" yj and at about 4% Ni the transformation occurs The adding of nickel (3%) is known to slow down the around room temperature. Although the quoted quench rates for rapid solidific8•Y2+ a reaction (9). Mat.
Res. Soc. Symp. Proc.
Vol.
28 (1984) CElsevier Science Publishing Co.,
Inc.
426
ation refer to the liquid-solid transformation (ignoring recalescence), there is ample evidence that the cooling rate can drop by several orders of magnitude during the subsequent s
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