Mechanisms of internal friction in a cu-zn-ai shape memory alloy
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I.
INTRODUCTION
THE internal friction of shape memory alloys has increasingly become a focus of both theoretical and practical interests, due to its usefulness in studying the phase transformations, I~.2'3J as well as the potential of these alloys in high damping applications. 14'5j With the increasing awareness on environmental protection and control, the problem of containing severe vibration and noise in mechanical devices, either operating at high speed or during impact loading, has received considerable attention. In order to develop these shape memory alloys for high damping application, mechanisms of mechanical damping or internal friction in these alloys must be thoroughly understood. This study is intended to achieve a better understanding of such mechanisms by simultaneously measuring the internal friction and the shape change occurring during cooling and heating through the martensitic transformation. II.
EXPERIMENTAL
The nominal composition of the alloy used in this study is Cu-13 wt pct Zn-9 wt pct AI. It was prepared by induction-melting pure (99.9 pct) metals of Cu, Zn, and A1, followed by casting and extrusion. The wire specimen of 1.2-mm diameter • 20-ram length was homogenized in the 13 parent phase by keeping it at 850 ~ for 10 minutes before quenching into water held at 18 ~ The internal friction and shape change of the specimen were then studied in a three-point bending configuration using a PERKIN-ELMER* dynamic mechanical ana*PERKIN-ELMER is a trademark Electronics, Eden Prairie, MN.
of Perkin-Ehner
Physical
lyzer, t3r The cylindrical sample, placed horizontally on two knife edges, was sinusoidally deformed by a V-shaped probe tip held at the midpoint of the sample but orthogonal to its length. The instrument allowed continuous determination of the position of the probe tip, and thus, any concomitant deflexion (resulting from the phase change, for example) during the measurement of internal T. XIAO, Research Associate, and G.P JOHARI, Professor, are with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada LSS 4L7. Manuscript submitted July 22, t994. METALLURGICAL AND MATERIALS TRANSACTIONS A
friction was monitored by the vertical displacement of the probe's mean position during a sinusoidal oscillation. All measurements were made at a frequency of 1 Hz and a heating or cooling rate of 20 ~ III.
R E S U L T S AND DISCUSSION
To study the effects of stress amplitude, plots of internal friction v s temperature (cooling) and probe position v s temperature (cooling and heating) were obtained at progressively higher stress amplitudes, as shown in Figures 1 and 2, respectively. From these plots, Figure 3 was drawn, where the error bars for the internal friction peak curve were based on repeat measurements at a fixed stress amplitude, which gave a standard deviation of 5 • 10 -3 '
Figure 3 shows that when the stress amplitude was initially at a level not high enough to induce stress-oriented transformation, and was later progressively increased, th
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