Determination of the sources of acoustic emission generated during the deformation of titanium
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I.
INTRODUCTION
IT is
well known that crystalline materials deform by specific deformation mechanisms and processes, and that detectible AE may be generated if the deformation processes are accompanied by a sudden stress relaxation and/or strain accommodation. Alpha-phase (HCP) titanium has been observed to deform by twinning, dislocation slip, and grain boundary sliding, although the latter process has been shown not to be a significant source of AE. ~ By simultaneously measuring a wide range of AE parameters along with other experimental variables sensitive to specific deformation mechanisms, it is hoped that the measured AE can be ascribed to specific deformation processes with some degree of confidence. The purpose of this paper is to present data and analyses which can be used to identify the sources of AE generated during deformation of alphaphase titanium.
II.
crystal c axes tended to be oriented radially but pointing at some small angle above the cross-sectional plane. The preferred orientation was less pronounced in the Marz material. Metallographic analyses showed that the grains were uniformly equiaxed and on the order of 10 microns in diameter in all of the materials tested. Microholes, similar to those described by Zhu Zu-Ming et al., 3 were found in both grades of material in the as-received condition. Due to the rod geometry, tensile specimens could be machined only with the gauge length parallel to the rod axis. The tensile samples had a 2.54 cm gauge length, 0.26 cm 2 circular cross-section, and a flat, 0.02 cm deep, on the gauge length which served as a seat for the transducer. Compression samples were machined into 1.15 cm • 0.26 cm 2 parallelepipeds oriented in the axial direction. All observable traces of twins and microholes were removed prior to testing by vacuum annealing the test specimens at 810 K for four hours after which they were allowed to furnace cool.
E Q U I P M E N T AND P R O C E D U R E
The data gathering and analyzing system used throughout this investigation is based on a Dunegan-Endevco (D/E) S140B high sensitivity transducer, D/E 3000 series signal processing equipment, a Hewlett-Packard 3400A voltmeter, a Motorola M6800 microprocessor, and a Dillon Universal testing machine. A detailed description of the experimental apparatus is given elsewhere.2 With this experimental set-up it was possible to measure and record simultaneously in digital form the applied load, the sample displacement, the RMS voltage (analog only), the AE count rate, the AE event rate, the AE energy rate, and the AE signal amplitude distributions. All deformation tests were carried out at constant crosshead speeds with the total system gain at 100 db or 112 db. Two purity grades of alpha-titanium were tested: both VP grade (impurities =.
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High strain emlsalon peak
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0 2.5
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Total strain ( % )
o
17
Fig. 5 - AE energy rate, plastic strain, and plastic strain rate as a function of total strain for a C-V type test.
yield, increased with increasing strain rate. The data were fo
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