Analysis of Acoustic Emission During the Melting of Embedded Indium Particles in an Aluminum Matrix: A Study of Plastic
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INTRODUCTION
RECENT study of the aluminum-indium system has shown that equilibrium melting of the indium particles can be detected by acoustic emission (AE) techniques.[1] AE results from rapid energy release that creates elastic pressure waves in a material. According to the literature, displacive solid-state transformations generate AE resulting from the shear mechanism of transformation and diffusive transformations normally occur too slowly to generate AE.[2] In steels, martensite[2] and bainite[3] generate AE, but formation of allotriomorphic ferrite or the eutectoid product pearlite does not.[2] Formation of Widmansta¨tten ferrite has been suggested to also generate AE.[3] Consequently, displacive or martensitic-like solid-state transformations are often distinguished from diffusion controlled phase transformations by the presence of AE.[4] However, liquid-solid transformations are also known to exhibit AE as the solid contracts, i.e., most materials exhibit AE upon solidification but not melting.[5] The exact cause of solidification AE is debated,[6] but may be due to frictional noise between solid crystals,[7] cluster addition or subtraction from the solid-liquid interface,[8] or perhaps casting separation from the mold wall. AE is detected in MICHAEL M. KUBA, Graduate Research Assistant, and DAVID C. VAN AKEN, Curator’s Professor, are with the Department of Materials Science and Engineering, Missouri University of Science and Technology, 223 McNutt Hall, 1400 N. Bishop, Rolla, MO 654090330. Contact e-mail: [email protected] Manuscript submitted June 26, 2012. Article published online October 10, 2012 3444—VOLUME 44A, AUGUST 2013
crystallizing polymers because of cavitation in areas of occluded liquid where shrinkage stresses overwhelm the cohesive strength of the melt and void formation occurs.[9] AE is also detected during tensile tests for dislocation creation and motion associated with a yield point drop[10] and for void nucleation at nonmetallic inclusions during ductile fracture processes.[11] However, even a small amount of prior cold work has been shown to drastically decrease the AE response from dislocation movement in aluminum during tensile tests.[10] The presence or the absence of AE in aluminum is dependent upon the slip distance, and a maximum pre-yield dislocation density of 2.34 9 106 cm2 for detectable AE upon yielding is predicted.[10] Thus, the presence of AE during phase transformations provides powerful insight into the mechanism of the transformation because the sources of AE are well documented. Malhotra and Van Aken[12] have proposed a strain energy effect associated with the 2.5 pct volume change upon melting for embedded indium inclusions in aluminum. The calculated increase in melting temperature (~6 K) was in good agreement with superheat measured by differential scanning calorimetry (DSC) and that observed by internal friction. Malhotra and Van Aken[12] also demonstrated that the internal friction peaks observed during melting are dependent upon applied test frequency and heating rate;
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