Acoustic emission is used here to study melting and solidification of embedded indium particles in the size range of 0.2 to 3 µm in diameter and to show that dislocation generation occurs in the aluminum matrix to accommodate a 2.5% volume change. The volume averaged acoustic energy produced by indium particle melting is similar to that reported for bainite formation upon continuous cooling. A mechanism of prismatic loop generation is proposed to accommodate the volume change and an upper limit to the geometrically necessary increase in dislocation density is calculated as 4.1 x 10 9 cm -² for the Al-17In alloy. Thermomechanical processing is also used to change the size and distribution of the indium particles within the aluminum matrix. Dislocation generation with accompanied acoustic emission occurs when the melting indium particles are associated with grain boundaries or upon solidification where the solid-liquid interfaces act as free surfaces to facilitate dislocation generation. Acoustic emission is not observed for indium particles that require super heating and exhibit elevated melting temperatures. The acoustic emission work corroborates previously proposed relaxation mechanisms from prior internal friction studies and that the superheat observed for melting of these micron-sized particles is a result of matrix constraint.
I. INTRODUCTIONRecent 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 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 Widmanstätten 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 crystallizing polymers due to cavitation in areas of occluded liquid where shrinkage stresses overwhelm the cohesive strength of the melt and void formation occurs [9].Acoustic emission 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 w...