Processing-Structure-Property Relations of Polymer-Polymer Composites Formed by Cryogenic Mechanical Alloying for Select
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ABSTRACT Cryogenic mechanical alloying (CMA) has been shown to be an effective means for producing composite powders for selective laser sintering (SLS). Unlike composite particles made by a coating process, both phases are continuous throughout the particles formed by CMA. Consolidation of these composite particles via SLS offers the possibility of forming parts with a co-continuous microstructure. In this research, the microstructure of mechanically alloyed polymer-polymer composites for use in the SLS process is investigated using transmission electron microscopy. By varying the charge ratio and milling time of the CMA process, the phase domain size of the resulting composite powder can be manipulated. This ongoing work explores the microstructural evolution as the composite powders are consolidated via SLS into macroscopic parts, as well as the relationships between microstructure and bulk properties. INTRODUCTION The mechanical alloying (MA) process was originally developed in the late 1960s for solid state processing of dispersion-strengthened metal powders with fine microstructures. Pan and Shaw', pioneers in the field of mechanically alloyed polymers, assert that the "mechanical alloying technique promises to provide the ability to make almost infinite permutations of polymeric alloys. This means that once the process is better understood the properties of the alloy may be specifically designed resulting in a truly EngineeredMaterial." The mechanically alloyed materials are produced using a ball mill. The initial materials (in powder or pellet form) are placed in the ball mill's vial with two or more metallic or ceramic balls (Figure 1). In a vibratory ball mill, high-energy impacts between the balls and the material occur when the mill's motor vigorously shakes the vial, trapping material between the balls (and between the balls and the vial walls) with each agitation (Figure 2).
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Figure 1. Schematic of vibratory ball mill vial and balls2 .
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Figure 2. High-energy ball-powder-ball collision, resulting in welding (B), extensional flow, and fracture (A)2 . 75
Mat. Res. Soc. Symp. Proc. Vol. 625 © 2000 Materials Research Society
As MA occurs, the particles are repeatedly fractured, deformed, and fused together. This process of repeated, fracturing and cold-welding causes a refinement in microstructure with milling time. The result is a two-phase lamellar or plate-like microstructure with an interlamellar distance dependent on processing time 3 (Figure 3). Other processing parameters which affect the composite microstructure include the energy input, which can be controlled by manipulating the ratio of the total ball mass to the powder mass (charge ratio), milling temperature, ball mill design, and number and size of balls used. The milling temperature can be critical because of its affect on material ductility, recrystallization kinetics, and thermally-aided diffusion across interfaces. Extrusion and injection molding require the polymers to flow on a macroscopic level, which would destr
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