Micro-strain Evolution and Toughening Mechanisms in a Trimodal Al-Based Metal Matrix Composite
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INTRODUCTION
OVER the past few decades, nanocrystalline (NC) and ultrafine-grained (UFG) materials have drawn attention due to the improved mechanical properties and the unusual grain structures.[1–5] A wide variety of synthesis techniques have been reported to fabricate bulk NC or UFG materials.[6,7] Among them, cryomilling is a promising synthesis method for making NC or UFG materials in commercial quantities (30 to 40 kg).[8–10] During cryomilling, gas-atomized metallic powders undergo severe plastic deformation via high-energy ball milling in cryogenic liquid slurry. During cryomilling these powders are sheared, fractured, and cold-welded back together refining the grain size to the NC regime. The grain refinement process during milling can be divided into three major stages[11]: (1) localization of high dislocation densities into shear bands, (2) low-angle grain boundaries (LAGBs) and subgrains evolving from dislocation rearrangement at particular strain levels via YUZHENG ZHANG, Research Assistant, and STEVEN R. NUTT, Professor, are with the Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089. Contact e-mail: [email protected] TROY D. TOPPING, Assistant Professor, is with the Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, CA 95616, and also with the Department of Mechanical Engineering, California State University, Sacramento, Sacramento, CA 95819. HANRY YANG, Research Assistant, ENRIQUE J. LAVERNIA and JULIE M. SCHOENUNG, Professors, are with the Department of Chemical Engineering and Materials Science, University of California, Davis. Manuscript submitted August 27, 2014. Article published online January 7, 2015 1196—VOLUME 46A, MARCH 2015
recovery, and (3) LAGBs transforming to high-angle grain boundaries (HAGBs) with excessive deformation by GB sliding and rotation. Cryogenic temperature effectively dissipates the heat generated from milling and thus limits recovery and grain growth.[9] In this study, Al alloy (AA) 5083 was selected as base material because of its potential application in automobile, aerospace and marine structures. Cryomilled Al alloy exhibits enhanced thermal stability due to the creation of nanodispersed aluminum nitrides that pin grain boundaries and allow for the NC or UFG microstructure to be preserved during thermomechanical processing and consolidation.[9,12–16] Since cryomilled AA 5083 retain its refined grain structures, the strength of the resultant bulk products are significantly enhanced according to the Hall–Petch strengthening mechanism.[9,17,18] However, NC or UFG materials usually suffer from limited ductility and low toughness owing to minimal work hardening associated with NC or UFG regions.[19,20] This limitation precludes the use of NC and UFG materials from most engineering applications. In an effort to enhance plasticity and mitigate the brittle behavior of cryomilled AA 5083 (Al-4.4Mg0.7Mn-0.15Cr wt pct) coarse grain (CG) regions were introduced into a UFG matri
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