Microstructure Evolution - Springer Link

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Oct 29, 2011 - CHRIS HAINES, Senior Metallurgist, JOSEPH PARAS,. Materials Engineer, DAROLD MARTIN, Senior Materials Engineer, and DEEPAK ...
Spark Plasma Sintering of Cryomilled Nanocrystalline Al Alloy - Part I: Microstructure Evolution YUHONG XIONG, DONGMING LIU, YING LI, BAOLONG ZHENG, CHRIS HAINES, JOSEPH PARAS, DAROLD MARTIN, DEEPAK KAPOOR, ENRIQUE J. LAVERNIA, and JULIE M. SCHOENUNG Aluminum alloys are widely used because they are lightweight and exhibit high strength. In recent years, spark plasma sintering (SPS) technology has emerged as a viable approach to sinter materials due to its application of rapid heating and high pressure. In this study, SPS was chosen to consolidate dense ultrafine-grained (UFG) bulk samples using cryomilled nanostructured Al 5083 alloy (Al-4.5Mg-0.57Mn-0.25Fe, wt pct) powder. Both bimodal microstructure and banded structure were observed through transmission electron microscopy (TEM) investigation. The evolution of such microstructures can be attributed to the starting powder and the process conditions, which are associated with the thermal, electrical, and pressure fields present during SPS. A finite element method (FEM) was also applied to investigate distributions in temperature, current, and stress between metallic powder particles. The FEM results reveal that the localized heating, deformation, and thermal activation occurring at interparticle regions are associated with the formation of the special microstructure. DOI: 10.1007/s11661-011-0933-3  The Minerals, Metals & Materials Society and ASM International 2011

I.

INTRODUCTION

IN recent years, spark plasma sintering (SPS) technology has emerged as a viable approach to sinter composites, nanocrystalline materials, and amorphous alloys.[1–5] Much of this work has been motivated by the premise that the rapid heating, and hence reduced sintering times, that are possible with SPS should help retain the initial microstructure, and thereby avoid or minimize undesirable reactions, such as coarsening or crystallization. The technique is similar to traditional hot pressing; however, in this case, the sample is heated by a high-intensity, low-voltage pulsed DC electric current flowing directly through the sample (if electrically conductive) and a graphite die, and by simultaneously applying a uniaxial pressure during consolidation. The SPS process has the advantages of sintering samples at a high heating rate and with high pressure. Thus, samples

YUHONG XIONG, YING LI, and BAOLONG ZHENG, Postdoctoral Researchers, ENRIQUE J. LAVERNIA, Distinguished Professor, and JULIE M. SCHOENUNG, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact e-mail: [email protected] DONGMING LIU, Visiting Assistant Researcher, is with the Department of Chemical Engineering and Materials Science, University of California, and is also an Associate Professor, with Department of Materials Science and Engineering, Shandong University, Jinan, Shandong 250061, P.R. China. CHRIS HAINES, Senior Metallurgist, JOSEPH PARAS, Materials Engineer, DAROLD MARTIN, Senior Materials Engineer, and DEEPAK KAPOOR, Group Leader, are with the United States Army, RDECOM-ARDEC, Picatinny Arsenal, NJ 07806. Manuscript submitted February 24, 2011. Article published online October 29, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

can be sintered at comparatively lower temperature in a shorter time and can retain finer microstructure than is achievable with conventional methods. Aluminum alloys, such as Al 5083 (Al-4.5Mg0.57Mn-0.25Fe, wt pct), are widely used because they provide a combination of light weight and high strength. Nanostructured and ultrafine-grained (UFG) Al alloys, in which grain sizes are typically from less than 100 nm to 800 nm, have the potential to revolutionize traditional materials design via atomic-level structural control to tailor engineering properties. Consideration of recent advances and discoveries indicates that such materials provide an exciting new approach for the development of advanced systems suited to military applications. For instance, UFG Al 5083 consolidated using cryomilled nanostructured powder has demonstrated an ultimate tensile strength (UTS) greater than 700 MPa (over twice that of a conventional Al 5083), and preliminary ballistic data showed that the UFG Al 5083 has a V50 that is 33 pct higher than conventional Al 5083.[6] Improvements to material properties greatly depend on the corresponding microstructure of the materials. Relevant studies for bulk Al 5083 samples made by SPS and other more conventional powder metallurgical methods have been reported.[3,7,8] Mainly, a bimodal microstructure (i.e., bimodal distribution of grain size[9,10]) and a banded structure were observed in the bulk samples before or after mechanical tests using a mixture of nanostructured powder and micrometergrained powder as the feedstock material. The presence of UFGs is responsible for improvements in strength, whereas enhanced ductility is attributed to crack tip blunting from ductile, coarse grains. In this study, cryomilled nanostructured Al 5083 powder (without VOLUME 43A, JANUARY 2012—327

coarse-grained powder) was used to sinter bulk samples by SPS, so that the microstructure in the UFG region that develops merely from this powder can be studied in detail. The mechanisms that control the evolution of the unusual microstructure have not been well studied experimentally. A number of simulations related to SPS processing have been reported using different methodologies.[11–13] However, most of these studies focus only on the effect of temperature and current distributions during SPS and consider only two-dimensional (2-D) geometry. Research into the role of powder contact area on microstructure evolution is very limited. In this study, a mechanism for microstructure evolution is proposed and verified by developing and implementing a three-dimensional (3-D) numerical simulation using Comsol Multiphase software. In a subsequent article (Part II),[14] we investigate the influence of SPS processing conditions on the densification behavior and mechanical behavior of the consolidated materials.

II.

EXPERIMENTAL METHODS

In this work, cryomilled nanostructured feedstock Al 5083 powder was used to synthesize bulk samples by SPS. The cryomilled Al 5083 powder was produced by DWA Aluminum Composites (Chatsworth, CA). Detailed information on the cryomilling process can be found in the literature by Newbery et al.[15] The bulk samples were made in a Dr. Sinter SPS-825S system using sieved powder (powder size