Effects of Alloy Composition on Microstructure and Mechanical Properties of Iron-Based Materials Fabricated by Ball Mill
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INTRODUCTION
DIFFUSION-BONDED powders based on the Fe-Cu-Ni-Mo systems have been used commercially to produce powder metallurgy parts through single-press/ single-sinter processing.[1,2] The sintered parts produced from diffusion-alloyed powders exhibit a heterogeneous microstructure because some of the elements, such as Ni and Mo, diffuse slowly during sintering cycles. The heterogeneous microstructure influences significantly the fracture behavior and causes unsatisfactory mechanical properties. Moreover, incomplete diffusion means that the full benefits of those expensive alloys will not be obtained. The effective approach to solve this problem is to use fully pre-alloyed steel powders. One of the most attractive approaches used to produce fully pre-alloyed powders is ball milling of elemental powders.[3] The ballmilled powders are characterized by not only a homogeneous element distribution, but also a grain-refined microstructure. It is well known that the strength of materials can be increased by decreasing their grain size to sub-micron or nano meter scale. In spite of its advantages, good consolidation of ball-milled powder to YONGQUAN YE, Ph.D. Candidate, XIAOQIANG LI, SHENGGUAN QU and YUANYUAN LI, Professors, and KE HU, Postdoctoral Fellow, are with the National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials, South China University of Technology, 381 Wushan Road, Guangzhou 510640, P.R. China. Contact e-mail: [email protected] Manuscript submitted May 9, 2014. Article published online October 15, 2014 476—VOLUME 46A, JANUARY 2015
bulk material will suffer difficulties. First, because of the solid-solution strengthening effect of the alloying elements, ball-milled powders are less compressible than those of chemically similar but produced by diffusion bonded or powder mixes.[4] The poor compressibility may limit the maximum sintered density of the parts obtained via traditional P/M compaction and sintering techniques, partly reducing the benefits of a homogenous distribution of the alloying elements. Secondly, for conventional sintering high-density steel parts can be obtained only at high temperature, resulting in grain growth and loss of the initial nano or ultrafine-grained structure of the starting powders.[5] Thus, it requires a special consolidation technique for the sintering of ballmilled powders. Spark plasma sintering (SPS) is a novel rapid sintering technique that utilizes direct current along with uniaxial load.[6] In comparison with the conventional sintering, SPS can consolidate powders to near-full density at a relatively lower temperature and in a shorter sintering time. Hence, SPS is thought to be an effective way to consolidate ball-milled powders. In previous investigations,[7,8] fully dense bulk Fe-2Cu-2Ni-1Mo steels were successfully fabricated by spark plasma sintering of ballmilled powders. Specifically, the effects of milling time and sintering parameters on the densification behavior of SPS specimens were studied in detail. As we know, the addition of alloying elements has
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