Mechanical Properties and Microstructural Behavior of a Metal Matrix Composite Processed by Severe Plastic Deformation T
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Mechanical Properties and Microstructural Behavior of a Metal Matrix Composite Processed by Severe Plastic Deformation Techniques Shima Sabbaghianrad1 and Terence G. Langdon1,2 1 Departments of Aerospace and Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, U.S.A 2 Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, U.K. ABSTRACT A severe plastic deformation (SPD) technique was applied to an Al-7075 alloy reinforced with 10 vol.% Al2O3. This processing method of high-pressure torsion (HPT) was performed at room temperature under a pressure of 6.0 GPa through a total number of up to 20 turns. The metal matrix composite (MMC) showed a significant grain refinement from an initial average grain size of ~8 µm to ~300 nm after processing by HPT through 20 turns which led to an increase in the average values of Vickers microhardness at room temperature. INTRODUCTION Severe plastic deformation (SPD) is a processing technique regularly applied to metals and alloys to achieve ultrafine-grained (UFG) materials having grain sizes in the submicrometer and nanometer range [1]. Several SPD methods are now available to achieve grain refinement, and thereby to improve the mechanical properties and increase the strength of the materials. The most attractive SPD processing methods are equal-channel angular pressing (ECAP) [2] and high-pressure torsion (HPT) [3]. In processing by ECAP, the material is in the form of a rod or bar and it is pressed through a die constrained within a channel that is bent though an abrupt angle. In processing by HPT, a thin disk is held in the shallow depression between anvils under a high pressure and subjected to concurrent torsional straining. The first application of HPT is traced back almost 70 years [4]. However, the technique has attracted considerable attention only within the last decade due to its ability to produce materials having exceptionally small grain sizes, generally smaller than those processed using ECAP [5,6]. Therefore, processing by HPT provides an opportunity for achieving very high strength and high hardness in various materials. Hardness homogeneity is also often achieved by increasing the numbers of revolutions in the HPT processing [7-10]. Metal matrix composites (MMC) consist of at least two chemically and physically distinct phases and are used to achieve properties that are not obtainable with any one of the phases individually. Metal matrix composites are widely used in structural applications and aerospace industries [11-14]. The first application of HPT processing was performed on an Al6061 + 10 vol.% Al2O3 up to a strain of ~7 under an applied pressure of 3.5 GPa [15]. This process led to a significant increase in the hardness value of the sample from Hv ≈ 650 MPa in the received sample to Hv ≈ 1600 MPa after an imposed strain of ~7. Application of severe plastic deformation techniques was later investigated on other MMC materials which led to high
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