Mechanical Properties of High Strength Aluminum Alloys Formed by Pulsed Laser Deposition
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ABSTRACT Very high-strength alloys of AI(O) have been formed using a pulsed laser deposition (PLD) system to deposit from alternating targets of Al and A120 3 . Ion beam analysis and transmission electron microscopy show that the deposited material is uniform in composition with up to 33 at.% 0 and has a highly refined microstructure consisting of a fine, uniform dispersion of -1 nm diameter 7-A120 3 precipitates. Ultra-low-load indentation testing combined with finiteelement modeling is used to determine the mechanical properties of the layers. Yield stresses as high as 5.1 GPa have been measured in these materials, greatly exceeding the strengths of aerospace Al alloys (-0.5 GPa) and even high strength steels. The key to the properties of these materials is the dispersion of small, hard precipitates spaced only a few Burgers vectors apart, dislocations are apparently unable to cut through and must bow around them. INTRODUCTION Aluminum alloys are important for a number of applications and would be even more widely applied if not for relatively low strength and poor tribological properties. We have been investigating plasma-based synthesis methods for forming new alloys of aluminum with higher strength [ 1-3], motivated by the earlier discovery at our laboratory of precipitate-hardened AI(O) alloys using 0 implantation into bulk AI.[4-7] In the present paper we describe the formation of AI(0) alloys using a pulsed laser deposition (PLD) system; films of these alloys are formed by alternating depositions of Al and A12 0 3, with each cycle kept under a monolayer.[1] Ion beam analysis shows that the resulting layers are of uniform composition in depth, with up to 33 at.% 0, while transmission electron microscopy (TEM) shows a highly refined microstructure consisting of a uniform dispersion of -1 nm diameter 7-A120 3 precipitates in small grains (5-25 nm) of fcc Al. This microstructure is similar to that formed earlier in the 0-implanted Al materials, where testing showed yield stresses of 1-3 GPa [4-7] and improved tribological properties were observed.[7,8] Similar alloys have also been formed at our laboratory using deposition in an Electron Cyclotron Resonance system, another plasma-based method.[2,3] We use nanoindentation testing combined with finite-element modeling of the indentation process to determine the mechanical properties of these layers. The finite-element modeling is essential because the deposited layer thickness is generally only a few times the maximum depth of indentation, so the load vs. depth response in the test is necessarily a combination of the properties of the substrate and of the layer. Using modeling, we have determined yield stresses as high as 5.1 GPa in these PLD-formed materials, greatly exceeding the strengths of aerospace Al alloys (-0.5 GPa) and even high strength steels. Young's modulus is also determined by the modeling: values range from 120 to 160 GPa, versus 71.9 GPa for pure Al. The modeling procedure we have developed is expected to be widely applicable to other materials
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