Mechanical behavior of fine-grained Mg-6.5Li at elevated temperature
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Oleg D. Sherby Department of Materials Science and Engineering, Stanford University, Stanford, California 94305 (Received 7 September 1993; accepted 3 February 1994)
A Mg-6.5 wt. % Li alloy containing 80% hep alpha phase and 20% bec beta phase was processed to achieve an average grain size of 5.9 /xm. Strain-rate-change tests were performed in the temperature range from 398 K to 573 K. Two types of creep behavior were observed. A stress exponent of five, obtained at low temperatures and high stresses, is attributed to a diffusion-controlled dislocation creep process in the alpha matrix. A stress exponent of three, obtained at high temperatures and low stresses, is attributed to a solute-drag controlled dislocation creep process in the alpha matrix.
I. INTRODUCTION Magnesium-lithium alloys represent promising structural materials because of their high specific stiffness. There is a two phase region in the magnesiumlithium system in the composition range of 5.4 to 10.9 wt. % lithium. These phases are a hexagonalclose-packed magnesium-based phase (a) and a bodycentered-cubic lithium-based phase (yS). Previous studies have been made of a two-phase alloy containing 9 wt. % lithium (Mg-9Li) where /3 is the continuous phase.1"3 Fine grain sizes were obtained in these alloys through a foil-metallurgy approach. These materials exhibited superplastic properties for fine grain sizes (2.7 < d < 14.2 yttm) and strain rates below e = 10~2 s"1 over temperatures ranging from 373 K to 523 K. Studies of high temperature deformation have not previously been made for a two-phase magnesiumlithium alloy in which a is the dominant phase. It would be expected that such an alloy would have markedly different behavior from alloys with a continuous /3 phase because of the higher strength of the hexagonal-closepacked a phase. It is the purpose of this investigation to study the elevated-temperature mechanical properties of a fine-grained alloy of magnesium with 6.5 wt. % lithium, in which a is the continuous phase.
II. PROCESSING AND EXPERIMENTAL PROCEDURE The Mg—6.5Li material was supplied as a casting by the Naval Surface Weapons Center, Silver Spring, MD. Mg-6.5Li contains approximately 80% a phase and 20% fi phase at room temperature. The melting point of the alloy is 861 K.4 This alloy was processed 1392 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 6, Jun 1994 Downloaded: 13 Mar 2015
by the same foil-metallurgy method that was used for the Mg-9Li alloy. 13 The Mg-6.5Li material was received as a cylindrical casting with a diameter of 3.9 cm and a length of 22 cm. This casting was cut into 3.7 cm sections that were pressed across their diameter at 423 K—453 K, flattening to a height of 1.5 cm. The sections were warm rolled at 373 K until a reduction in thickness of 74 to 1 was achieved. After polishing and cleaning these resulting foils, the thickness was found to be 0.11 mm. The polished and cleaned foils were stacked and press-bonded at a temperature of 483 K, achieving an additional reduction of 4.3 to 1. This laminat
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