The effect of strain rate on the properties of an aluminium-lithium alloy

Abstract

The flow stress of metals is generally observed to increase with strain rate• The literature on the dependence of properties on strain rate is extensive, but for new alloys, and high-strength materials in particular, data are far more difficult to obtain. Commercial AI -L i alloys show combinations of strength and toughness that are comparable with, and in some cases superior to, traditional highstrength A1 alloys [1]. Investigations of properties at high strain rate have shown an increase in strength with increasing strain rate in the range of strain rates 10-5-103 s -1, with the effect being far more pronounced in tension than in compression [2]. Chiem et al. [2] showed that fracture strain increases with strain rate at high strain rates. However , their values reported at the high strain rates are well below the quasi-static ductilities• Kobayashi et al. [3], on the other hand, showed that the elongation at high strain rates is superior to that measured at quasi-static strain rates for A1-Li alloys• This letter contributes to the database on AI -L i alloys by characterizing the properties of an alloy in compression over the strain rate range 10 .3 to 6 x 103 S 1 . The AI L i alloy used in this study was Alcan 8090-T82551 in the form of rectangular extruded bar of dimensions 100 m m x 25 mm. The alloy has nominal values of 0.2% proof stress, ultimate tensile stress and elongation of 475 MPa, 525 MPa and 5.0%, respectively, which concur with values given in [1]. At the lowest strain rate (10 .3 s -1) compression tests were performed with the cylinder axis in the extrusion, short transverse (thickness) and long transverse (width) directions, and using two different lubrication conditions, viz. a Teflon tape and a Teflon grease (Dupont Kryotox 240 AC). At the higher strain rates all tests were with the specimen axis in the short transverse direction and using only the Teflon tape. The range of strain rates was achieved using a screw-type hydraulic drop weight with a piezoelectric load cell, and direct impact Hopkinsons bar techniques, which were described in detail in [4-6]• Fig. 1 illustrates four typical stress-strain curves for specimens with the cylinder axis in the short transverse direction• The two curves at the strain rate of 1.3 x 10 .3 s -I are typical results for the two types of lubrication. Up to the maximum load the curves were identical within observed specimen-tospecimen scatter• Beyond a maximum load the Teflon tape-lubricated specimens most commonly failed in shear at a strain of the order of 0.2, whereas 600