Microstructure-property relationships in low-density Al-Li-Mg alloys
- PDF / 3,763,242 Bytes
- 11 Pages / 597.28 x 785 pts Page_size
- 75 Downloads / 218 Views
I.
INTRODUCTION
THE benefits o f lowering the density and increasing the elastic modulus o f aluminum alloys by lithium additions are well known. Experimental observations show that the beneficial effects o f Li additions continue to accrue when the Li concentration exceeds the solubility limit, t~-6] However, nearly all AI-Li alloy development efforts have focussed on alloy compositions that do not exceed the Li solubility limit. This is a consequence o f the desire to avoid insoluble A1Li (S) particles, w h i c h have deleterious effects on fracture toughness and electrochemical properties, t4-14] Unfortunately, requiring that the Li concentration not exceed the solubility limit imposes a l o w e r limit o f about 2.5 M g / m3 on the density o f practical AI-Li alloys. Conventional w i s d o m suggests that further density reductions are necessarily associated with degraded engineering properties as a result o f coarse, insoluble A1Li particles that form during solidification. However, we note that several successful engineering alloys contain substantial volume fractions o f insoluble particles: cast irons and AI-Si alloys, for example. These examples suggest that development o f satisfactory engineering properties may be possible in A1-Li alloys that achieve very l o w density through Li additions that exceed the solubility limit. Meschter and co-workers I~3,14] investigated A1-Li-Cu alloys containing 14 to 17 at. pct Li and 0 to 0.9 at. pct C u , with the goal o f determining the volume fractions o f various phases that could be tolerated without excessive degradation o f mechanical properties. In particular, these investigators explored the potential benefits o f high T.E. BUCHHEIT, Graduate Student, and J.A. W E R T , Professor, are with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903-2442. Manuscript submitted March 1 6 , 1992. METALLURGICAL TRANSACTIONS A
solution treatment temperatures w h i c h decrease the volume fraction o f insoluble particles (referred to as constituent particles in A1 alloys) but increase the volume fraction o f nonequilibrium A13Li that precipitates during subsequent aging treatments. Both A1Li constituent particles and A13Li precipitate particles have been shown to have deleterious effects on the tensile ductility o f A1-Li alloys, t4'5'13-181 Rapid solidification processing o f the A1-Li-Cu alloys produced microstructures with small grain sizes, a microstructural feature thought to mitigate the deleterious effects o f slip localization caused by A13Li particles3 tS,tSl The results reported by Meschter e t a l . t~3"~41 show that rapidly solidified and consolidated alloys having volume fractions o f A13Li up to 0.42 retained acceptable tensile ductility, but only 0.05 volume fraction o f A1Li constituent particles could be tolerated without unacceptable degradation o f tensile ductility. Since other engineering alloys are known to contain more than 0.05 volume fraction o f insoluble particles, the conclusion reached by Mesc
Data Loading...
