Mechanical behavior of Al-Li-SiC composites: Part I. Microstructure and tensile deformation
- PDF / 2,159,576 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 22 Downloads / 257 Views
I.
INTRODUCTION
THE presence of lithium in aluminum alloys significantly improves their specific stiffness. Each percent weight of lithium reduces the density by 3 pct and increases the elastic modulus by approximately 6 pct.[1] In addition, lithium helps to strengthen these alloys through the precipitation of the ordered d ' (Al3Li) phase, coherent with the matrix,[2,3] and Al-Li alloys exhibit excellent yield and tensile strengths. This set of properties is particularly interesting for aerospace applications, where good specific properties are always at a premium. However, these improvements are accompanied by a significant reduction in ductility and toughness. This behavior has been associated with the d ' precipitates, whose resistance to dislocation motion decreases as they are sheared by dislocations. This results in planar slip along preferential crystallographic planes and a tendency toward strain localization in slip bands. The intense slip bands can lead to a strain concentration at grain boundaries and may promote intergranular fracture when the macroscopic plastic strain is still low.[4] Commercial AlLi alloys try to minimize the negative effects of planar slip through the addition of zirconium, which refines the grain size, and by forming incoherent precipitates, such as the S ' (Al2CuMg) or u ' (Al2Cu) phases, which resist shearing and, thus, induce homogeneous slip. These modifications improve the behavior of Al-Li alloys, but it is known that commercial Al-Li alloys aged to the peak strength usually fail by intergranular fracture and exhibit poor ductility and fracture toughness as compared to conventional aluminum alloys of comparable strength.[5,6] P. POZA, Assistant Professor, and J. LLORCA, Professor, are with the Department of Materials Science, Polytechnic University of Madrid, 28040 Madrid, Spain. Manuscript submitted February 5, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
Further improvements in the elastic properties can be attained by reinforcing Al-Li alloys with ceramic particles.[7,8] Al-Li/SiC metal-matrix composites can be processed following standard metallurgy techniques and exhibit elastic moduli over 100 GPa, with relative densities around 2.6, leading to a specific stiffness 50 pct above that of standard aluminum and titanium alloys.[8] The ceramic reinforcements also improve other properties, such as the creep and wear resistance and, to a minor extent, the strength,[7,9] and the Al-Li/SiC system presents an excellent performance/price ratio for applications where the specific stiffness is critical. The microstructure[10,11,12] and mechanical properties[13,14,15] of these composites have been concisely reported in several articles, which indicate that the potential benefits are reduced by the embrittlement induced by both Li and SiC. Whether these problems may be overcome is uncertain because of the lack of a systematic investigation on the relationship between the microstructure and the mechanical behavior. This investigation was performed on a commercial 8090 Al alloy
Data Loading...
