Composite Materials Research Progress

Recent Advances in Discontinuously Reinforced Aluminum… 291
stress into a modified creep equation yields a true stress of ~ 2 and a true activation energy
close to that for boundary diffusion for Al, indicating that the rate controlling process is
related to grain boundary sliding. In other words, the grain boundary activity becomes
dominant in an UFG 5083 Al alloy during creep deformation at high temperatures.
Processing of DRA composites through SPD is an effective route to refine the grain size
of composites to submicrometer level and to disperse the reinforcing particles homogeneously
within the UFG matrix. Langdon and coworkers studied microstructural development in an
Al-6061 composite reinforced with 10 vol.% Al2O3 particulates by means of the HPT and
ECAP techniques [23]. The average size of particulates is ~ 10 μm. For HPT, the samples
were strained at room temperature to a total strain of ~ 7 under a pressure of 3.5 GPa. For
ECAP, samples were pressed for eight passes at 673 K, and two additional passes at 473 K,
giving a total strain of ~ 10. Substantial grain refinement of aluminum alloy matrix can be
achieved using both techniques, i.e., a mean grain size of ~0.2 μm is attained after HPT and
~0.6 μm after ECAP. The microstructures of strained composites consist of an array of very
small grains with poorly defined boundaries. There is no refinement for the alumina
microparticles after HPT or ECAP treatment. The strength of the ECAP 6061 Al/Al2O3
composite is increased by almost two fold by ECAP and close to a factor of ~3 by torsion
straining due to the grain refining of aluminum alloy matrix [23]. In general, ECAP treatment
does not cause fracture of reinforcing particles during plastic straining, especially for finer
particulates [28]. However, limited particle cracking is found for the 6061 Al composite
reinforced with large alumina particulate (7.4 μm) subjected to ECAP at room temperature
[58].
Figure 18. Temperature dependence of the compressive yield strength for the tri-modal 5083Al/10 wt%
B4C composite. The inset shows true stress-strain curves tested at elevated temperatures [15].