1987 Volume 28 Issue 5 Pages 392-405
Creep tests at high temperatures have been carried out in an Al–Al2O3 binary alloy and an Al–Al2O3–Si ternary alloy with different grain sizes. Dislocation structures after the steady-state creep have been observed in the temperature range from about 473 K to 873 K. The creep behaviour and the contribution of grain boundary sliding in a low stress range at temperatures above 773 K have been examined in detail in the Al–Al2O3 alloy.
Dislocations were generally distributed rather uniformly, being pinned to dispersed particles when crept at temperatures below 673 K, though incomplete subboundaries were occasionally observed in a high stress range. On the other hand, when crept at temperatures above 773 K, sharp subboundaries, which appeared to be formed associated with the grain boundary sliding, were often observed with the uniform distribution of dislocations.
At temperatures above 773 K, the Al–Al2O3 alloy was deformed at relatively large creep rates under stress lower than the threshold value which was estimated at temperatures below 673 K, that is, the temperature compensated creep rate \dotεs⁄D in the low stress range deviated upwards from the \dotεs⁄D vs σ⁄E relation obtained at temperatures below 673 K, where \dotεs was the steady state creep rate, D the self-diffusion coefficient of aluminium, σ the applied stress and E the Young’s modulus. The deviation increased with increasing testing temperature and with decreasing grain size. The contribution of grain boundary sliding to the total creep strain ranged from about 10 to 30% when crept at temperatures above 773 K, and the mechanism of creep deformation occurring in this temperature range was discussed.
