Comparison of creep deformation rates during load and strain controlled multi-step creep ageing tests on AA7050

Abstract

In this work, the constant load and constant strain controlled creep behaviour of a precipitation hardenable aluminium alloy 7050 during a two-step ageing treatment have been experimentally studied. The two-step temperature treatment is strictly in accordance with the T74 ageing temperature profile (120 °C × 6 h+ 177 °C × 7 h) for 7050 alloys. It has been found that at 120 °C creep strain generation was extremely small, whilst at 177 °C significant creep strains were developed for both the load and strain controlled creep tests. The samples during the load controlled creep ageing tests reached the tertiary creep regime under a constant stress of 230 MPa within 14 h, while remained in the secondary creep regime under stresses ranging from 150 − 215 MPa for this time. The strain controlled creep ageing tests can prevent the samples from reaching their tertiary creep regimes through reducing the initial high stresses (up to 320 MPa) to a stress lower than 230 MPa during the tests. The creep strain rates obtained from strain controlled creep ageing tests are found smaller than the minimum creep strain rates from load controlled creep tests. The steady state stress exponent n was found ∼3.3 for load controlled creep ageing tests. Similar n values was obtained for strain controlled creep ageing tests when stress is larger than 200 MPa, while this value jumped to ∼15 when stresses reduced to < 200 MPa. All these findings suggest that load and strain controlled creep exhibit different behaviours and thus a proper calibration is required if using the constant load creep data to simulate the constant strain stress relaxation curves under the same ageing conditions.