Theoretical and practical implications of creep curve shape analyses for 2124 and 2419

Abstract

Creep and creep fracture properties are presented for two commercial aluminum alloys, 2124 and 2419, tested at stresses giving creep lives up to about 1000 hours at 373 to 463 K. The results are analyzed using the methodology, which quantifies the shape of individual creep curves and the variations in curve shape with changing stress and temperature. The relationships are shown to allow reasonable prediction of 100,000-hour stress rupture values, while also explaining the complex creep behavior patterns widely observed with precipitation-hardened alloys.

Theoretical and practical implications of creep curve shape analyses for 8090

“…As reported for 2124 and 2419, [1] normal creep curves were recorded under all test conditions studied for 8090, with the curve shapes varying systematically with changing stress and temperature. The patterns of curve shape change are illustrated in Figure 1(a) by plotting the true creep strain () normalized through the total creep strain to failure ( f ) as a function of time (t) normalized through the time to fracture (t f ).…”

“…To allow direct comparisons to be made with the data obtained for 2124 and 2419, creep tests were again carried out using the constant truestress machines and procedures described previously. [1] However, as noted for 2124 and 2419, although high-precision equipment was employed, [3] the creep property values determined for 8090 showed significant scatter. For this reason, extended stress ranges were selected to give test durations from about 6 to over 6000 hours at 373 to 463 K, allowing unambiguous identification of the trends in creep and creep fracture properties with changing stress and temperature.…”

“…Although the curve shapes vary (Figures 1 and 2), the creep properties of metals and alloys are usually described only through the dependences of on stress () and temperature (T), generally using power-law equations of the form [1] where the values of the constant (A), the stress exponent (n), and the activation energy for creep (Q c in units of J mol Ϫ1 when the gas constant R ϭ 8.314 J K Ϫ1 mol Ϫ1 ) differ in different stress/temperature regimes. Adopting this approach and treating the log plots in Figure 3(a) as straight lines, n decreases from about 32 to 5 as the temperature increases from 373 to 463 K, with Q c increasing from around 140 to 220 kJ mol Ϫ1 as the applied stress is reduced.…”

“…On this basis, increases from ϳ1.5 toward 3 or more as the test durations increase at 427 K and above for 8090 ( Figure 6), with a similar trend for 2124 attributed to particle coarsening initiating the tertiary stage. [1] Hence, using a high-resolution transmission electron microscope (TEM), precipitate coarsening studies were undertaken for 8090. Table I lists the mean diameters of the spherical ␦Ј (Al 3 Li) precipitates distributed uniformly throughout the grains, as well as the average widths of the precipitate-free zones at grain boundaries ( Figure 7).…”

Analysis, representation, and prediction of creep transients in Class I alloys

“…However, non-steady creep behaviors hold a wealth of information which can often better reveal the fundamental mechanisms of creep deformation. Recent examples of exploiting non-steady data from full creep curves using the θ-projection method [35,36] are found in the work of Wilshire and colleagues [37,38]. In hot forming applications using AA5083 materials, such as QPF, loading conditions are also non-steady and, thus, are quite different from those of the creep tests to which the θ-projection method is typically applied.…”