13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- microstructure, instead of having the more common microstructure with γʹ′-cuboids in a γ-matrix [15, 16]. The aim of this study is to investigate the long-term stress relaxation behaviour of a Ni-based single-crystal superalloy. Due to anisotropy and the well-known tension/compression asymmetry, three different crystal orientations are tested in both tension and compression. 2. Experimental procedure The Ni-based single-crystal superalloy MD2 with chemical composition Ni-5.1Co-6.0Ta-8.0Cr-8.1W-5.0Al-1.3Ti-2.1Mo-0.1Hf-0.1Si in wt.%, was used in this study. Prior testing, the material was solution heat treated at 1275°C for 8 h followed by a two-stage aging process with 3 h at 1100°C and 24 h at 850°C. Test specimens were machined from cast bars and the deviation from the ideal orientation was less than 10° for all specimens. The tests were performed in an Instron servo-hydraulic TMF machine, and were conducted as strain controlled TMF cycles with a temperature range from 100°C to either 750°C or 950°C. During each cycle, a hold time of 100 h was applied at maximum temperature (Tmax) and maximum strain. After the 100 h hold time, the temperature was lowered to 100°C and zero strain before the second cycle was initiated. Each specimen was subjected to two TMF cycles, and no specimens were cycled to fracture. Consequently, a hold time of total 200 h at Tmax was applied for each specimen. Fig. 1 displays a plot of the TMF cycle. For a Tmax of 750°C, the 〈001〉, 〈011〉 and 〈111〉 directions were tested. However, for a Tmax of 950°C, only the 〈001〉 and 〈011〉 directions were tested. The total mechanical strain range (Δεmech) was chosen in order to obtain approximately the same inelastic strain for all specimens. Since the different crystal orientations have different Young’s modulus, different Δεmech were used for the different crystal directions. The 〈001〉 oriented specimens were subjected to a Δεmech of 1%, the 〈011〉 specimens to 0.7%, and the 〈111〉 specimens were subjected to 0.6%. For each direction and Tmax, one specimen was subjected to an in-phase TMF cycle (IP-TMF), and the other specimen was subjected to an out-of-phase TMF cycle (OP-TMF). An IP-TMF cycle implies tensile stress relaxation at Tmax while OP-TMF implies compressive stress relaxation at Tmax. Strain! Temperature! 100°C! 750°C or 950°C! 100h! 100h! 0%! ± 0.6, 0.7 or 1%! 5 min! Figure 1. The TMF cycle used in the stress relaxation tests.
RkJQdWJsaXNoZXIy MjM0NDE=