Creep Mechanisms of a Ni-Co-Based-Wrought Superalloy with Low Stacking Fault Energy
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INTRODUCTION
NI-BASED superalloys have been widely used as turbine blades and disks in aircraft engines and industrial gas engines for their excellent high-temperature mechanical properties, such as good tensile and creep properties.[1–4] With the development of the engines, the creep resistance of the disks at elevated temperatures is a major concern for the engineers to design new disk alloys. In order to achieve good high temperature creep performance of modern engines, many turbine disk alloys such as Rene 95, RR1000, and ME3 were developed.[5–7] Because these alloys were designed by increasing the volume fraction of c¢ phase and the levels of refractory elements contents, they could only be produced via the powder metallurgy (P/M) process route. However, the high cost of making clean powder and thermo-mechanical processing steps during P/M route leads to difficulties in broad applications of these alloys. Hence, the advanced Ni-Co-based superalloys known as TMW alloys for disk applications have been developed via the normal cast-and-wrought (C&W) route.[8–10] The newly developed TMW alloys were designed based on combining the good characteristics of Ni- and Co-based alloys, where Ni-based alloys provide basic mechanical properties for disk applications and Co-based alloys play an important role in improving the high-temperature mechanical performance and widening the processing window of the TMW alloys. Compared to the commercial U720Li alloy, TMW alloys have a higher Ti and Co contents. Ti would substitute Al in c¢ (Ni3Al) and promote the formation of Ni3(Al, Ti) acting as the anti-phase boundary (APB) strengthening.[11] To date, TMW alloys perform better creep properties than U720Li alloy at 998 K (725 C).[12,13] For example,
CHENGGANG TIAN and LING XU, Doctors, and CHUANYONG CUI and XIAOFENG SUN, Professors, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] Manuscript submitted November 15, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
TMW-4M3 alloy obtained about a six times creep life compared to U720Li alloy at 998 K (725 C)/630 MPa. The improvement in creep life was attributed to the different creep mechanisms dominated in U720Li alloy and TMW-4M3 alloy at 998 K (725 C)/630 MPa:[14–16] Orowan looping process, which combines dislocation slip and climb, and partial dislocations shearing precipitates were the main creep mechanisms for U720Li alloy. On the other hand, the addition of Co content in TMW-4M3 alloy could decrease the SFE, resulting in the deformation microtwinning process, which caused its enhanced creep resistance compared to U720Li alloy under the same creep condition. Generally speaking, three main processes may occur between dislocations and c¢ precipitates in Ni-based superalloy: (1) Orowan looping, (2) thermally activated dislocation climbing, and (3) shearing. Depending on which process is activated during creep deformation depends on the temperature and the loading stress. However, previous studies ma
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