Precipitate Behavior, Mechanical Properties and Corrosion Behavior of an Al-Zn-Mg-Cu Alloy during Non-Isothermal Creep Aging with Axial Tension Stress

Theprecipitate behavior, mechanical properties and corrosion behavior of an Al-Zn-Mg-Cu alloy during non-isothermal creep aging were investigated. The results show that diffraction patterns of GPI zones gradually disappear and those of η′ phases are strengthened during the heating stage. More importantly, the size and volume fraction of precipitates increase with aging temperature increasing, which greatly enhances the mechanical properties of the alloy. The hardness and tensile strength of the alloy with H210 aging condition are 165 HV and 564 MPa, respectively. During the cooling stage, in addition to the diffraction pattern of η′ phase, that of GPI zones can be observed again. Furthermore, the size of the precipitates decreases, and the volume fraction reaches a maximum. The hardness and tensile strength of the alloy with C120 aging condition reach 185 HV and 580 MPa, respectively. Furthermore, the characteristics of the grain boundary reveal that the width of precipitation free zones (PFZ) first increases during the heating stage and then decreases during the cooling stage. In the C120 condition, the newly generated secondary precipitates and the coarsening of undissolved precipitates around the grain boundary lead to the further narrowing of PFZ, but the coarse grain boundary precipitates (GBPs) are still not continuously distributed in the grain boundary. Hence, the alloy with C120 condition exhibits the most excellent corrosion resistance.

Characterizing the Influence of Cyclic Re-Priming on the Prediction of Long Term Creep Damage for Gas Turbine Components

Industrial Gas Turbines have traditionally been designed for base load operation. However, modern applications require flexibility as well as availability. These requirements lead to components in the hot section experiencing both cyclic and hold time loading, at high temperatures. These loading profiles inevitably lead to damage from both fatigue, due to engine cycles, and creep, due to dwells at load, as well as the interaction of these two damage mechanisms over the duration of the service interval. It is these interactions which can lead to higher average creep rates and more damage than expected. This paper presents the results of a study into the influence of cyclic loading on the average creep rate for a proprietary single crystal nickel based alloy, which is based on the chemistry of INCONEL® 792, a relatively high chromium, gamma/gamma-prime strengthened superalloy. Creep tests have been conducted isothermally with reload cycles of varying duration (or dwell) that result in an unexpected reduction in creep life and an increase in overall creep rate when compared to the continuous isothermal creep tests performed at constant stress without reloading. A hypothesis is presented which attributes the increase in overall creep rate to the influence of a recovery potential stress. The dwell period of each reload cycle is critical to calculating the recovery potential and subsequent creep rate. Results show that tests with relatively short dwell periods exhibited lower lives and higher creep rates than tests with fewer cycles and longer dwell periods. The implication of these findings are significant when considering actual operation, where variations in engine cycles and dwell periods, could influence the accumulation of creep damage. Therefore, an initial approach is presented which accounts for cycles as part of the creep damage calculation.