Challenges in Life Prediction of Gas Turbine Critical Components

Peer reviewed: YesNRC publication: Ye

Brazing and Wide Gap Repair of X-40 Using Ni-Base Alloys

NRC publication: Ye

Challenges in Life Prediction of Gas Turbine Critical Components

NRC publication: Ye

Analysis of Semi-Elliptical Cracks at a Notch Root Using Phenomenological and Crystallographic Constitutive Models for Single-Crystal Superalloys - ABSTRACT ONLY

NRC publication: Ye

Fatigue properties of narrow and wide gap braze repaired joints

With the increasing utilization of braze repair in the gas turbine industry, the properties of braze joints under simulated service conditions become vital in selecting braze repair over other processes. While braze repair has often been claimed to deliver mechanical properties equivalent to that of the parent material, this is largely based on the results of tensile or accelerated creep tests for most gas turbine hot section components failure occurs as a result of thermal fatigue or thermomechanical fatigue. The damage that occurs under such conditions cannot be assessed from tensile or creep testing. This study was undertaken to characterize the fatigue properties of narrow and wide gap brazed X-40 cobalt-based superalloy and compare these properties to that of the X-40 parent material. Butt joint narrow gap and wide gap specimens were vacuum brazed using BNi-9 braze alloy. X-40 and IN-738 were used as additive materials in wide gap braze joints. To characterize the fatigue properties of the braze joints and parent material, isothermal fatigue tests were conducted at 950\ub0C and under load control using a fully reversed sinusoidal wave form having stress amplitude of 75% of the yield strength of the parent material. The braze specimens were fatigue tested in the as-brazed condition. The fatigue test results showed that the fatigue lives of the brazed specimens were lower than that of the parent material, particularly for the narrow gap samples and wide gap samples containing IN-738 additive alloy. All fatigue failures in the brazed samples occurred in the braze joints. An analysis of the fracture surfaces using a scanning electron microscope revealed that porosity was the major contributing factor to fatigue failures in the wide gap braze joints. The testing life debit observed in the narrow gap braze samples can be attributed to the presence of brittle boride phases in the braze joint. This study also included examination of techniques for reducing the aforementioned porosity and presence of brittle intermetallic phases. \ua9 2011 American Society of Mechanical Engineers.Peer reviewed: YesNRC publication: Ye

Braze Repair of Cobalt Based Superalloy X-40

NRC publication: Ye

Cyclic dwell fatigue behaviour of single crystal Ni-base superalloys with/without rhenium

In this study, compressive dwell (C-D) and no-dwell (N-D) low-cycle fatigue (LCF) behaviours of several single crystal Ni-base superalloys, including CMSX-4, LSC-11 and LSC-15, were studied under strain-controlled zero-compression (RE = -co) loading at 1100\ub0C. LSC-11 and LSC-15 are new alloys developed by 1H1 Corporation, Japan with 0.8 wt% Re and without Re addition, respectively, as reduced-cost alternatives to the second generation single crystal Ni-base superalloys. The fatigue experiments were conducted with or without a two-minute dwell (hold) in compression and total strain ranges of 0.7%, 0.6% and 0.5% on uncoated specimens in the [001] orientation. Examination of the cyclic stress-strain behavior revealed that the initially compressive mean stress relaxed to approximately zero stress in N-D tests, while compressive hold resulted in the development of a tensile mean stress during C-D fatigue. Cyclic stress softening was observed under all test conditions. Microstructural analysis of tested specimens showed that N-D fatigue promoted isotropic coarsening of the \u3b3\u2032 precipitates, while C-D loading resulted in the formation of discontinuous \u3b3\u2032 rafting parallel to the loading direction. Fatigue cracks initiated from the specimen surface from regions of localized oxide attack. All alloys were compressive dwell sensitive. C-D fatigue lives were 4-15 7 shorter than N-D when the same alloys were considered. CMSX-4 exhibited 1.5-3 7 N-D fatigue life advantage over alloys LSC-11 and LSC-15. Under C-D fatigue the life advantage of CMSX-4 was 20-50% greater than alloys LSC-11 and LSC-15. The differences in these behaviours could be attributed to Re content and oxidation.Peer reviewed: YesNRC publication: Ye

Numerical Analysis of Semi-Elliptical Cracks in Single-Crystal Superalloy Specimens - ABSTRACT

Peer reviewed: YesNRC publication: Ye

Isothermal and Thermomechanical Fatigue Behaviour of IN738LC

The isothermal low-cycle fatigue (IT-LCF) and the in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) life of IN738LC nickel-base superalloy was investigated in the temperature range of 750\ub0C to 950\ub0C. The experimental results indicate that fatigue life depends on strain-temperature phasing and mechanical strain range. In-phase TMF tests exhibited lives that were similar to the strain-life data obtained from IT-LCF tests conducted at 950\ub0C. A crossover of IP and OP-TMF strain-life curves, where OP-TMF resulted in the longest life at high-strain ranges and shortest life at low-strain ranges, was also observed. Under all test conditions fatigue cracks nucleated at surface connected grain boundaries and propagated along grain boundaries. Extensive creep cavitation and intergranular damage were observed in IP-TMF test specimens. A higher density of surface-connected fatigue cracks was observed for IT-LCF and IP-TMF loading conditions than for OP-TMF.Peer reviewed: YesNRC publication: Ye

Cyclic Dwell Fatigue Behaviour of Single Crystal Ni-Base Superalloys with/without Rhenium

In this study, compressive dwell (C-D) and no-dwell (N-D) lowcycle fatigue (LCF) behaviours of several single crystal Ni-base superalloys, including CMSX-4, LSC-11 and LSC-15, were studied under strain-controlled zero-compression (R, = -co) loading at 1100\ub0C. LSC-11 and LSC-15 are new alloys developed by IHI Corporation, Japan with 0.8 wt% Re and without Re addition, respectively, as reduced-cost alternatives to the second generation single crystal Ni-base superalloys. The fatigue experiments were conducted with or without a two-minute dwell (hold) in compression and total strain ranges of 0.7%, 0.6% and 0.5% on uncoated specimens in the [001] orientation. Examination of the cyclic stress-strain behavior revealed that the initially compressive mean stress relaxed to approximately zero stress in N-D tests, while compressive hold resulted in the development of a tensile mean stress during C-D fatigue. Cyclic stress softening was observed under all test conditions. Microstructural analysis of tested specimens showed that N-D fatigue promoted isotropic coarsening of the \u3b3' precipitates, while C-D loading resulted in the formation of discontinuous \u3b3' rafting parallel to the loading direction. Fatigue cracks initiated from the specimen surface from regions of localized oxide attack. All alloys were compressive dwell sensitive. C-D fatigue lives were 4-15x shorter than N-D when the same alloys were considered. CMSX-4 exhibited 1.5-3* N-D fatigue life advantage over alloys LSC-11 and LSC-15. Under C-D fatigue the life advantage of CMSX-4 was 20-50% greater than alloys LSC-11 and LSC-15. The differences in these behaviours could be attributed to Re content and oxidation. \ua9 2012 The Minerals, Metals, & Materials Society. All rights reserved.Peer reviewed: YesNRC publication: Ye