Assessment of Fatigue Resistance of Aluminide Layers on MAR 247 Nickel Super Alloy with Full-Field Optical Strain Measurements
This work presents the results of fatigue tests of MAR 247 alloy flat specimens with aluminides layers of 20 or 40 µm thickness obtained in CVD process. Fatigue test was conducted at amplitude equal to half of maximum load and ranging between 300 and 650 MPa (stress asymmetry ratio R = 0, frequency f = 20 Hz). Additionally, 4 of the tests, characterized by the highest amplitude, were accompanied with non-contact strain field measurements by means of electronic speckle pattern interferometry and digital image correlation. Results of these measurements allowed to localize the areas of deformation concentration identified as the damage points of the surface layer or advanced crack presence in core material. Identification and observation of the development of deformation in localization areas allowed to assess fatigue-related phenomena in both layer and substrate materials.
Keywordsaluminide layer fatigue testing full-field optical strain measurements super nickel alloy
The load analysis of the constituents of aviation engines such as turbine blades shows that material durability is dependent on crack initiation and development in thermo-mechanical fatigue, high cycle fatigue, low-cycle fatigue or creep conditions. Fatigue resistance of Ni alloys (such as CMSX, Rene, Inconel) is in the scope of interest of numerous research centers. Many authors analyzed influence of protecting layers on fatigue resistance (Ref 1-7). However, these studies were limited to determination of influence of the layer thickness on material/layer system durability. In some of them, the change in number of cycles to break of layer deposited on material in the mechanical notch location has been investigated (Ref 4-6). This approach enabled localization of the crack initiation area and its development observation; nonetheless, this strategy does not concern changes of the layer quality in a notch area and the real loading conditions are not fully replicated. Moreover, crack initiation and development observations require subsequent interruptions of fatigue tests (for example every 200,000 cycles) (Ref 4-6) for microscopic observations. This results in change of the loading conditions—due to the need of removing samples out of grips—and may have negative influence on the tests repeatability and accuracy, thus limiting its reliability. The usage of optical methods of strain field measurements gives an opportunity to observe whole samples’ gauge sections. They are fixed in loading device during testing, and this allows to determine the areas of crack localization occurring naturally. The most popular methods of strain field measurement are Electronic Speckle Pattern Interferometry (ESPI) and Digital Image Correlation (DIC). The main advantage of ESPI method is its high displacement measurement resolution (about 10−6). However, the method is vibrations sensitive and it enables resolving only a limited amount of displacement between consecutive loading steps, inducing the requirement for stopping tests and conducing measurements under the static load. On the other hand, DIC measurements are much less sensitive to vibrations at the expense of lower displacement measurement accuracy.
In this paper application of both methods is presented, especially considering their performance in fatigue tests of MAR 247 alloy with aluminides layers of 20 and 40 µm thickness.
Samples of MAR 247 alloy were produced in casting process of uniform crystallization performed in ceramic molds.
Fatigue tests were made for two series of MAR 247 alloy samples with coarse-grained core and with 20 or 40 µm aluminides layers thickness. Loading cycles had sinusoidal shape, and the maximum stress was in the range of 300-650 MPa, loading frequency f = 20 Hz and cycle asymmetry ratio R = 0. The samples geometry, shown in Fig. 2, was chosen to enable both DIC and ESPI measurements for a whole gauge section area.
Fatigue tests were made by using universal hydraulic MTS 810 testing stand with ±100 kN loading range, equipped with TestStar II computerized controller. In case of ESPI-assisted tests, the loading procedure included the stages of stepped loading for a part of samples by means of manual loading system. This approach was necessary due to ESPI vibrations sensitivity excluding the use of hydraulic feed system. In the case of DIC assisted tests, the cyclic loading processes have not been interrupted. Fractures after fatigue tests were examined by means of Scanning Electron Microscopy (SEM).
Fatigue Tests with DIC
DIC is a method of non-contact strain/displacement measurement based on digital images analyses developed in the 80 s (Ref 8). DIC measurements require the presence of speckle pattern on the surface of observed objects, in the case of presented investigation made by applying firstly white suspension mixture for penetrative testing followed by a graphite paint spraying.
Fatigue tests with 2D DIC measurements were conducted with relatively high stress range (600 MPa) and with frequency of 20 Hz. Loading amplitude range was selected to ensure finishing the tests in relatively short time of few hours. Trigger of the camera (AVT PIKE F-505C) used for DIC measurement images acquisition was synchronized with the maximum load occurrence in the cycles of fatigue tests. Every 100th image (snapped every 5th second) was registered on the hard disk for minimizing image storage space and enhancing strain maps processing time. Vic2d (Correlated Solutions, USA) software has been used for strain distribution maps processing.
Fatigue Tests with ESPI
ESPI strain field measurements are based on the analysis of the phase changes of the light wave by using interference fringes images resulted from deflection of appropriate oriented monochromatic light beams reflected from specially prepared object surface (Ref 9-13).
A series of 5-12 strain field maps have been obtained for each sample with coating, and these maps have been used for observations of damage initiation and development.
Eddy current measurements by means of Nortec device 600 Olympus have been done for one sample tested with ESPI assistance for which crack presence was revealed in the advanced stage. The results of this test allow to confirm the location of flaws generated during fatigue testing delivered from ESPI.
Results and Discussion
Results of high cycle fatigue tests of investigated samples
No. of specimen
Maximum stress, MPa
80,000 (test terminated before sample break)
DIC-based strain field measurements were relatively easy to implement in the fatigue strength investigation of samples with aluminides coatings of different thickness. However, limited strain measurement resolution allowed only for the detection of damages at the last stage of crack development, especially for sample with thinner layer. The improvement of measurements resolution would require limiting of the area of observation at the expense of resigning from the observation of the whole sample surface or using a camera with sensor of larger resolution.
Metallographic inspection of places predetermined by ESPI measurements revealed change in crack direction after crossing layer/core material interface. In aluminides layers, cracks propagated perpendicularly to sample surface, while in core material cracks were deflected by approximately 45° (sometimes with the change of deflection direction to −45°) to the initial direction. This crack behavior may explain considerably early detection of strain localization areas (especially in case of ESPI measurements). This was caused by the emergence of the largest layer discontinuities oriented perpendicularly to the surface and followed by the period of small changes in strain maps related with deflected crack propagation in the core material.
High-quality aluminides layers on MAR 247 alloy have been obtained in CVD process. Fatigue strength of samples with layers of 20 and 40 µm thickness was examined.
The results of fatigue tests showed slightly better performance of samples with 20 µm layers. It might be explained by the much faster strain localization in samples coated with 40 µm layers revealed by both, DIC and ESPI measurements. Nonetheless, for the room temperature fatigue conditions the most crucial aspect for the investigated material/layer system seems to be the initiation and propagation of the crack in core material.
The analysis of strain field maps obtained from DIC and ESPI measurements during fatigue tests of samples with aluminides layers on nickel alloy allowed investigation of the influence of coating thickness and stress level based on the identification of the strain localization and its development during tests. The areas of initial localization of strain showed where cracking of coating commenced because of the supposed structural notches or origins related to surface roughness. Further increase of registered strain values and it localization in smaller areas showed the last stage of deflected cracks development in core material.
Each of the analyzed non-contact strain measurement techniques requires different testing procedures. They also deliver measurements results with different level of resolution. In cases of investigations requiring lower resolution, more vibration tolerant DIC method might be applied without the need of pausing fatigue tests. On the other hand, when higher resolution of measurements is required ESPI might detect smaller changes on observed surfaces, but more complicated testing procedure has to be applied. Nonetheless, application of any optical strain field measurements in fatigue tests of modern coated materials leads to an enhancement of the understanding of their degradation mechanisms.
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