© 2013 Siemens Energy, Inc. All rights reserved. longitudinal and transverse directions are interested and investigated. However, the diagonal direction is very often used as loading components during actual service; therefore more studies concerning this particular direction should be done. The current DS material is a newly cast and wrought nickel-base directionally solidified (DS) superalloy, to meet the requirement for development of more efficient engine operation with higher operating temperatures and stresses. In the current paper, FCG behaviors of DS material have been evaluated at T0 and T0+250 0C, and in longitudinal, transverse and diagonal orientations. The effects of temperatures and grain orientations on the FCG behaviors are investigated in associated with fractographic analysis (For confidential, some of the test data are normalized). 2. Material and experimental procedure The nominal chemical compositions of the material are listed in Table 1. The original slabs were directionally solidified to produce a longitudinal direction which corresponds to [001] crystal orientation. The directionally solidified direction along the axis of the bar was determined to be within 50 of dispersion using the X-ray diffraction technique. Table 1. Nominal compositions of the current DS material (wt.%) Al B C Cr Mo Ta Ti W Ni 3.40 0.0125 0.08 11.6 1.65 4.80 3.90 3.50 Bal. FCG tests were conducted by using the direct-current electric potential drop method (DC-PDM) described in ASTM E647. Standard compact-tension (CT) specimens (48*50*10) were machined using electron-discharge machining techniques from the heat treated slabs. Prior to standard FCG tests, all specimens were pre-cracked at room temperature with a frequency of 10 Hz to eliminate the effect of machining notches. Lengths of all pre cracks were in a range of 2-3 mm. FCG tests were conducted in accordance with the requirements of ASTM E647 [7]. All tests were running loading control by a computer-control, closed-loop servo-hydraulic testing machine, with a fixed load ratio of 0.1 and frequency of 10 Hz. At both temperatures, two repeated specimens were tested in each grain orientation, one with standard FCG tests with constant loading, the other with threshold testing by decreasing loading, whose derating ratio is 0.1. The crack length data in region II of each specimen, with or without threshold value test, are computed by two methods, i.e., the secant method (also known as point-to-point technique) as well as the seven-point incremental polynomial method, as described in [7]. Conventionally, the secant method gives a result that most reflects the actual conditions, such as crack growth rate change, fluctuation and so on, while the polynomial method produces a more smooth da/dN curve. In the subsequent analysis, results from the seven-point polynomial method will be used to compare the temperature as well as orientation effects. For the standard C(T) specimens, the stress intensity factor ranges were calculated by the following equation as:
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