Experimental Study of High Temperature Fracture Behavior of A286 Superalloy at 650°C

In this paper an experimental work is done for investigation of high temperature fracture properties of A286 superalloy at 650°C. Stress intensity factor K and parameter C* for this superalloy are determined experimentally. For estimation of these parameters, an instrument is developed for investigation of high temperature fracture properties. For estimation of stress intensity factor, compliance method is used. For this purpose four different compact tension specimens are tested and the parameter K is estimated. Creep tests are done for the selected specimens and parameter C* is determined by semiempirical relationships at 650°C. In these tests it is concluded that the specimens are placed near the plane stress condition. Crack growth behavior of this alloy is also studied. High incubation time (600 h) leads to overaging and therefore this alloy after this time showed very ductile creep properties, and fast creep crack growth was the major result of this overaging phenomenon. Finally the obtained results are compared with well-known nonexperimental methods for determination of these parameters. The obtained results showed that the results are in good agreement with each other.


INTRODUCTION
A286 alloy is an iron-base superalloy used in gas turbine jet engines, superchargers and also in various applications at moderately elevated temperatures such as turbine wheels and blades, frames, casings, afterburner parts and fasteners. This alloy is strengthened by the ordered fcc γ′-Ni 3 (Al, Ti) precipitates that are coherent with the austenite matrix and are formed during ageing at 730°C. This alloy exhibits good mechanical properties, i.e., flow stress, at service conditions as well as at room temperature, and also good corrosion resistance due to the elevated chromium content. This alloy, however, is unstable after short ageing treatments at 730°C because the metastable γ′ dissolves, and the stable η phase (hcp Ni 3 Ti) forms, which degrades the good mechanical performance of the material. Thus, A286 alloy must be used at temperatures below that critical temperature to prevent from overaging process [1,2].
The early attempts by Landes and Begley [3] to correlate da/dt with * C under creep conditions met only with limited success for creep resistant A286 alloy. In [46] it is showed that generally the net section stress approach may be used for creep ductile materials and specifically was proved to be suitable for alloy GH2132 (equivalent to A286) [7]. The crack growth behavior of an iron based superalloy GH2132 (equivalent to A286) under creep-fatigue conditions has been studied and the three stages I, II and III of crack growth rate curve da/dtσ n have been analyzed in [8]. In [2] the precipitation-hardened alloy A286 has been characterized as a function of ageing treatment, and the creep behavior has been studied in the temperature range of 600700°C and at 230740 MPa. The mechanism of η phase precipitation in A286 superalloy during heat treatment is studied in [9]. Study of influence of γ′ and η phases on corrosion behavior of A286 superalloy by using electrochemical potentiokinetic techniques is done in [10]. Potentiodynamic study of the influence of γ′ and η phases on pitting corrosion of A286 superalloy is performed in [11].
In this paper this alloy is investigated at 650°C and at this temperature different tests are performed. The stress intensity factor is determined experimentally. The instrument developed here, originally is designed for creep testing of materials. In this work, hardware and software facilities of the machine are improved for high temperature fracture testing purposes. The original creep testing machine is AMSLER DSM 6100-. made in Zwick company. This machine has the capability of applying force up to 50 kN.
The first work is preparation of compact tension (CT) specimens with proper dimensions for this testing machine. The parametric dimensions of CT specimen according to ASTM E1457 [12] are shown in .ig. 1. Here, actual dimensions are based on limitations in the furnace and gage length of the creep testing machine.
According to the parametric dimensions shown in .ig. 1, actual CT specimens are manufactured using electrodischarge machining (EDM) method from A286 material considering w = 20 mm, B = 10 mm and a = 10 mm as shown in .ig. 2. The width of EDM precrack is 0.1 mm [12].
The second hardware task is design and manufacturing of a proper fixture for gripping the CT specimens. This fixture is designed according to recommendations in ASTM E1457 [12] and limitations in the dimensions of the furnace and the gage length of the standard round bar specimen used for creep testing. The manufactured fixture is shown in .ig. 3.
The material used for the fixture is IN713LC which is a nickel-based superalloy with excellent creep properties. Because the fixture has proper rigidity and also it has enough creep resistance, the linear variable dif-ferential transformer (LVDT) measures the load-line displacement with a good accuracy.
The third hardware improvement is implementation of the direct current potential drop (DCPD) method for measurement of creep crack growth. As it can be seen in the .ig. 4, in this method a proper amplitude of current is sent to the test specimen and a fraction of the input signal is exited from the specimen into the A2D converter. .or this purpose a proper direct current source is used. This power source sends the current into the test specimen. This current source has output amplitude with 8.5 V in voltage and 35 A in current. This high current power supply is ideal for elimination of probable noises in the system. .or reading output voltage from the specimen, a four channel A2D converter with 20 bit resolution is used. This converter is connected to the computer by a RS232 cable and the data is transferred to the special software designed for this purpose. This converter with high resolution has the capability of detection of output voltages with the resolution of 1.4 µV if the input voltage is up to 600 mV.
The fourth hardware improvement is optical measurement of creep crack growth. This method is rarely used for measurement of crack length at high temperatures because it has complexities and difficulties. But using this method, the crack length can be visually monitored and is a good choice for materials that they do not have enough sensitivity to current induced by the DCPD method. Also using two methods simultaneously, the results obtained could be more reliable.
In the optical hardware system, a camera with special lens is used for online monitoring of crack length. Also a high temperature lamp is used which is placed at the bottom of the testing machine furnace for lighting the specimen in the furnace. A balancing light is also needed for compensating the light at low ambient light especially at night. The picture of the specimen can be seen on the monitor as it is shown in the .ig. 5. Since creep testing is a time consuming test, the high temperature lamp used for lighting the specimen could be damaged because it is exposed to high temperatures for long times. .or elimination of this shortcoming, a digital timer is used in the designed equipment. This timer lights the lamp every 5 min for 10 s and during this 10 s that the lamp is lighting the furnace, a picture is taken from the specimen and saved in the computer memory by the special software.

DETERMINATION O. STRESS INTENSITY .ACTOR
.or experimental estimation of stress intensity factor, compliance method is used. In this method variation of compliance with respect to crack size is measured and then the stress intensity factor can be determined.
Consider a cracked body as shown in .ig. 6 which is subjected to load P. The following equations are used for determination of stress intensity factor , V CP = (1) where P is the applied load, V is the load-line displacement, and C is the compliance.
It can be shown that [13] 2 d , where G is the energy release rate, B is the specimen thickness, and a is the crack size.
Considering the relation between G and K as following equation where K is the stress intensity factor and E is the Young modulus of the material. Combining Eq. (2) with Eq. (3), we find the following relationship between K and change in compliance with the crack size where ( ) . a w is the shape factor or the nondimensional stress intensity factor. In the above equation, the term CBE represents nondimensional compliance.
.or determination of stress intensity factor, four different CT specimens with different crack sizes are selected. The basic dimensions of the actual CT specimens are shown in Table 1.
As mentioned earlier, for determination of stress intensity factor, we need to determine compliance. According to Eq. (1), for determination of the compliance, the load-line displacement should be measured. In this work the LVDT, that used for measurement of axial elongation in simple creep test specimens, is employed. .or adaptation of the LVDT with the fixture assembly, the fixture is so designed that the gage  length is kept the same as for simple creep testing (100 mm).
Using this method, there is no need to any extra instrument for measurement of the load-line displacement. The test temperature is 650°C, at this temperature Young modulus of the material is 150 GPa.
In .ig. 7 the compliance diagrams for all specimens are shown, and in .ig. 8 the nondimensional compli-ance versus nondimensional crack length is presented. Using Eq. (4) the nondimensional stress intensity factors are computed and presented in Table 2.
.or verification of the obtained results, the following equations for determination of stress intensity factor for CT specimen are used [14]: In Table 2, a comparison between the results obtained by experimental procedure proposed here and the above equation is made. As it can be seen in Table 2, the differences between two methods are in good agreement with each other.

ESTIMATION O. THE PARAMETER C * .OR A STATIONARY CRACK
In this section four CT specimens with the same geometry and different applied loads shown in Table 3 are tested at various loading conditions. Using Eq. (8) that is a semi-empirical equation [12] for the measurement of the parameter * C for specimens with a w > 0.4, the parameter * C is estimated for these specimens by measuring the load-line displacement at steady-state condition. The test temperature is 650°C and the ma-  terial properties of A286 superalloy at this temperature is shown in Table 4, where E is the modulus of elasticity, ν is Poison ratio, 0 σ is yield strength of material, A and n are creep material properties according to Nortons creep law. The creep material properties of this alloy (A and n) are extracted from .ig. 9 at 649°C.
The following equation represents the Nortons creep law as constitutive equation for secondary creep regime: ss , n A ε = σ (7) where ss ε is steady-state strain rate and σ is the applied stress which can be found in the .ig. 9.
Using the following equation which is a semiempirical relationship, we can estimate the parameter * C by measurement of steady-state load-line deflection rate [12] * ss (2 0.522(1 )) , where P is the applied load and ss V is the steady-state load-line displacement rate which can be measured at steady-state condition as shown in .ig. 10. The geometry and loading conditions for the selected specimens are presented in Table 3.
During the test, the crack length is monitored by the DCPD and the optical method and it is observed that there was no change in crack length in all creep tests. The crack length is kept constant and hence the parameter * C in this condition is steady state. .or comparison of the obtained results with 2D estimation of the parameter * , C the following equation is used [16] A w a h a w n P The above equation is known as the GE/EPRI relationship, where 0 σ is the yield strength of the material and 0 P is the limit load which can be determined by the following equations [13]: Bw a = η − σ (10) for plane strain condition, Bw a = η − σ (11) for plane stress condition, 1 ( , ) h a w n can be obtained by finite element solutions and it depends on the stress state. This parameter is tabulated in plane strain and plane stress conditions.
The experimental parameter * C is determined using Eq. (8) by measuring the steady-state load-line displacement. Equation (9) is used for the comparison of experimental results with 2D numerical solutions. This comparison is shown in Table 5. Here we define the normalized parameter * C by the following relations: * * * ns exp pl stress . C C C = (14) Using the above equations the normalized parameter * C is calculated and presented in Table 6. As it can be seen in the Table 6, the parameter * C determined in this experiment is approximately 6 times the parameter * C in plane strain condition and 0.7 times the pa-   rameter * C in plane stress condition. It can be concluded that these specimens are placed near the plane stress condition.

INVESTIGATION O. CRACK GROWTH BEHAVIOR
In this section, crack growth behavior of A286 is investigated. .or this purpose, CT specimens made of this alloy are tested at 650°C and under different levels of loadings.
In terms of heat treatment of this alloy it can be said that A286 alloy is a heat and corrosion resistant austenitic iron-base material which can be age-hardened to a high strength level. The alloy is also used for low temperature applications requiring a ductile, nonmagnetic high strength material at temperatures ranging from above room temperature down to at least 196°C. The alloy may be used for moderate corrosion applications in aqueous solutions. A286 alloy is an agehardening heat resisting alloy which attains optimum strength properties by solution heat treatment followed by aging heat treatment. A286 alloy may be solution treated at either 899 or 982°C depending upon properties desired. The 899°C solution treatment results in a finer grain size and superior room and elevated temperature short time tensile properties. The 982°C solution treatment develops a slightly coarser grain size with superior creep and stress rupture properties. Large sections, such as plate, are generally oil quenched from the solution heat treatment. Thin sections, such as sheet or strip, may be air cooled from the solution heat treatment. An aging treatment of 718°C for 16 h followed by air cooling is conducted after either solution heat treatment. The aging treatment develops the high strength of A286 alloy [15].  In this work, the A286 alloy is selected in the condition of AMS 5732, i.e. solution heat treated at 982°C and aging treatment of 718°C for 16 h followed by air cooling after solution heat treatment. Under this condition, the important material properties of this alloy are shown in the .ig. 11 and Tables 7 and 8.
Here the CT specimens tested at 650°C with the following geometric and loading condition. The major part of time in this test is elapsed for crack extension (almost 600 h) and only 21 h of the test time is elapsed for crack propagation. Some other tests are also performed by applying other forces. In the case of forces lower than 3 kN no crack propagation occur and the specimens were fractured with no evidence in crack propagation. In the other hand, for specimens with loading condition higher than 3 kN, the crack propagation time was very small and finally fast fracture is observed at 15 kN. So at 3 kN the crack propagation was seen and traced. In .ig. 12 the crack propagation at different times after crack extension is shown. .ractography analysis results are shown in .igs. 13 and 14. As it can be seen, in the CCG region the intergrannular cracking is obviously observed.
As mentioned earlier, this alloy is an austenitic ironbase material and by age-hardening heat treatment this alloy is aged and the creep strength and creep ductility should be improved. But fast crack propagation implies that this is not true. .or investigation of this paradox, .ig. 15 is presented. Aging effect is shown on the microhardness of this alloy. At 730°C aging time, 16 h the material has good mechanical properties. But after 16 h, microhardness will decrease. This phenomenon is called overaging and causes the mechanical properties to decrease. .or 650°C overaging occurs almost at 170 h, in our test the incubation time is almost 600 h and is greater than this time.

CONCLUSIONS
Alloy A286 CP specimens are tested at 650°C and the stress intensity factor, the parameter * C and creep crack propagation are studied. .or this purpose an instrument is developed. Using this instrument, the stress intensity factors are determined using the compliance method. The obtained results were compared with the well-known relations for stress intensity factors. The parameter * C for four CT specimens with the same geometry and different loading condition is estimated. In these tests it is concluded that the specimens were located near the plane stress condition. .inally, crack propagation behavior is studied. High incubation time (almost 600 h) for this alloy leads to overaging process and so this alloy showed very ductile behavior at these condition. The crack extension time was limited to 21 h.