PSI - Issue 23

M.A. Artamonov et al. / Procedia Structural Integrity 23 (2019) 257–262 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

258

2

Introduction

The powder heat-resistant Ni-based alloys [1] are often used for manufacturing of turbine disks of the gas turbine engines (GTE) and aero-derivative engines. The maximum operating temperatures of parts made of these materials reach 650 °C and higher. In case of crack in these parts, subject to operating conditions (stress level, loading mode, temperature), the fracture most likely occurs due to the low-cycle fatigue mechanism (LCF). Therefore, it is important to know the fatigue characteristics of the material and factors influencing on the development of fracture in the material. The researches in this area are quite extensive [2,3,4,5]. This information allows, to determine the number of the crack propagation cycles in the part before failure, and determine such an important characteristic, whitch ensures the economical and safe operation of the product, as the overhaul life. For the purpose of research, the cylindrical specimens were made of EP741NP superalloy and tested for LCF. After the LCF tests the fractographic analysis of fracture surfaces of the broken specimens was carried out. The purpose of this study was to determine the kinetics of the fatigue fracture development in specimens, which made it possible to identify the implemented durability of specimen. The knowledge of durability and lifetime, in its turn, made it possible to determine the period of the fatigue crack initiation in the material. The kinetics of the fatigue crack growth (FCG) was determined fractographically by measuring the pitch of fatigue striations. There are a lot of studies that demonstrate the correspondence of the fatigue striation size to the distance which the crack propagates in one loading cycle [6, 7, 8]. The kinetics of the fatigue crack propagation is characterized by the C and m coefficients of Paris equation related to the relationship between the FCG rate da/dN and magnitude of the stress intensity factor (SIF) ∆K for the second, stable stage of the fatigue crack propagation [9]. The material of specimens is the powder Ni-based superalloy EP741NP. The powder is obtained by the method of the vacuum-induction melting of metal with the subsequent spraying into granules of the spherical shape using the centrifugal disintegration of a fast-rotating casting (PREP). The size of granules of the superalloy, the samples are made, does not exceed 140 microns [10]. The specimens were cut from the high-pressure turbine disk. After the heat treatment, the microstructure of superalloy is the γ -Ni solid solution with the average grain size of ~ 40 µm, along the grain boundaries there are large particles of the strengthening intermetallic γ' - phase up to 3 µm in size. The billets for specimens were cut from the hub of HPT disk in the axial direction. The cylindrical specimens were used for testing. These specimens had the diameter of 4.37 mm and the gauge length of 13 mm. The surface of samples was ground with the subsequent polishing in axial direction to prevent the formation of transverse scratches along the circumference of sample. The specimens were tested with the use of the Instron 8862 series test machine in the wide range of temperature from 20 °C to 750 °C. Prior to testing, the Young’s modulus was measured at the room temperature and test temperatures T = 350, 450, 550, 650 and 750 °C. The exposure time of the system with installed specimen after reaching the specified temperature was 1 hour before the start of the test. The LCF tests were carried out in accordance with the state standard specification under strain control (fixed strain range). The parameters of loading during the tests of specimens: form of cycle – sinusoidal; loading frequency – 1 Hz; controlled variable during the loading – range of deformation in the cycle ∆ε=1; cycle asymmetry ratio R ε = ( ε min /ε max ) = 0 and 0.5. The fractographic analysis was carried out with the use of the JEOL JSM-IT300LV scanning electron microscope. During the fractographic analysis, the location of fracture origin and type of the fatigue failure of specimens were determined. In case of finding several origins, the origin to which the largest area was chosen for the subsequent analysis (Fig.1). The search was made for the fracture sites at which the formation of fatigue striations could be observed (Fig. 1c). The width of fatigue striations characterizes the FCG rate in one loading cycle at observed fracture site. The sites being investigated should be located along one straight line and corresponded to the Material under examination and experimental methods Methods of fractographic analysis