PSI - Issue 23

Ivo Šulák et al. / Procedia Structural Integrity 23 (2019) 161–166 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

The phenomenon of fatigue crack initiation from a smooth surface in pure single- and polycrystalline FCC metals has strongly attracted researchers ’ attention for decades with the aim to clarify the mechanism of a surface roughening, its evolution in time and more importantly, its relationship with fatigue crack initiation (Mughrabi et al., 1983; Polák, 1998; Šulák and Obrtlík, 2017a, 2017b) . Distinctive surface slip markings, called also persistent slip markings (PSMs), arising on the surface of cyclically loaded materials are the first indicator of starting fatigue damage, namely the localization of plastic deformation into persistent slip bands (PSBs) within the bulk material. An irreversible motion of dislocations within areas of intensive cyclic slip leads to a formation of pronounced surface relief consisting of local elevations and depressions, known as extrusions and intrusions, respectively, at places where PSBs intersect the originally flat surface. For more details on the PBSs see also the review of Buque (Buque, 2001). It is generally accepted that fatigue cracks initiate at PSMs (Polák et al., 2016) . In this work, the results of the study of surface relief formation in individual grains of polycrystalline nickel-based superalloy MAR-M247 fatigued with a strain range of 1% at room temperature are presented. A quantitative and qualitative data on the PSMs morphology obtained using atomic force microscopy (AFM) are compared with results acquired by a novel microscopic technique – correlative probe and electron microscopy (CPEM) which combine the advantages of conventional AFM with high resolution scanning electron microscopy (SEM). Dislocation arrangement in an underlying structure was documented by means of transmission electron microscopy (TEM). The material used in this investigation was nickel-based superalloy MAR-M247 with the following chemical composition (in wt.%): 0.15 C, 8.37 Cr, 9.91 Co, 9.92 W, 0.67 Mo, 5.42 Al, 1.01 Ti, 3.05 Ta, 0.04 Fe, 0.015 Br, 1.5 Hf and Ni bal. The microstructure of MAR-M247 is characterized by dendritic grains with an average grain size of 2.1 mm. It consists of the face-centered cubic (fcc) γ matrix and tri-modal γ  precipitates. For details concerning the microstructure of MAR-M247 see (Šulák et al., 2016) . The superalloy was supplied by PBS Velká Bíteš, a.s. , as a cast cylindrical rods of 22 mm in diameter. The rod was machined to the dimensions of the cylindrical test specimen with a gauge length and diameter of 15 mm and 6 mm, respectively. Prior to the fatigue test, the gauge part of the specimen was carefully mechanically and subsequently electrochemically polished. For easier orientation on the specimen surface, fine circular marks were engraved on the central part of the cylindrical specimen. Interrupted low cycle fatigue (LCF) test was carried out in a symmetrical push-pull cycle ( R ε = – 1) under total strain control mode with a constant str ain rate of 2×10 -3 ×s -1 at room temperature in air. For microscopic surface observations, the cyclic straining in a total strain range of 1% was repeatedly interrupted at given numbers of cycles corresponding to 1%, 2%, 5%, 10%, 20%, 40%, and 80% of the fatigue life. The nature of early fatigue damage, crack initiation and subsequent fatigue crack propagation was observed by AFM Accurex IIL (Topometrix) in contact mode and completed with a correlative probe and electron microscopy (CPEM) developed by NenoVision s.r.o. This unique combination – AFM in tapping imaging mode placed in the chamber of high resolution dual beam SEM Lyra3 XMU from Tescan Co. – enables advantageously parallel imaging of the same object by two different techniques. SEM was equipped with a focused ion beam (FIB) and the profiles of intrusions and early crack propagation paths were exposed by producing FIB cuts across PSMs as described in detail in work of Polák et al. (Polák et al., 2017) . 3. Results and discussion To reveal the nature of fatigue crack nucleation and early crack propagation mechanism and their relationship to PSMs the fatigue test was periodically interrupted and a specimen was removed from the testing system for systematic microscopic observations. Fig. 1 shows details of PSMs appearance for a different number of cycles as revealed by SEM and AFM. The first interruption of the test was performed at 10 cycles (1% of N f ) and increased attention has been paid to the surface relief formation which has been proven to be an essential feature in fatigue 2. Experimental