PSI - Issue 23

Tomáš Babinský et al. / Procedia Structural Integrity 23 (2019) 523–528 Babinský & Pol ák / Structural Integrity Procedia 00 (2019) 000 – 000

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temperature environment. The need for high operating temperatures is driven by an effort to increase the performance and to lower the fuel consumption in jet engines. (Reed (2006), Donachie M. and Donachie S. (2002)) The highest resistance to external loads is exhibited by precipitation-strengthened superalloys. The structure usually consists of γ matrix with coherent Ni 3 Al-type γ’ precipitates. Depending on the chemical composition, superalloys are used either in cast or wrought form. In the present work, examples of both cast (Inconel 713LC) and wrought (Rene 41) alloys have been investigated. A study of mechanisms in Rene 41 is of great interest as very little about its low-cycle fatigue response is known. 2. Experimental Inconel 713LC ( “ LC ” stands for “low carbon”) was provided in as-cast state by PBS Velká Bíteš, Rene 41 was provided in wrought state (water-quenched) by VDMMetals USA. Both alloys were supplied in the form of cylindrical rods of 200 mm in diameter. The chemical composition of both alloys is shown in Table 1. Specimens were cylindrical with axis parallel to the rod. The gauge length and the diameter was 12 mm and 6 mm respectively. Specimens were mechanically grinded and electrolytically polished. Mechanical tests were performed in computer-controlled electrohydraulic MTS system and were controlled with total strain amplitude 0.8 %. Specimens were subjected to fully-reversed cyclic straining (R = -1) at constant strain rate 2∙10 -3 s -1 at room temperature. In order to compare hysteresis loop shapes of specimens cycled under various conditions, 2000 data points were recorded in each cycle. Recorded hysteresis half-loops were transferred into relative coordinates, digitally smoothed and differentiated. First and second derivatives of both tensile and compressive half loops were calculated. Internal microstructure was observed using TESCAN Lyra3 scanning electron microscope (SEM). After the cycling was finished, fracture surfaces and surface reliefs were observed using SEM. Table 1. Chemical composition of 713LC and Rene 41 superalloys in wt. %. (Donachie M. and Donachie S. (2002)) Alloy Cr Ni Co Mo Ti Ta Al Fe Zr C B IN713LC 12 75 - 4,5 0,6 4 6 - 0,1 0,05 0,01 Rene 41 19 55 11 10 3,1 - 1,5 <0,3 - 0,09 0,01 Recorded data were analyzed using the statistical theory of the hysteresis loop. According to Masing (1923), a bulk material is composed of microvolumes with different yield stresses. Yield stress can be separated into two components: internal stress σ i and effective stress σ e , which have an additive effect = + . ( 1 ) The internal stress component is determined by long-range stress fields from an internal structure of a material, e.g. dislocation density and dislocation arrangement. Internal stress opposes the external stress, its value increases linearly until the internal critical stress is reached. During subsequent loading the value of internal stress remains constant and the dislocation can move further. The effective stress component arises from overcoming local obstacles such as P-N stresses. Effective stress accelerates a moving dislocation and it can be overcome by thermal activation. (Polák and Klesnil (1982), Polák (1991)) In addition to the Masing (1923) hypothesis, Polák and Klesnil (1982) defined the state of the material by the probability density function (PDF) of internal critical stresses f(σ ic ) , which describes a distribution of microvolumes of certain internal critical stresses. The macroscopic value of internal critical stress can be calculated by integration over all microvolumes. According to quasi-elastic approximation, effective stress increases linearly until the saturated effective stress σ es is reached. The slope of the σ e – ε dependency is determined by effective Young’s modulus E eff 3. Analysis of the hysteresis loop