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Effect of high temperature deformation on the structure of Ni based superalloy

Nowotnik A.

Journal of Achievements in Materials and Manufacturing Engineering

2008

Vol. 27, nr 2

115-122

Purpose: A study on the hot deformation behaviour and dynamic structural processes (dynamic precipitation operating during deformation at elevated temperatures) of nickel based superalloy was presented. Design/methodology/approach: Compression tests were carried out on precipitations hardenable nickel based superalloy of Inconel 718 at constant true strain rates of 10-4, 4x10-4s-1 within a temperature range of 720-1150° C. True stress-true strain curves and microstructure analysis of hot deformed alloy were described. Microstructure examination has been carried out on the compressed samples of Inconel 718 alloy using an optical microscope - Nikon 300 and in the scanning electron microscope HITACHI S-3400 (SEM) in a conventional back-scattered electron mode on polished sections etched with Marble's solution. Findings: Structural observations of deformed at high temperatures, previously solution treated Inconel alloy revealed non uniform deformation effects. Distribution of molybdenum-rich carbides was found to be affected by localized flow within the investigated strain range at relatively low deformation temperatures 720 - 850° C. Microstructural examination of the alloy also shown that shear banding, cavities growth and intergranular cracks penetrating through the whole grains were responsible for decrease in the flow stress at temperature of 720, 800 and 850° C and a specimen fracture at larger strains. On the basis of received flow stress values activation energy of a high-temperature deformation process was estimated. Mathematical dependences (σ pl -T and σ pl - ) and compression data were used to determine material's constants. These constants allowed to derive a formula that describes the relationship between strain rate (ε), deformation temperature (T) and flow stress σ pl. Research limitations/implications: Even though, the light optical microstructure observation of deformed samples revealed some effects of heterogeneous distribution of the phase components, in order to complete and confirm obtained results it is recommended to perform further analysis of the alloy by using transmission electron microscopy technique (TEM). Practical implications: Interaction of precipitation process developed during deformation below solvus temperature and heterogenuos deformation (flow localization) can become a significant aspect of high temperature performance of precipitation hardenable alloys and may perhaps also allow produce specific microstructures of such deformed materials. Originality/value: It is a scarcity of data which are to describe specific features of phase transformation processes in precipitation hardenable alloys. In addition, existing data do not allow to simplify structural features of dynamic precipitation and simplifying structural description of the process. The compression tests on age hardenable alloys and the analysis of dynamic precipitation process have got a practical meaning.