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1

Mechanical and structural aspects of high temperature deformation in Ni alloy

Nowotnik A.

Journal of Achievements in Materials and Manufacturing Engineering

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2008

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Vol. 26, nr 2

143-146

EN

Purpose: Experimental results on hot deformation and dynamic structural processes of nickel based alloy were reviewed. The attention was given to the analysis of dynamic structural processes which operate during hot deformation of the material. Design/methodology/approach: Hot compression tests were performed on solution treated precipitations hardenable nickel based superalloy of Inconel 718 within a temperature range of 720-1150*C at constant true strain rates of 10 -4, 4x10 -4 s -1. The flow stress curves and microstructure of deformed nickel based superalloy were presented. Findings: During hot compression of solution treated material, highly localized flow was observed at relatively low deformation temperatures 720-850*C. The particle distribution and their morphology were not found to be affected by localized flow within the investigated strain range. At low strain rate the shear banding and intergranular cracks and cavities growth were found to be responsible for the observed flow stress decrease at 720, 800 and 850oC and might result in a sample fracture at larger strains. Research limitations/implications: In spite of intense strain hardening due to deformation and phase transformation overlapping, light optical microstructure observation of deformed samples did not reveal significant effects of heterogeneous distribution of the phase components. Therefore, 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: An interaction between dynamic precipitation and flow localization may become an important feature of high temperature performance and may also allow producing specific structures of materials. Originality/value: The contribution of flow localization to the strain hardening or flow softening and the flow stress-strain behavior during hot deformation of precipitation hardenable alloys is still a subject of extensive researches. The interaction between the flow localization and dynamic precipitation process was a subject of a very limited research works only.

2

Identification of dynamically precipitated phases in hot-working Inconel 718 alloy

Nowotnik A., Sieniawski J., Mrówka-Nowotnik G.

Journal of Achievements in Materials and Manufacturing Engineering

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2008

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Vol. 31, nr 2

275-280

EN

Purpose: The main purpose of this paper was to analyze how localized flow/structural inhomogeneties that may develop during hot deformation can affect a precipitated phases and to determine what kind of particles are present in the microstructure of hot-worked Inconel 718 superalloy. 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 and 4x10 -4 s -1 at temperature of 720 and 850° C. The dynamic behaviour were explained through observation of the microstructure using standard optical, scanning and transmission electron microscopy. Precipitated phases were identified using EDS technique and based on selected area diffraction pattern. Findings: Microstructural observations of deformed at high temperatures, previously solution treated Inconel superalloy revealed non uniform deformation effects. Distribution of molybdenum- and niobium-rich carbides affected by localized flow was found. Microstructural examination of the alloy also shown shear banding penetrating through the whole grains. Practical implications: The experiments on hot deformation of age hardenable Inconel 718 superalloy and the analysis of dynamic precipitation process have a practical aspect. This interaction could become an important feature of high temperature performance and may also influence the production of specific structures of this material. Originality/value: Even though the number of research has focused on the hot deformation behaviour of Inconel 718, there is still scarcity of data referring to the analysis of dynamic structural processes which operate during hot deformation of this precipitation hardenable alloy: in particular dynamic precipitation and dynamic particles coarsening.

3

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

Nowotnik A.

Journal of Achievements in Materials and Manufacturing Engineering

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2008

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Vol. 27, nr 2

115-122

EN

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.

4

Wpływ parametrów odkształcania na procesy wydzieleniowew nadstopie niklu typu Inconel

Nowotnik A., Kubiak K.

Hutnik, Wiadomości Hutnicze

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2008

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Vol. 75, nr 8

432-434

PL

Na podstawie wyników badań określono zmiany mikrostruktury przesyconego stopu Inconel 718 odkształconego z prędkością odkształcania 10~4 i 4x104 s1 w zakresie temperatury wydzielania faz umacniających (fazy y" i węglików). Stwierdzono, że podczas odkształcania w zakresie temperatury 72CH850 °C występuje lokalizacja odkształcenia powodującą niejednorodny rozkład wydzielających się w czasie odkształcania węglików Mo i Nb, sprzyjająca pękaniu i niszczeniu stopu. Średnia wartość energii aktywacji Q procesu odkształcania stopu Inconel 718 w zakresie temperatury 95CK1100 °C z prędkością 104 i 4x104 s1 wynosi 450,8 kJ/mol.

EN

Hot compression tests were performed on the solution treated Inconel 718 superalloy within temperature range characteristic for precipitation of hardenable phases (y"9 carbides) with constant true strain rates of Id3 and 10r4 s'1 in order to evaluate micros tructure changes due to overlapping of deformation and dynamic precipitation process. It was found that relativelu low deformation temperature (720+850 °C) is conducive to strain localization and non-homogenous distribution of carbides precipitated during deformation what may cause the fracture of the samples. The mean value of activation energy Qfor deformation process of Inconel 718 alloy in the temperature range of 900+1150 °Cat strain rate of 1(I4 and4xld4 r1 is 450.8 kJ/mol.

5

Phase transformation during hot deformation of 0.16% C steel

Nowotnik A., Błaż L., Siwecki T., Sieniawski J.

Archives of Metallurgy and Materials

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2006

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Vol. 51, iss. 2

187-192

EN

Effect of hot deformation process and transformation Austenite(A)→ Ferrite(F) and Austenite → Pearlite(P) on a carbon steel (0.158%C) microstructure was studied. Hot compression test at constant strain rate were performed during controlled cooling of the sample within the temperature range related to the start and finish of these phase transformations. Attention was paid to structural effects of dynamic precipitation and resulted morphology of structural components dependent on expected localization of phase transformation. It was shown that the flow localization during hot deformation and preferred growth of the pearlite colonies at shear bands was very limited. The most characteristic feature of the microstructure observed for hot deformed samples was the development of carbides that nucleated along elongated ferrite grains.

PL

W pracy przedstawiono wyniki badań wpływu odkształcania wysokotemperaturowego i przemiany Austenit(A) → Ferryt(F) oraz Austenit → Perlit(P) na mikrostrukture stali węglowej (0.158%C). Przeprowadzono próby ściskania w warunkach kontrolowanego chłodzenia ze stałą prędkością odkształcania. Odkształcenia realizowano w czasie przejścia przez temperaturowy zakres przemiany fazowej. Uwagę szczególną zwrócono na obserwację mikrostruktury i morfologię składników fazowych mogących potwierdzić proces niejednorodnego wydzielania związanego z lokalizacją przemiany. Wykazano, że w warunkach dynamicznych nieciągła przemiana stali praktycznie nie charakteryzuje się skłonnością do lokalizacji spowodowanej niejednorodnością odkształcenia. Najistotniejszą cechą mikrostruktury stali po odkształceniu w zakresie przemiany jest wydzielanie węglików na granicach odkształconych ziarn ferrytu.