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Cyclic oxidation resistance of Co–9Al–9W new cobalt-based superalloy

Moskal Grzegorz, Tomaszewska Agnieszka, Migas Damian, Niemiec Dawid

Inżynieria Materiałowa

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2019

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Vol. 40, nr 2

39--45

EN

Cobalt-based superalloys are class of new heat-resistant materials for components of turbine engines. The γ/γʹ phase microstructure similar to nickel-based analogue, provide excellent creep resistance as well as resistance to corrosion and oxidation. Superior high temperature resistance drives intensive development of Co-based superalloys. The aim of paper is assessment of high temperature oxidation behaviour of Co–9Al–9W alloy in as-cast state. The cyclic oxidation performance of Co–9Al–9W alloy was studied at temperature 800°C and 900°C. The cyclic oxidation test was carried out under laboratory air atmosphere with time of exposure 25 hours and multiplicity of it. The scale morphology after cyclic oxidation test was investigated. The evaluation of scale concerned macrostructure, microstructure, chemical and phase composition of surface. Surface of tested alloys after oxidation was characterized using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and light microscopy (LM). Obtained data revealed that Co–9Al–9W alloy showed acceptable resistance to cyclic test at temperature 800°C. The oxide scale at this variant showed mainly Co oxides as an dominant phase. At higher temperature – 900°C the strong effect of spallation of scale was observed as results of CoWO4 complex oxide formation.

PL

Celem pracy była charakterystyka odporności na utlenianie nowego typu stopów na bazie kobaltu umacnianych fazą Co3(Al, W). Ocenie poddano stop Co–9Al–9W (% at.) w stanie po odlaniu i homogenizacji. Wykonano próby odporności na utlenianie cykliczne w cyklach 25-godzinnych w atmosferze powietrza. Zakres prezentowanych badań obejmował pomiary masy próbek po kolejnych cyklach, ocenę stanu powierzchni z uwzględnieniem procesów jej odpadania, jak również charakterystykę składu chemicznego i fazowego powstałej zgorzeliny oraz jej budowę morfologiczną.

2

Oxidation performance of Co–Al–W and Co–Ni–Al–W new type of γ-γʹ cobalt-based superalloys

Moskal G., Migas D., Tomaszewska A., Mikuszewski T., Maciąg T., Godzierz M., Niemiec D.

Inżynieria Materiałowa

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2017

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Vol. 38, nr 4

163--169

EN

Cobalt-based superalloys are class of new heat-resistant materials for components of turbine engines. The γ-γʹ phase microstructure similar to nickel-based analogue, provide excellent creep resistance as well as resistance to corrosion and oxidation. Superior high temperature resistance drives intensive development of Co-based superalloys. The aim of paper is assessment of high temperature oxidation behaviour of Co–9Al–9W and Co–20Ni–7Al–7W alloys in as-cast state. The non-isothermal oxidation performance of Co–9Al–9W and Co–20Ni–7Al–7W alloys was studied using thermogravimetric method up to 1200°C. The thermogravimetric analysis was carried out under argon atmosphere with heating rate 5°C/min. The scale morphology after high temperature oxidation test was investigated. The evaluation of scale concerned macrostructure, microstructure, chemical and phase composition of surface after high temperature oxidation tests. Surface of tested alloys after oxidation was characterized using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and light microscopy (LM). Co–20Ni–7Al–7W was characterized by slightly better high temperature oxidation resistance than basic Co–9Al–9W alloy. The surface stability in elevated temperature is important factor in case of components undergoing elevated temperature. This paper shows possibilities of these alloys for high temperature service.

PL

Intensywny rozwój silników lotniczych napędza poszukiwania nowych materiałów żarotrwałych, zdolnych do pracy w coraz trudniejszych warunkach. Efektem tych poszukiwań są nowe nadstopy typu γ-γʹ na bazie kobaltu, charakteryzujące się większą odpornością na utlenianie w wysokiej temperaturze od obecnie stosowanych odpowiedników na bazie niklu. Celem pracy było scharakteryzowanie utleniania nadstopu kobaltu Co–20Ni–7Al–7W w podwyższonej temperaturze i porównanie go z nadstopem bazowym Co–9Al–9W. Praca objęła badania kinetyki utleniania wysokotemperaturowego w warunkach nieizotermicznych metodą grawimetryczną oraz analizę składu chemicznego, fazowego i mikrostruktury powstałej zgorzeliny.

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Characterization of primary microstructure of γ–γʹ Co–Al–W cobalt-based superalloy

Mikuszewski T., Tomaszewska A., Moskal G., Migas D., Niemiec D.

Inżynieria Materiałowa

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2017

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Vol. 38, nr 5

217--223

EN

Present paper concerns primary structure of cobalt-based superalloys Co–9Al–9W with γ/γʹ structure, casted via induction melting process. Common problems described in literature, concerning difficulties in obtaining uniform distribution of tungsten, particularly in interdendritic areas were reason of utilization of modified casting process. Alter method is based on tungsten dosing into liquid metal bath of cobalt and aluminium during melting. This method allows to obtain microstructure characterized by considerably decreased microsegregation, especially in case of tungsten. This type of primary structure results in simplified and more economical homogenization performance of casts before further technological processes. Used method of casting give the possibility to obtain samples with small effect of segregations of alloying elements. Observed effect of interdendritic segregations were related to higher level of aluminium concentration and lower cobalt, but in both cases the measured difference was 2 to 4 % at.

PL

Celem artykułu jest charakterystyka mikrostruktury pierwotnej stopu Co–9Al–9W w stanie bezpośrednio po odlaniu, z uwzględnieniem rozmieszczenia składników stopowych w rdzeniach dendrytów i obszarach międzydendrytycznych w różnych strefach odlewu oraz profilu mikrotwardości. Ważną kwestią była jednorodność rozmieszczenia wolframu w obszarach rdzeni dendrytów i w przestrzeniach międzydendrytycznych, co jest typowym problemem w stopach z dodatkiem wolframu.