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» TEM analysis of creep mechanisms of single-crystalline nickel-base superalloys
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Beata Dubiel  Aleksandra czyrska-Filemonowicz
Single crystal (SC) superalloys are especially designed for gas turbine
blades in jet aircraft engines and stationary turbines generating
electricity. The basis criterion in material selection for gas turbine
blades is a high creep strength, because they have to withstand centrifugal
forces acting during operation.
The main creep mechanisms in polycrystalline superalloys are:
-- dislocation creep by combined slip and climb of dislocations,
-- diffusion creep by Nabarro-Herring or Coble mechanism [1, 2].
As it was shown for many superalloys [3÷5], in the range of temperature
and stresses characteristic for service conditions of turbine
blades, grain boundary diffusion by Coble mechanism is the main
factor controlling the creep rate. Therefore the elimination of grain
boundaries is necessary for improvement of creep resistance of
superalloys. It can be achieved by a directional solidification with
a grain selector, as was demonstrated first by B. Piearcey from Pratt
& Whitney in 1970 [6]. Single crystal superalloys exhibit the highest
creep resistance among metallic materials. The development of
chemical composition of SC superalloys resulted in elaboration of
four generations of these alloys [7, 8] and progress continues already
towards the fifths generation [9].
The as-cast single crystal superalloys exhibit a dendritic structure
with the eutectic areas between dendrite arms, which can be
homogenized by heat treatment. The typical microstructure consists
of about 70% of cuboid γʹ precipitates, about 400 nm in size,
separated by γ matrix channels about 30 nm wide. Residual eutectic
areas contain large irregular γʹ precipitates. Such two-phase microstructure
guarantees the high creep resistance of single-crystalline
turbine blades. The γʹ precipitates are an ordered Ni3Al-base intermetallic
phase with the lattice parameter only slightly smaller than
the Ni-base γ solid solution. This produc[...]
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INŻYNIERIA MATERIAŁOWA 2010/3
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» Zmiany mikrostruktury podczas eksploatacji łopatki turbiny gazowej z monokrystalicznego nadstopu niklu CMSX-4
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bEata DUbiEL aLEKsanDra CZyrsKa-fiLEMonowiCZ
Metodami skaningowej i transmisyjnej mikroskopii elektronowej zbadano zmiany mikrostruktury w .opatce z monokrystalicznego
nadstopu niklu CMSX-4 po 12 700 godzinach pracy w energetycznej turbinie gazowej. Zaobserwowano wyra.ne ro.nice w mi-
krostrukturze u podstawy, w .rodku wysoko.ci i przy wierzcho.ku piora .opatki. Szczego.owa analiza podstruktury dyslokacyjnej
umo.liwi.a okre.lenie ro.nych mechanizmow pe.zania dzia.aj.cych w poszczegolnych strefach .opatki.
Scanning and transmission electron microscopy investigations of microstructural changes in CMSX-4 single crystal blade after
exploitation for 12 700 hours in stationary gas turbine have been performed. The pronounced differences in microstructure at the bottom,
in the middle of the height and at the blade tip have been observed. The detailed analysis of the dislocation substructure enabled
determination of different creep mechanisms acting in particular zones of the blade.
S.owa kluczowe: monokrystaliczne nadstopy niklu, pe.zanie, .opatki turbin gazowych
Key words: single crystal nickel-base superalloys, creep, gas turbine blades
S. 307 UHTNIK-WIADOMO.CI HUTNICZE Nr 4 Stereoscan oraz transmisyjnego mikroskopu elektro-
nowego JEOL JEM-2010 ARP.
3. Wyniki bada.. 3.1. Zmiany kszta.tu wydziele.
fazy ƒÁf. Obserwacje mikrostruktury na
przekrojach rownoleg.ych do kierunku dzia.ania
si.y od.rodkowej ujawni.y zro.nicowanie kszta.-
tu i wielko.ci wydziele. fazy ƒÁf przy podstawie,
w .rodku i na wierzcho.ku piora .opatki (rys. 2).
U podstawy .opatki, gdzie podczas pracy tempera-
tura by.a najni.sza (807 ‹C), a napr..enia najwy.-
sze (189 MPa), mikrostruktura nadstopu CMSX-4
obserwowana na obrazach SEM by.a podobna, jak
w stanie obrobionym cieplnie. Na obrazach SEM nie
zaobserwowano zmian w mikrostrukturze spowodo-
wanych dzia.aniem napr..e. w wysokiej temperatu-
rze. Wzrost temperatury w .rodku piora .opatki do
927 ‹C i jednoczesny spadek [...]
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HUTNIK - WIADOMOŚCI HUTNICZE 2011/4
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» Characterisation of nanocomposite nc-WC/a-C and nc-WC/a-C:H coatings on oxygen hardened Ti-6Al-4V alloy
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TOMASZ MOSKALEWICZ BOGDAN WENDLER Aleksandra Czyrska -Filemonowicz
Titanium and its alloys are often applied in aeronautics, chemical,
petrochemical, marine industries and medicine as structure materials
due to their high specific strength and excellent corrosion resistance
[1, 2]. However, titanium alloys are characterized by poor
tribological properties especially in sliding situations. These include
high and unstable friction coefficients, severe adhesive wear,
susceptibility to fretting wear and a strong tendency to gall when
in rubbing contact with itself and other surface [3]. Deposition of
a low friction and wear resistant coatings by different surface engineering
methods is one of the most prospective means to improve
the tribological properties of the titanium alloys.
Nanocomposite coatings were found to be a prospective for improvement
of tribological properties of materials. Hard and soft
phase were often mixed in order to get a compromise between mechanical
and tribological properties [4]. In this field, many nanocrystalline
metal carbide particles (nc-MeC, where Me = transition
metal) have been used as a hard phase and amorphous carbon,
hydrogen-free (a-C) or hydrogenated (a-C:H) have been used as
a second phase [4÷8].
Carbon-based nanocomposites, where nanocrystalline phase is
embedded into a-C matrix, are considered as a new class of protective
material and found wide range of engineering applications,
such as cutting and machining tools, bearing, pumps, machine and
engine parts [9, 10]. These coatings exhibited high hardness, toughness,
good wear resistance and very low friction in ambient air due
to the lubricious a-C matrix [11÷13].
However, when such coatings are deposited on ‘soft’ substrates,
like commercially pure titanium or Ti-6Al-4V alloy, when the applied
load is high enough the softer substrate can undergo plastic
deformation [14]. Therefore, in a present study nanocomposite
coatings were deposited on a ‘hard’ substrate - oxygen hardened
Ti-[...]
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INŻYNIERIA MATERIAŁOWA 2010/3
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