Thermal expansion in the Ni−Cr−Al and Co−Cr−Al systems to 1200°C determined by high-temperature X-ray diffraction

Lowell, C. E.; Garlick, R. G.; Henry, Bert

doi:10.1007/bf03186796

Cited by 8 publications

(7 citation statements)

References 1 publication

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“…Analogously, the increase in the Al content of the g phase is associated with an increase in the lattice parameter of the g phase (Table IV), and a decrease in the volume fraction of the g phase (the volume fraction of NiY-rich phases is smaller than 1 pct and is virtually unaffected by LSM). The lattice parameter changes are in agreement with literature data for ternary NiCrAl alloys, [27,28] which show an increase in the lattice parameters of the g and b phases if Cr is replaced by Al (Al has a larger atomic radius than Cr and Ni [29] ). The increase in the volume fraction of b phase within the LSM zone corresponds well with the reduced partitioning of Al, Cr, and Co between the g and b phases (i.e., the image *Taking into account the coarse b, and ignoring the needlelike g precipitates ( Figure 3).…”

Section: Table IV Lattice Parameter a (In Pm) Of The ␥ And ␤ Phases supporting

confidence: 90%

Microstructure refinement of NiCoCrAIY alloys by laser surface melting

Kwakernaak

Sloof

Nijdam

2006

Metall Mater Trans A

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The surface of a cast g ϩ b alloy is melted with 75 MW/m 2 laser pulses for five different laser irradiation times and two different alloy surface conditions (different initial alloy reflectivity). Detailed analysis of the chemical composition, microstructure, and phase constitution of the laser-melted and original alloy reveals that after laser surface melting (1) the average size of the b dendrites is much smaller, (2) the Y is distributed more homogeneously as smaller NiY-rich precipitates along g/b phase and g/g grain boundaries, (3) the alloy contains a higher volume fraction of the b phase, (4) the b phase contains less Al and more Cr and has a lower lattice parameter, and (5) the g phase contains more Al and less Cr and has a higher lattice parameter. Based on both experimental results and model calculations, it is shown that the maximum depth of the melt pool increases with increasing laser irradiation time and decreasing initial alloy reflectivity. The arm spacing of the secondary b dendrites, which also increases with increasing laser irradiation time, is related to the average alloy cooling rate according to the power of Ϫ1/3, in agreement with theories on solidification.

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Section: Table IV Lattice Parameter a (In Pm) Of The ␥ And ␤ Phases supporting

confidence: 90%

Microstructure refinement of NiCoCrAIY alloys by laser surface melting

Kwakernaak

Sloof

Nijdam

2006

Metall Mater Trans A

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“…It is known that the presence of a separate alloy α-chromium phase with its very different coeffi- cient of thermal expansion 15 can promote spallation during temperature cycling.…”

Section: Cyclic Oxidationmentioning

confidence: 99%

Oxidation - Nitridation of Ni-Cr-Al alloys

Han

Young

2004

Mat. Res.

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A series of alloys containing 24-36 wt pct Cr and 13.5 -25.0 wt pct Al was reacted with air at 1100 °C for 260 h. The products of isothermal reaction were scales of α-Al 2 O 3 plus small amounts of Cr 2 O 3 . These grew according to parabolic kinetics, interrupted by episodic weight losses caused by partial spallation. No nitridation occurred during the isothermal exposures. Reaction during thermal cycling for up to 260 one hour cycles was much more severe. Repeated scale spallation led to subsurface alloy depletion in aluminium and, to a lesser extent, chromium. This caused transformation of the prior alloy three-phase structures (α-Cr+β-NiAl+γ-Ni) to single-phase γ-nickel solution. Destruction of the external scale allowed gas access to this metal which was able to dissolve both oxygen and nitrogen. Inward diffusion of the two oxidants led to development of a complex internal precipitation zone: Al 2 O 3 and Cr 2 O 3 beneath the surface, then Al 2 O 3 then AIN, then AIN + Cr 2 N and finally AIN alone in the deepest region. Diffusion-controlled kinetics were in effect initially, but mechanical damage to the internal precipitation zone led to more rapid gas access and approximately linear kinetics in the long term.

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“…Lowell et al 51 reported the coefficients of thermal-expansion (CTE) for the different phases in the Ni-Cr-Al system using high-temperature X-ray diffraction during the course of a single run. The mean CTEs reported for a,c/c¢, and b phases in a similar alloy were 13 Â 10 )6 , 17 Â 10 )6 , and 19 Â 10 )6 /°C, respectively.…”

Section: Resultsmentioning

confidence: 99%

In Situ Study of Oxidation-Induced Growth Strains In a Model NiCrAlY Bond-Coat Alloy

et al. 2007

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Synchrotron radiation has been used to study in situ the evolution of growth strains in an Al 2 O 3 scale (the so-called TGO or thermally grown oxide) on a model bond-coat alloy (Ni-19.7 Cr-19.2 Al-0.1 Y at.%) as oxide growth proceeds in air at 950-1100°C, and the changes in these strains due to thermalexpansion mismatch as the samples are cooled. Tensile growth stresses develop in the oxide scales during the initial stages of oxidation, a result of initially formed transition aluminas converting to the stable a-Al 2 O 3 form, but large residual compressive stresses are present at room temperature due to thermalexpansion mismatch between the scale and the bond-coat.

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Thermal expansion in the Ni−Cr−Al and Co−Cr−Al systems to 1200°C determined by high-temperature X-ray diffraction

Cited by 8 publications

References 1 publication

Microstructure refinement of NiCoCrAIY alloys by laser surface melting

Microstructure refinement of NiCoCrAIY alloys by laser surface melting

Oxidation - Nitridation of Ni-Cr-Al alloys

In Situ Study of Oxidation-Induced Growth Strains In a Model NiCrAlY Bond-Coat Alloy

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