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Phase transformation strengthening of high-temperature superalloys

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ABSTRACT

Decades of research has been focused on improving the high-temperature properties of nickel-based superalloys, an essential class of materials used in the hot section of jet turbine engines, allowing increased engine efficiency and reduced CO2 emissions. Here we introduce a new ‘phase-transformation strengthening' mechanism that resists high-temperature creep deformation in nickel-based superalloys, where specific alloying elements inhibit the deleterious deformation mode of nanotwinning at temperatures above 700 °C. Ultra-high-resolution structure and composition analysis via scanning transmission electron microscopy, combined with density functional theory calculations, reveals that a superalloy with higher concentrations of the elements titanium, tantalum and niobium encourage a shear-induced solid-state transformation from the γ′ to η phase along stacking faults in γ′ precipitates, which would normally be the precursors of deformation twins. This nanoscale η phase creates a low-energy structure that inhibits thickening of stacking faults into twins, leading to significant improvement in creep properties.

No MeSH data available.


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Phase transformation strengthening in ME501.(a) Schematic of an isolated SESF in a γ′ precipitate. (b) γ formers (Co, Cr, and Mo) segregated along the SESF in ME3. (c) Two more dislocations have interacted at the γ-γ′ interface near the SESF in ME3. (d) Two more Shockley partials shear along the SESF forming a four-layer twin that is able to shear both the γ and γ′ precipitates. (e) η formers (Co, Ta, Ti, and Nb) segregated along the SESF in ME501. (f) Two more dislocations have interacted at the γ-γ′ interface near the SESF in ME501. (g) Given results in Fig. 5 the dislocations are not able to form a twin in ME501.
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f6: Phase transformation strengthening in ME501.(a) Schematic of an isolated SESF in a γ′ precipitate. (b) γ formers (Co, Cr, and Mo) segregated along the SESF in ME3. (c) Two more dislocations have interacted at the γ-γ′ interface near the SESF in ME3. (d) Two more Shockley partials shear along the SESF forming a four-layer twin that is able to shear both the γ and γ′ precipitates. (e) η formers (Co, Ta, Ti, and Nb) segregated along the SESF in ME501. (f) Two more dislocations have interacted at the γ-γ′ interface near the SESF in ME501. (g) Given results in Fig. 5 the dislocations are not able to form a twin in ME501.

Mentions: The implications from the DFT calculations in Fig. 5 are summarized in Fig. 6. In the case of ME3, γ formers (Co, Cr, Mo) segregate to the fault, transforming the fault to a γ-like region, as shown in Fig. 6b. New Shockley partials interact at the γ-γ′ interface where the SESF has formed. These partials are able to enter the γ′ precipitate and shear along the SESF, with little energy penalty. The formation of a γ-like phase along the SESF removes nearest-neighbor violations, promoting further shearing by partials due to the subsequent lower energy barrier. Consequently, twins which shear through both γ and γ′ phases (Fig. 6c,d) can form, thereby defeating the effectiveness of the strengthening γ′ precipitates. This explains the high frequency of twins observed in ME3.


Phase transformation strengthening of high-temperature superalloys
Phase transformation strengthening in ME501.(a) Schematic of an isolated SESF in a γ′ precipitate. (b) γ formers (Co, Cr, and Mo) segregated along the SESF in ME3. (c) Two more dislocations have interacted at the γ-γ′ interface near the SESF in ME3. (d) Two more Shockley partials shear along the SESF forming a four-layer twin that is able to shear both the γ and γ′ precipitates. (e) η formers (Co, Ta, Ti, and Nb) segregated along the SESF in ME501. (f) Two more dislocations have interacted at the γ-γ′ interface near the SESF in ME501. (g) Given results in Fig. 5 the dislocations are not able to form a twin in ME501.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5121413&req=5

f6: Phase transformation strengthening in ME501.(a) Schematic of an isolated SESF in a γ′ precipitate. (b) γ formers (Co, Cr, and Mo) segregated along the SESF in ME3. (c) Two more dislocations have interacted at the γ-γ′ interface near the SESF in ME3. (d) Two more Shockley partials shear along the SESF forming a four-layer twin that is able to shear both the γ and γ′ precipitates. (e) η formers (Co, Ta, Ti, and Nb) segregated along the SESF in ME501. (f) Two more dislocations have interacted at the γ-γ′ interface near the SESF in ME501. (g) Given results in Fig. 5 the dislocations are not able to form a twin in ME501.
Mentions: The implications from the DFT calculations in Fig. 5 are summarized in Fig. 6. In the case of ME3, γ formers (Co, Cr, Mo) segregate to the fault, transforming the fault to a γ-like region, as shown in Fig. 6b. New Shockley partials interact at the γ-γ′ interface where the SESF has formed. These partials are able to enter the γ′ precipitate and shear along the SESF, with little energy penalty. The formation of a γ-like phase along the SESF removes nearest-neighbor violations, promoting further shearing by partials due to the subsequent lower energy barrier. Consequently, twins which shear through both γ and γ′ phases (Fig. 6c,d) can form, thereby defeating the effectiveness of the strengthening γ′ precipitates. This explains the high frequency of twins observed in ME3.

View Article: PubMed Central - PubMed

ABSTRACT

Decades of research has been focused on improving the high-temperature properties of nickel-based superalloys, an essential class of materials used in the hot section of jet turbine engines, allowing increased engine efficiency and reduced CO2 emissions. Here we introduce a new ‘phase-transformation strengthening' mechanism that resists high-temperature creep deformation in nickel-based superalloys, where specific alloying elements inhibit the deleterious deformation mode of nanotwinning at temperatures above 700 °C. Ultra-high-resolution structure and composition analysis via scanning transmission electron microscopy, combined with density functional theory calculations, reveals that a superalloy with higher concentrations of the elements titanium, tantalum and niobium encourage a shear-induced solid-state transformation from the γ′ to η phase along stacking faults in γ′ precipitates, which would normally be the precursors of deformation twins. This nanoscale η phase creates a low-energy structure that inhibits thickening of stacking faults into twins, leading to significant improvement in creep properties.

No MeSH data available.


Related in: MedlinePlus