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| EFFECT OF VOLUME FRACTION AND SIZE OF FINE γ' ON CREEP STRENGTH OF A DS NICKEL-BASE SUPERALLOY |
| Lin Donglian (T. L. Lin); Yao Deliang (Shanghai Jiaotong University); Ltn Xianjin (Shanghai Institute of Iron and Steel); Sun Chuanqi (Beijing Institute of Aeronautical Materials) |
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Cite this article:
Lin Donglian (T. L. Lin); Yao Deliang (Shanghai Jiaotong University); Ltn Xianjin (Shanghai Institute of Iron and Steel); Sun Chuanqi (Beijing Institute of Aeronautical Materials). EFFECT OF VOLUME FRACTION AND SIZE OF FINE γ' ON CREEP STRENGTH OF A DS NICKEL-BASE SUPERALLOY. Acta Metall Sin, 1982, 18(1): 104-140.
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Abstract The effect of volume fraction and size of fine γ' on creep strength of a directionally solidified nickel-base superalloy——DSK3 at 760℃ has been examined. DSK3 was solution heat treated at 1100—1270℃ to homogenize the alloy and dissolve coarse γ' and eutectic γ—γ' constituents which subsequently reprecipitated in the form of a uniform fine γ' dispersion with various volume fractions (0.25—0.63) and different sizes (0.1—0.3μm) upon cooling and aging. The size and amount of fine γ' increased with the increase of solid solution temperature. The creep rupture life increased and the secondary creep rate decreased as the solution temperature increased and the relationship t_f and e at 760℃ can be expressed as:(?)~αt_f=c where a=1 and c≈5.5. The improvement of creep rupture life was found to be due to a decrease of secondary creep rate and an extension of secondary stage of creep. The secondary creep rate (?) is strongly dependent on size a, interparticle spacing λ and volume fraction v_f of fine γ', and it takes the form:(?)∝α/v_f~(2/3)(1-v_f~(1/3)) or (?)∝λ~2/αThe dislocations structure and morphology of γ' of the alloy produced by creep to primary, secondary and tertiary stage at 760℃ under 73.8 kgf/mm~2 and 78 kgf/mm~2, and at 950℃ under 25 kgf/mm~2 was examined in TEM. During the primary stage of creep at 760℃ under 73.8 kgf/mm~2 or 78 kgf/mm~2 dislocations are moving between the γ' cuboids shearing the γ matrix. Two sets of dislocations intersecting or reacting each other in the γ matrix are visible, but no dislocation is ever observed inside the γ' cuboids. A dense 3-dimensional dislocation network has formed in the γ matrix of the alloy during secondary creep. A few superlattice dislocation pairs were found in the γ' at 760℃ under 78 kgf/mm~2, but still no dislocation in the γ' under 73.8 kgf/mm~2 is observed in secondary stage of creep.The morphology and size of γ' are changing during creep at 950℃ under 25 kgf/mm~2, especially at the end of secondary stage of creep and during tertiary stage of creep. In the early primary stage of creep dislocation structure is similar to that at 760℃ but two sets of α/2<110> dislocations can react more easily and form a 2-dimensional dislocation networks covering the γ—γ' interface in the later primary stage of creep. γ' particles start to coarsen and become a plate shape by lateral merging of cubes without noticeable thickening of the plates from secondary stage of creep.Direct observation of dislocation structure and γ' morphology in relation with secondary creep rate suggests a high temperature creep model for the nickel-base superalloy in the range of temperature and applied stress where shearing of the γ' phase does not control the straining process. During secondary creep, strain is mainly the result of climb rate of dense 2- or 3-dimensional dislocation networks and the free path of dislocation glide in γ. Secondary creep rate (?) can be described as:(?)=NAbR where N is the density of dislocation sources; A the sweeping area of a dislocation, b the magnitude of the Burgers vector of the dislocation and R the climb rate of the dislocation over γ' particle or the number of critical link length of dislocation network can be developed to operate in a unit time, i. e., the number of operating sources of the dislocation in a unit time.The volmne fraction v_f and size of γ' will play an important role in secondary creep rate (?) by changing the configuration and density of the dislocation and influencing the process of the dislocation climb (recovery) in γ matrix. Applying the following equation relating the applied stress and dislocation density ρ to creep process:σ= σ_0+αGbρ~(1/2) where G is the shear modulus, α the strengthening proportional constant by the interaction of dislocations and σ_0 flow stress due to all causes other than dislocation-dislocation interaction, and combining climb model given by Anscll and Weertman the equation for secondary creep rate (?) can be derived as:(?)=K/G~3T λ~2/α(σ-σ_0)~n exp(-Q_s/RT) or (?)=K/G~3T α/v_f~(2/3)(1-v_f~(1/3))~2(σ-σ_0)~n exp (-Q_3/RT) where K is a constant, Q_s the activation energy for self-diffusion, and n=4—5. The expression relating λ, α, v_f and (?) predicted by the theory agrees satisfactorily with experimental results.
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Received: 18 January 1982
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