Effect of modifying surface layers on the creep of molybdenum

Kolobov, Yu. R.; Ratochka, I. V.

doi:10.1007/bf00767549

Cited by 3 publications

(4 citation statements)

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“…The results of a number of experimental works [1, 5,6] testify to the fact that grain-boundary diffusion fluxes of impurity from an external source under certain conditions give rise to activation of GB sliding. This effect is observed in polycrystals of bcc-and fcc-metals, and at random boundaries in bicrystals having fcc-and bcc-lattices [1, 5,7,8].…”

Section: Role Of Directed Diffusion Fluxes Along Gbs In Activating Gbmentioning

confidence: 83%

“…It is to be noted that the apparent energy of creep activation Q is decreasing from the values being approximately equal to that of volume diffusion activation Q controlling the processes of lattice dislocation climb to the values approximating the activation energy of GB diffusion Q governing the processes of GBS [2]. On the other hand, it was shown in [6] that the contribution from GBS under creep in a coarse-grained polycrystalline material into the total deformation could be increased without changing the grain size by transferring GBs into non-equilibrium (high-energy) state using grain-boundary diffusion fluxes of impurity atoms from an external source. In this case, we also observe a decrease in Q down to the values close to those for grain-boundary diffusion.…”

Section: Influence Of Grain-boundary State and Grain Size On Physicommentioning

confidence: 98%

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Characteristic features of diffusion-controlled processes in ordinary and ultrafine-grained polycrystaline metals

et al. 2004

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An analysis of manifestation of the effect of acceleration of creep in polycrystalline metals and alloys in the presence of grain-boundary diffusion fluxes of an impurity from an external medium (coating) is performed. The influence of the softened (due to impurity diffusion fluxes) layer thickness on the creep character in molybdenum is discussed for the case of its diffusion contact with nickel. An analysis of the effects of the regime of impurity grainboundary diffusion on the value of strain-rate sensitivity and on the behavior of grain-boundary migration as a function of external stress is made. Using submicrocrystalline nickel as an example, the influence of grainboundary state on properties of metallic materials in submicrocrystalline state is investigated. Direct measurements, theoretical calculations and indirect estimations of the differences in grain-boundary diffusion coefficients in coarse-and ultrafine-grained states are performed. The role of dispersion hardening in stabilizing the submicrocrystalline structure and improving the creep resistance is addressed, including the case where the grain-boundary fluxes of impurity atoms are forced from an external medium.

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Section: Role Of Directed Diffusion Fluxes Along Gbs In Activating Gbmentioning

confidence: 83%

Section: Influence Of Grain-boundary State and Grain Size On Physicommentioning

confidence: 98%

Characteristic features of diffusion-controlled processes in ordinary and ultrafine-grained polycrystaline metals

et al. 2004

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“…On the other hand, the estimates of the osmotic pressure made in [13] taking into account the concentrations of impurity in GB and the bulk provide the values of the order of 5 MPa, which is essentially lower than Р osm obtained in this work. Thus, the osmotic pressure developing in the presence of diffusion flows of impurity is comparable in its value with the external loading at which molybdenum is deformed under the conditions in questions at strain rates on the order of 10 -4 -10 -7 s -1 [14] and it is quite sufficient to give rise to dislocations at GBs. This type of dislocation origination was observed experimentally after molybdenum specimens were annealed without an external load in a diffusion contact with nickel at 1373 K for 4 hours [8].…”

Section: Russian Physicsmentioning

confidence: 91%

On physical nature of creep acceleration effect in submicrocrystalline materials

Ratochka

2004

Russ Phys J

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An analysis of special features of manifestation of the creep acceleration effect in polycrystalline metals and alloys in the presence of grain-boundary diffusion fluxes of an impurity from an external medium is made. It is noted that in certain cases where the diffusion properties of sub-boundary regions differ greatly from those within the boundary and in the bulk of the grain (e.g., in materials with submicrocrystalline structure (grain size 0.1-0.5 µm) and/or in the case of diffusion along the grain boundary of a strongly segregating impurity), conventional theoretical methods of calculation of the temperature-temporal range of the occurrence of the above effect have to be corrected. Based on the analysis of the experimental data, feasible solutions to the problem are discussed.It is well known that grain-boundary diffusion of impurity atoms from an external medium can, under certain conditions, give rise to activation of grain-boundary sliding, and, as a consequence, to creep acceleration in polycrystalline metals and alloys. It has been shown that this effect occurs in materials with bcc-, fcc-, and hcp-lattices and is manifested under the diffusion regime В 1 (according to a classification given in [1]) or under conditions close to this regime [2,3].According to the classification cited, diffusion processes for an immobile grain boundary (GB) in a bicrystal could be classified into five types: С, В 1 , В 2 , В 3 , and В 4 [1]. Under regimes С and В 1 , diffusion is largely non-stationarywhere i b с in the impurity concentration in GB and t is the time of diffusion annealing). Under regime С, diffusion of impurity occurs along GBs only. Under regime В 1 , a GB is being filled with impurity atoms while there is already an outflow of the impurity from the boundary into the bulk. Regime В 2 is characterized by a fast variation in impurity concentration in the sub-boundary regions ( i / 0 v c t ∂ ∂ ≠ , where i v с in the impurity concentration in the bulk of a grain). Regimes В 3 and В 4 correspond to volume diffusion. It should be noted that in the case of regimes В 2 , В 3 and В 4 , there is no variation in the impurity concentration in GBs ( i / 0 b с t ∂ ∂ = ) (quasi-stationary diffusion regimes). A formal characteristic evidencing the presence of this regime or the other is the value of asymptotic parameters λ and ν [1]where β is the GB diffusion width and D v and D b are the coefficients of grain-boundary and volume diffusion, respectively. Within recent years, it has been found out for submicrocrystalline (SMC) materials (grain size 0.1-0.5 µm) that the experimental temperature interval of manifestation of creep acceleration disagrees with the calculated data where В 1 operates for the respective system [4,5]. For instance, the effect under study in a Ni(Cu) system (shown in brackets is the impurity-diffusant) occurs at a temperature that is about 300 о lower than that calculated by the above-cited formulas. The temperature interval of creep acceleration effect for coarse-grained nickel under diffusion f...

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“…It is well-known [1,2] that the effect exerted by GB diffusion fluxes on the grain boundaries in polyand bicrystals is the reason for the diffusion-induced grain-boundary migration (DIGM) and GB sliding activation. Under certain conditions, grain-boundary sliding activation may lead to an essential decrease of creep resistance [2] and to superplasticity [3].…”

Section: Introductionmentioning

confidence: 99%

Diffusion and Properties of Bulk Nanostructured Metals and Alloys Processed by Severe Plastic Deformation

Kolobov

Grabovetskaya

Ivanov

et al. 2003

DDF

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Main characteristic features of grain boundary (GB) diffusion of substitution impurity from an external source in nanostructured nickel, copper, titanium in comparison with coarse-grained materials are considered. Physical reasons of the increase of grain boundary diffusivity in nanostructured state in comparison to the coarse-grained metals and and individual GBs are discussed. The creep acceleration effect of nanostructured metalsn in coarse-grained materials is discussed. It is established that the above effect is associated with the GB sliding activation. The pecularity of second phase growth on grain boundaries during high strain rate and low-temperature superplasticity in nanostrucutred Al-Mg-Li alloys and its interrelation with the diffusion process activation is examined.

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Effect of modifying surface layers on the creep of molybdenum

Cited by 3 publications

References 1 publication

Characteristic features of diffusion-controlled processes in ordinary and ultrafine-grained polycrystaline metals

Characteristic features of diffusion-controlled processes in ordinary and ultrafine-grained polycrystaline metals

On physical nature of creep acceleration effect in submicrocrystalline materials

Diffusion and Properties of Bulk Nanostructured Metals and Alloys Processed by Severe Plastic Deformation

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