If we assume the Brian Wilshire interpretation of the creep curve, being in only two sections: an initial curve at a decaying rate, and a final transient curve at an accelerating rate terminating in failure, (the so-called steady state creep part of the curve being merely the overlap of these two regimes) it leads to interesting predictions.

A second major assumption is that the single crystal turbine blade, although much fewer defects than a polycrystalline blade, still contains numerous defects. In particular, the defects are almost certainly bifilms entrained by the pouring of the liquid metal by the alloy maker and/or by the blade manufacturer. Bifilms, as cracks, will be expected to open under creep conditions, gradually opening up porosity which will eventually lead to accelerating creep rate and final failure.

Thus, if all this is true, the second part of the Wilshire curve is dominated by the development of voids and cracks opening up from bifilms. Grain boundary sliding would also be expected to be accelerated if the boundary was weakened by the presence of a bifilm.

After all this, it follows that if the blade could be manufactured without bifilms, therefore achieving an essentially defect-free matrix, the second failure curve is completely eliminated. Only the first Wilshire curve exists, so that under creep conditions an essentially defect-free metal will undergo progressively slower creep, and should never develop pores or cracks that could lead to failure. The component should not creep to any significant extent and should not fail.

These are shocking predictions. However, it should not be difficult to test their accuracy, and it is surely important to do so. All other metallurgical issues are, of course, secondary.

Dear Kaustav,

As already said in an answer to another question, I'm not an expert in your superalloys. However, I've seen a lot of micrographs in my life ... So, even if I'm not sure to be able to answer your question, I'd anyway like to look at your microstructures, if it is possible.

Therefore in order to fix ideas and to be sure of what is exactly meant, could you show a few micrographs of the kind "bimodal grains with bimodal precipitates" and of the kind "bimodal precipitate with coarse grain" typical for the superalloy under scope ? Could you also indicate the 3D-lattices (FCC, BCC, HCP, ... ?) for the bimodal grains and coarse grains and also the nature of the precipitates ? And what is the superalloy ?

Regards   

I am talking about Ni based superalloys with gamma matrix and gamma prime precipitates only

The following reference can be found helpful: R. W. Evans, B. Wilshire, "Creep of Metals and Alloys", Book nº 304, The Institute of Metals, London, 1985.

Kaustav,

Thanks for the additional information. Then I’d say - at first sight and because there are (assumed similar) bimodal precipitates in both cases - that the case with bimodal grains would be better for the creep behaviour. Not necessarily in terms of improved creep strength (the coarse grained structure would probably be right), but in terms of higher creep extension to failure. This is  because a well mixed bimodal distribution of grain sizes will probably be better for the « control » of grain boundary moving (increased grain boundaries area). But this is just a feeling at first sight (it depends probably also of other factors like the history of thermomechanical processing, the resulting shape of the grains, occurrence of orientation gradients, of serrated grain boundaries, twin content, precipitation and distribution of carbides, possibility of dynamic recrystallization, ….). But the last word will be given to the actual creep conditions.

Creep resistance is improved by eliminating grain boundaries, or at least the boundaries that are normal to the direction of stress. Hence the advantage with single crystals and directionally solidified microstructures.

> S. Tarafder,

Yes of course single crystals are best. But which kind of Ni based polycrystalline superalloys (with gamma matrix and γ’ precipitates only) would you favour on base of their microstructure : showing bimodal grains or coarse grains ?  And what about the role played by bimodal precipitates (γ’ precipitates are known to improve creep strength) ?

I am not sure bimodality of grain size distribution would necessary impart better creep resistance. It would improve toughness though.

Bimodal distribution of precipitates in a Ni-base superalloy could be indicative of a number of things, depending upon the composition and thermal processing the alloy is subjected to. If there were both γ’ and γ’' precipitates, they could have different mean sizes. Incomplete solutionizing could impose a dual γ’ mean size. A generalization therefore of desirability of bimodality in precipitate distribution may not be appropriate. What may be of greater significance is the volume fraction of γ’ (and/or γ’') in a Ni-base superalloy.

I agree with you. The volume fraction of γ’ and/or γ’' in a Ni-base superalloy is a key factor for creep resistance.

The bimodality of the grains with discrete carbides improves the creep resistance and toughness whereas the distribution and lattice mismatch between γ and γ’ plays an important role in further enhancing the creep resistance in polycrystalline Ni base superalloys.

It is pretty difficult to say. Theere are many factor which effects creep : grain size, precipitate, dislocations etc. exact mechanism is governed by the creep deformation maps. plenty of this maps are available on line for superalloy. you can find it 

If we assume the Brian Wilshire interpretation of the creep curve, being in only two sections: an initial curve at a decaying rate, and a final transient curve at an accelerating rate terminating in failure, (the so-called steady state creep part of the curve being merely the overlap of these two regimes) it leads to interesting predictions.

A second major assumption is that the single crystal turbine blade, although much fewer defects than a polycrystalline blade, still contains numerous defects. In particular, the defects are almost certainly bifilms entrained by the pouring of the liquid metal by the alloy maker and/or by the blade manufacturer. Bifilms, as cracks, will be expected to open under creep conditions, gradually opening up porosity which will eventually lead to accelerating creep rate and final failure.

Thus, if all this is true, the second part of the Wilshire curve is dominated by the development of voids and cracks opening up from bifilms. Grain boundary sliding would also be expected to be accelerated if the boundary was weakened by the presence of a bifilm.

After all this, it follows that if the blade could be manufactured without bifilms, therefore achieving an essentially defect-free matrix, the second failure curve is completely eliminated. Only the first Wilshire curve exists, so that under creep conditions an essentially defect-free metal will undergo progressively slower creep, and should never develop pores or cracks that could lead to failure. The component should not creep to any significant extent and should not fail.

These are shocking predictions. However, it should not be difficult to test their accuracy, and it is surely important to do so. All other metallurgical issues are, of course, secondary.

Interesting thoughts John Campbell.

But would there not be an equilibrium concentration of vacancies when casting a single crystal which, under creeping conditions, would aggregate together to form incipient defects that enlarge, ultimately, to produce the second part of the Wilshire pair of curves ?

Thanks John for the interesting note. I'd agree with Wilshire's view. Now, one could interpret "coarse grain" in the question as covering single crystals. However, "bimodal grains" correspond by definition to polycrystalline structure. In my opinion to be clear a distinction should be made between bimodal grains and bifilms when answering the question.

Dr Tarafder

I agree it seems quite reasonable to assume that vacancies could condense to form volume defects such as pores or cracks. However, perhaps surprisingly,  this never occurs. Vacancies always condense to form fully condensed features such as dislocation loops and stacking fault tetrahedral. Once, again, this is a clear demonstration of the powerful attractive forces between metallic atoms.

Dr Tarafder

I agree it seems quite reasonable to assume that vacancies could condense to form volume defects such as pores or cracks. However, perhaps surprisingly,  this never occurs. Vacancies always condense to form fully condensed features such as dislocation loops and stacking fault tetrahedral. Once, again, this is a clear demonstration of the powerful attractive forces between metallic atoms.

Is that because there is never that many of them to take them much beyond 2-d defects? Or is it thermodynamically not possible ?

If interatomic forces can be construed as an expression of thermodynamics, then it seems that condensation to defects thicker than 2D is not possible.

Yes..in mesocale may be the dynamics is different and may be this bimodal grain with bimodal ppt is possible to manufacture like coarse grain superalloy with bimodal precipitate..but I don't know whether ultrafine grain with precipitate bimodality is possible or not....because inhibiting grain growth and promoting precipitate growth..I don't know how this can be achieved..I am seriously learnt a lot from these discussion..seriously RG has some advantage..:-)