Does the Hall-Petch relation of finer the grain size higher the hardness hold to all Mg alloys? If some Mg alloys show reverse Hall-Petch relation, what could be the reasons?
I can only think of one condition that Hall-Petch gives counterintiutive result. Magnesium when fine grained (i.e. below about 8-10 micron grain size) shows a different deformation mechanism, that is superplasticity via grain boundary sliding. However, this mechanism only works under very slow deformation rates (say slower than 10 to power -2) or at high temperatures. Now, if you attempt to measure hardness under suitable conditions magnesium alloys with such grain sizes would give you unexpected values. Other than this scenario I fail to think of, let alone know, any magnesium alloy that disobeys Hall-Petch.
Yeah I do agree with the reason of very fine grain size under which reverse of Hall Petch relation is possible. but the condition to obtain such a fine grain size mentioned by Ali Kaya seems not possible due to domination of dynamic recovery and grain growth menchanism. would any body explanin and comment on it?
I am sorry but your original question does not mention any condition that could cause recrystallization and grain growth. If one assumes room temperature hold, then your counter argument collapses.
Order of nano-grains brings about reversed effect of HP relations. In some alloys it is order of 10-100 nm. The only source I have at hand now on this is: "The effect of alloy powder morphology on microstructural evolution of hot worked P/M {FeAl} / T. ŚLEBODA, K. DONIEC // Archives of Materials Science and Engineering. ISSN 1897-2764. — 2007 vol. 28 iss. 10 p. 613–616. or some other work of that author
Almost any material consists of grains boundaries which are themselves defects which may occur gap. In some cases (but not always) act sufficiently simple pattern, then smaller the grain size, the smaller the friction force therebetween, and thus easier to deform the material. In particular, under certain grain size material can transition from solid state to superplastic when even small and small heat loads can deform (stretch or compress) the material without destruction.
Dear all
There is a new work that tries to combine both the Hall-Petch and its inverse in one model through presenting a multiscale model that enables description of both the Hall-Petch relation and its inverse in one equation without the need of prior knowledge of the grain size distribution, for more details, please see the following link:
https://www.researchgate.net/publication/324507369_A_new_multiscale_model_to_describe_a_modified_Hall-Petch_relation_at_different_scales_for_Nano_and_micro_materials
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