well, yes and no.

different properties, for sure.. same material? it depends on the scale you are looking to. Not only strain but a lot defect crystal defect..

not so easy.. in supraconductor the properties are anisotropic (well if looking close to the boundary they all are..) so of course a grain boundary is a disruption of the properties. they are plenty of classification of grain boundaries..

so your question is much to general to be answered, just like this.

In my opinion, we can consider grain boundaries as region of large strain and defects. The two different crystal symmetries (of-course its same material and consequently same crystal structure but rotated in 3D space with respect to one another) of neighboring grains meet each other "head-on" and due to this mismatch, there are voids and vacancies and all sorts of crystal defects one can think of.

so properties are most definitely different from inside the grains (or we can say single crystal). They are the key difference between so called "bulk crystals" and "single crystal" and any response is usually a combination of responses from inside the grain (i.e. single crystal) and at grain boundary. (although one can argue about nature of this "combination" but more or less, we can define it like this)

The properties of a grain boundary are different form bulk since the grain boundaries (GBs) are considered as defects in a crystal structure. In addition, impurities can segregate to GB as well as many dislocations can form at the GBs. The strain is usually relaxed at the GBs. However, it depends on the structure and type of the GB.

Thanks all for your answers, Andriy is there a way to determine whether the strain is relaxed or not?

Theoretically, yes. However, you need to perform a quantitative strain analysis by e.g. EBSD in SEM or by CBED in TEM or by NBED in TEM.

Is there a word like 'domain boundary'? If so, what is the relation between this two? Is GB in a state of armpophous or not? Thank you in advance.

yes Doug Han,

there is a concept of domain boundary. and it is often related to as boundary of magnetic domains inside a grain. Essentially, magnetic domains have same crystal symmetry but only vary with respect to direction of magnetic (spin) moments. Domain boundaries consequently are where this magnetic (spin) frustration occurs between any two given domains. and if i'm not wrong, this is also accompanied with stresses.

and in my opinion, as amorphous materials do not have long range order, they lack the crystal symmetry through which we define grain boundary.

M. Farhan, I would disagree that grain boundaries are high strain regions. They don't necessarily have to be. In fact, often times there strain fields are virtually nonexistent. What they are is high disorder regions and, thus, locally one might even think of them as regions without defined local structure, i.e. akin to amorphous structures. Of course, this only on a very local scale. However, they are two-dimensional defects and, as such, don't have 'bulk' properties in the sense of the question that Edgar was asking. So the comparison is not possible on the terms of the question, only in terms of single (grain boundary free) and poly (with grain boundaries) crystals.

Grain boundaries (GBs) are basically regions with a slightly different arrangement of atoms with respect to the grain. This sounds simple but has a myriad implications: change of band structure, band gap, defect density, solubility and diffusivity of impurities. These in turn affect most macroscopic properties.

Also note there are two effects - that of the grain boundary itself, and that of the two grains it connects having different orientations. If the property you are interested in is anisotropic, then the latter will play a role too.

In general, grain boundaries have a more opens structure, hence less density than bulk and the number af atomic bonds are smaller (and perhaps weaker due to differnet bonding lengths).

J. Marian, sorry if I could not explain it in simple English, but what I wanted to say was that the stress in the crystals is sort of drifted towards the GB where it is relaxed by creating surfaces and defects.

also I totally agree with your point that the comparison in properties can not simply be made on the basis of single and poly crystalline arrangements; however, I was trying to elaborate the difference between grain boundary and domain boundary in reply to question asked by Doug Han.

The properties of a grain boundary are different form bulk (perfect) since the grain boundaries (GBs) are considered as 2D defects (imperfect) in crystal structures. You have to utilize some SEM, TEM, XRD and mechanical tests i.e,compression deformation to see ansd evaulate the difference.

Domains are already mentioned. More spectacular are twins which change the orientation and the local anisotropy of a property. It is a 2D defect as well but there is no lattice mismatch at all.

A grains is crystalline volumes with the same orientation. "Same" can be relative and depends on the respective problem, material etc. Therefore the classification of small and high angle boundaries has been formulated. But also these terms are not well defined and will be used to describe a qualitatively different anisotropic behaviour, i.e. deformation, condactivity etc..

From my point of view a common grain boundary is a defect which does not automatically cause strain. The atomar movement of atoms is more a "relaxation" of thsee atoms belonging to the interface. However, I havn't heart that they distort the lattice. It is more the opposit, i.e. in a material which will be deformed dislocation can only limited pass a common grain boundary so that one observes a higher dislocation density along grain boundaries. For recrystallized materials they usually disappear (if the energetically it has some benefit for the material). And for grown single phase materials only a strong anisotropic expansion coefficient could generate dislocations in a polycrystalline material if it cools down. Nevertheless, I havn't seen this personally.

Amorphous material have no far ordering like crystals, no orientation, so that the term grain is actually irritating for these materials.

Yes, the interface could be treated as another phase, see Interfaces in Crystalline Materials by A.P. Sutton and R.W. Balluffi..

Grain boundaries have a lower density, as in the same volume unit the number of atoms is reduced. See our article in researchgate: HREM-Studies of the microstructure of nano-crystalline Palladium, Scripta Met 1990. Hence, electrons considered as electromagnetic waves have different resonant frequencies. In the semiconductor community the space charge region (at p/n-interfaces) and enhanced energy barriers at grain boundaries are also discussed. Looking forward to learn about results of a changed Tc at the grain boundary region, it would be very interesting.