A crystal is the region within which the crystal lattice is continuous. In metallurgy, grain is usually used to have the same meaning as crystal. In other cases, grain can be used to denote a particle, e.g. in a powder, and once that happens it becomes more complicated because a powder particle can, but need not, consist of more than one grain. "Crystal" is well-defined but the meaning of "grain" probably has to be inferred from context in a given publication.
As mentioned by Dr. Nobre, additional complexity can arise when there are slight discontinuities in the crystal lattice, such as dislocations, stacking faults, or anti-phase boundaries, that are generally not considered to disturb the lattice enough to mark the end of one crystal and the beginning of the next (hence the talk about domains).
Dear Dr. Ameed:
Actually, there are an incredible number of publications and great part of them exhibits some multidisciplinary features. Also, specifics barriers of idiom can impose some limitations. Then, some words can exhibit a full understanding in a specific context, but seems or stay incomplete in another’s.
The word grain is proper to describe the smaller part of a polycrystalline or ceramic material. A grain can be single crystal or exhibits two or more crystalline domains, in which the crystalline lattice exhibit distinct crystallographic orientations, the boundaries of these domains define wall-domains or grain-boudaries.
In an ideal crystal, the crystalline body does not exhibits a priori multiple domains, despite that it is possible.
I suggest two chapters as basic in these concepts since this concept exhibits several nuances, and can turn complex:
Book 1:
Chapter 05 Surfaces, Interfaces and Grain boundaries
p.177-216
In: Introduction to Ceramics
Second Edition
W. D. kingery
H. K. Bowen
D. R. Uhlman
Ed. John Wiley& Sons
1990
Book 2
Chapter 2 Defects in Ceramics
p. 101-184
In: Physical Ceramics/Principles for Ceramic Science and Engineering
First Edition
Yet-Ming Chiang
Dunbar Birnie II
W. David Kingery
Ed. Wiley/MIT
1997
Best regards
Marcos Nobre
In the most easy way it can be said that crystal(s) appear during solidification, casting. Grain appears during recrystalization of crystals after deformation. So crystals for example -> dendrites, grains for example->pearlites. Of course in solid state can be the same phase in form of crystals and grains, it depend only what was the technology of manufacturing.
The particle size or garain size has recently gained more importance in nano materials where properties depend on particle size. This happens when particle size is around 10 nm. If you cut down the size of a bigger particle into nano sized particles then question comes whether the particle or we call it grain is one single crystal or has many small crystallites having boundaries and randomly oriented. If only single crystal is present, no difference between grain and crystallite size. If more than one crystal is present than grain is bigger unit than crystallites.
A crystal is the region within which the crystal lattice is continuous. In metallurgy, grain is usually used to have the same meaning as crystal. In other cases, grain can be used to denote a particle, e.g. in a powder, and once that happens it becomes more complicated because a powder particle can, but need not, consist of more than one grain. "Crystal" is well-defined but the meaning of "grain" probably has to be inferred from context in a given publication.
As mentioned by Dr. Nobre, additional complexity can arise when there are slight discontinuities in the crystal lattice, such as dislocations, stacking faults, or anti-phase boundaries, that are generally not considered to disturb the lattice enough to mark the end of one crystal and the beginning of the next (hence the talk about domains).
In my opinion, there is no difference. A grain or crystal in a polycrystalline material is separated by high- and low-angle grain boundaries.
Crystallite, a basic element in an element, size could be calculated from Scherrer's formula from the XRD pattern data. The grain might be made of few crystallites to form a particle (grain) whose size could be calculated from SEM images etc.,
As stated in some of the previous replies, in metallurgy we use the terms interchangeably. In our field the following logic applies, we view a crystal as being a region where the matrix atoms have formed a certain structure, typically BCC, FCC, or HCP, with a fixed orientation with respect to some external reference frame (in other fields, a much complex descriptions of the crystal structures can be required, and you will need familiarity with concepts such as the unit cell, which repeats to build up the crystal, the fourteen Bravais lattices, 32 point groups, and 230 space groups, symmetry elements, glides and operators). But returning to our simple cases, with respect to any arbitrary external reference system, certain directions in those crystal lattices will have this fixed orientation relationship with the external frame of reference, as stated above. Thus in a contiguous region where the repeating crystal pattern is relatively undisturbed (same/similar orientation), we would say the atoms belong to single grain of a fixed crystal orientation= a crystal. Now internally the crystal/grain might not be perfect, it may contain dislocations, point defects, twins etc. etc. Some defects may have even organised themselves into lower energy configurations, creating a small change in orientation between different internal regions of the same grain (i.e. sub grain boundaries). Even when that organisation of the dislocations is lacking, it is also possible for the crystallographic orientation to vary across a grain in a more, (macroscopically at least), smoother manner due to internal dislocations.
Anyway when crystallographic orientation changes abruptly, and by over a certain limit of misorientation, i.e. at a high angle grain boundary, then we enter another crystal/grain. Exactly where that misorientation limit should lie is not exactly defined, but 15 degrees is often used by EBSD practitioners, as being true high angle grain/crystal boundary. Boundaries are a whole topic in themselves, and as noted 15 degrees is not sacrosanct, (why 15, not 15.1, or 14.9 ?), so there is a measure of arbitrariness involved here. When we look at such structures optically (say to measure grain size) we often rely on acids to attack either the high angle boundaries themselves, or planes of different orientation at different rates. This is not always 100% effective and some boundaries are not revealed by such etching.
So when speaking to metallurgists, if you use the terms as stated above there should be no confusion. However, be careful because I suspect that this use of the terminology for grains and crystals, might not be universal across other disciplines.
DEAR DR.,
READ THIS LINK
http://nptel.ac.in/courses/113106032/5%20-%20Polycrystalline.pdf
BEST REGARD
Crystal is a geometrical concept: a seres of atoms which have a degree of order and symmetry, independently on whether they are part of a material or not. Grain is a physical portion of a material, normally a metallic material, which can havee one or more crytsals, eg different orientations due to stacking faults
Yes, the concept of crystals can obviously exist independently of there being any materials which fall into the framework which crystallographers over the years have devised to classify these materials with internally repeating structures/arrangements of atoms. But the construct was not abstract, rather it was devised to describe actual materials. According to certain authors the history of crystallography can be divided into two parts, pre 1912 and post 1912. Prior to 1912 optical methods had been used exclusively to investigate minerals and some organic compounds, laws were deduced, rules of symmetry established, 32 crystal classes identified and defined. Miller indices were developed, six crystal systems were recognised, principles of nodes/centres developed, and 14 Bravias lattices were described. Nomenclature was varied and fragmented. Then X-Ray diffraction came along and crystals were then investigated on an atomic basis. Now symmetry previously based on 'macro' properties could be based upon atomic configurations in the unit cell. No longer restricted to growth faces/external planes, the crystallographer could consider internal planes. The crystals were then further classified beyond the 32 'opticallly derived' classes. The 230 space groups among crystals could be identified, by the occurences of absences or extinctions.
It it thus legimate to say 'A' is a crystal, 'B' is also a crystal, and there is a highly sophisticated method to classify any crystal/crystalline material, which in order to understand requires a not only a knowledge of geometrical concepts, but also interaction of X-Rays with repeating atomic structures, in order to identify such crystals precisely.
And just so we do not confuse readers of these posts, common stacking faults in metals only create only a very local region with a different crystal structure (biggest is the annealing twin boundary which is visible to optical microscopy). The most common being associated with partial dislocations, nodal dislocation junctions, stacking fault tetrahedra, although larger examples can be found in stainless steels with a low stacking fault energy. Some nice examples can be found in the relevant sections of 'Electron Microscopy of Thin Crystals', P.B. Hirsch, A. Howie, R.B. NIcholoson, D.W. Pashley, and M.J Whelan. There are some nice technqiues described about how to operate the microscope to distinguish between extrinsic and intrinsic faults etc. etc. along with the relevant electron diffraction constrast theory.
But to be clear we do not generally describe these internal features within grains to be new crystals, but rather as faults within the crystal/grain, which give a local stacking of a different crystal structure. To be fair to Miguel some authors do describe them as a layer of say HCP crystal. (In the FCC case).
These stacking defects are 'invisible' to macro XRD. By using special techniques it is possible to dissasemble some polycrystalline (aka multi-grained) metals into single crystals. Each such grain would diffract to give a single crystal pattern, maybe twinned patterns if annealing twins had been present due to low stacking fault energy of the alloy.
A grain with an annealing twin is called a twinned crystal. The choice has been made to refer to the whole as a single crystal/grain, the two parts of which are linked by the twinning orientation across the twinning plane. Accepted use is not to call this two crystals or two grains.
Of course you are free not to use these accepted protocols, but you would be in the minority, and be speaking a different language to that of metallurgists and thus be misunderstood in that field.
On the other hand there could be other disciplines where a different lexicon applies, I can not help you there.
Dear Ahmed Ameed
The foregoing answers fit to your question. Nevertheless I dare to make some additional remarks.
1. The concept of grains can be applied to amorphous solid materials as well, whereas the concept of a crystal is not compatible with the amorphous state.
2. A crystal is borne by a lattice that on principle occupies the available space.
3. In procecessing of ceramics or metals by sintering the particle size determines to a large extent the properties of the final product. In some cases the particle size corresponds to the grain size, but this is not necessarily true. One can state that grain size is equal or smaller than the particle size.
4. Polymers are solid as well. Most thermoplastic polymers are semi-crystalline, which means that amorphous regions co-exist with crystalline regions; see for a more detailed discussion of polymer crystallinity
Materials Science in Design and Engineering
http://www.vssd.nl/hlf/m017.htm
As have been said by several researchers grain usually refers to a part of a crystalline material that has the same crystallographic structure and orientation, i,e, a crystal. The very thin region region between grains or crystals is called grain boundary. An exception is in powdermetallurgy where the term grain is usually used to denote a powder particle which may often contain several crystals, i.e. grains.
Micro-structure of a crystalline material composes of number of grains with their boundaries and each grain formed by one or more crystal. These materials exhibit different crystal structure " lattice structure" which defines the arrangement and the orientation of the material atoms such as HCP, BCC, and FCC crystal. Usually the physical properties of the materials are determined by their lattice structure.
In Physical Metallurgy, a single crystal implies either specially grown or naturally obtained solid piece enclosed by mostly faceted smooth surfaces. The grain implies a region in a polycrystalline and amorphous solid, which is separated from the rest by the grain boundaries. The crystal definitely implies the lattice structure with translational symmetry. The grain in a polycrystalline solid has a lattice structure similar to the single crystal but in the case of amorphous solid (like metallic glasses ) the grain has only short range order but no long range order.
In theory also we consider the crystal is ideal single orientation and structure, but in reality formed crystal metallic in two steps the first is germination or nucleation and second crystal growth in different orientation our crystal is constituted from grain crystal in different orientation.
I think most of you are not material scientists or engineers having few courses on modern physical metallurgy otherwise you guy would ask what is the difference between grains and solid materials? without specifying what do you mean by solids; the solids having crystal structure in the form of single or polycrystalline aggregates, or composite materials are those in which individual phases are bounded together as as such a manner that the average properties are determined by the individual properties of each phase. In polycrystalline metals and alloys we don't make any distinction in calling individual entities or domains as grain or crystalline grain, but as physical metallurgists we prefer to call grains, grain boundaries, grain boundary diffusion etc. In composite materials whether individual phases are crystalline or not still the grain terminology is used to describe those region having their own physico chemical identity separated from the rest by well defined boundaries (i.e., the l grain boundaries) . But the dispersed phase particles are not normally considered composite because the dispersed phase affects properties by interacting with the primary phase, rather than by contributing on its own.
When atoms or ions are arranged in a pattern that repeats itself in three dimensions, they form a solid which is said to have a crystal structure and is referred to as crystalline material. Upon solidification of metals, several stages do occur: a) formation of nuclei, b) growth of nuclei into crystals and c) joining together of crystals to form grains and associated grain boundaries. Typically grains are randomly oriented. Solidified metal containing many crystals is said to be polycrystalline. The crystals in the solidified metal are called grains and the surfaces between them are named grain boundaries.
I hope this answers your question.
Fernando Costa Oliveira
Lisboa, 3rd June 2015
Dear colleague Ameed,
Crystallite Size is Different than Particle Size. A particle may be made up of several different crystallites or just one crystallite so in this case (particle size = crystallite size)
Crystallite size often matches grain size, but there are exceptions
Crystallites are coherent diffraction domains in X-ray diffraction.
Particles are chunks/pieces (usually very small, below 1 mm) of solid matter, ensembles of atoms. Particles can be as small as two atoms (the nitrogen particle for example, N2)
Grains are volumes, inside crystalline materials, with a specific orientation.
Particles can be polycrystalline, single crystal or amorphous. A 100 nanometer particle of gold, for instance, can be made of:
a single gold crystal,
many grains with a grain size
Grain : Atoms are grouped together having same orientation.
Grain boundry : Intersection of different oriented grains.
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