It is generally known that lower stacking fault energy materials promotes growth of annealing twins during recrystallisation. Why should stacking fault energy influence recrystallization behaviour in low SFE materials?
There's a simple, straightforward answer: A twin is basically half a stacking fault. Suppose you're looking at an fcc material. The pure crystal has a stacking order ABCABCABC..., while for a stacking fault you skip a beat and then continue the pattern, e.g. ABCBCABCABC... So notice that near the beginning you have two B planes that are only two steps apart from each other and two C planes that also are two steps apart, unlike the 3 steps apart that like planes have in fcc. So you have two planes of atoms that are coordinated as hcp instead of fcc. While the distributions of first-neighbor distances are the same, the distributions of second-neighbor distances are not, and there's a difference in energy because of second-neighbor interactions. In a low-SFE material, the difference in energy between hcp coordination and fcc coordination is small.
In a twin, the stacking order just reverses at some point, e.g. ABCABCBACBA. In this case you have only one hcp-coordinated layer, namely the C in the middle that's surrounded by two B's. So in terms of second-neighbor interactions, a twin boundary is half a stacking fault. If one has low energy, so does the other. Low twin energy, low activation energy to form a twin, high propensity to twin during recrystallization.
In my example I'm talking about a coherent twin in an fcc metal, but similar sorts of things happen in other crystals.
High SFE materials are expected to show quick and easy
dynamic recovery from Arhenius equation than lower SFE materials and low SFE materials can also be expected to undergo twinning which means more strain hardening. However, low dynamic recovery results in high stored energy because of the strain hardening. This high energy combined with twinning accentuate nucleation of new grains during the recrystalliation process resulting in fine grains and high strength from Hall- Petch equation.
There's a simple, straightforward answer: A twin is basically half a stacking fault. Suppose you're looking at an fcc material. The pure crystal has a stacking order ABCABCABC..., while for a stacking fault you skip a beat and then continue the pattern, e.g. ABCBCABCABC... So notice that near the beginning you have two B planes that are only two steps apart from each other and two C planes that also are two steps apart, unlike the 3 steps apart that like planes have in fcc. So you have two planes of atoms that are coordinated as hcp instead of fcc. While the distributions of first-neighbor distances are the same, the distributions of second-neighbor distances are not, and there's a difference in energy because of second-neighbor interactions. In a low-SFE material, the difference in energy between hcp coordination and fcc coordination is small.
In a twin, the stacking order just reverses at some point, e.g. ABCABCBACBA. In this case you have only one hcp-coordinated layer, namely the C in the middle that's surrounded by two B's. So in terms of second-neighbor interactions, a twin boundary is half a stacking fault. If one has low energy, so does the other. Low twin energy, low activation energy to form a twin, high propensity to twin during recrystallization.
In my example I'm talking about a coherent twin in an fcc metal, but similar sorts of things happen in other crystals.
The answer to this query involve certain terms which are briefly explained along with as follows(A lot more is available in the literature ).
[A]The two primary methods of deformation in metals are SLIP and TWINNING . SLIP occurs by dislocation glide of either screw or edge dislocations within a slip plane. It is by far the most common mechanism.
[B]TWINNING is less common but readily occurs under some circumstances.A TWIN is a very large stacking fault*. Twinning occurs when there are not enough slip systems to accommodate deformation and/or when the material has a very low SFE [Stacking –Fault Energy-γSFE] ( J/m^2). A material with high SFE such as Al(160- 200J/m^2)** will be less susceptible to twinning than materials with low stacking-fault energy like Cu(70-78J/m^2). Twins are abundant in many low SFE metals like copper alloys, but are rarely seen in high SFE metals like aluminum because when the SFE is low, the mobility of dislocations in a material decreases.
[C]IN FACT it is due to the two mutually related characteristics- Galling and Stacking-Fault Energy(SFE).Galling is a form of wear caused by adhesion between sliding surfaces so that some metal is pulled with the contacting surface due to a large force compressing the surfaces together causing simultaneous friction and adhesion in between the surfaces followed by slipping and tearing of crystal structure beneath the surface. This leaves some material stuck to the adjacent surface while the galled material appears as torn lumps. Some metals are more prone to galling due to their crystal str ucture and ductility. No doubt, both Al and Cu are FCC but Al being softer is easy to galling ,i.e. has a greater tendency to produce dislocations.
*The normal stacking pattern is ABCABC--- in FCC but the stacking fault may change it to ABCBC---. Lower SFE materials display wider stacking faults and have more difficulties for cross-slip and climb. The SFE modifies the ability of a dislocation in a crystalto glide onto an intersecting slip plane. When the SFE is low, the mobility of dislocations in a material decreases.
** Galling is more in crystals with high SFE.
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