What is a twin and what is a “twin”?

What is a twin and what is a “twin”?

ACTA 58 METALLURGICA, What is a twin and what is a “twin”?* Hitherto crystallographers and metallurgists have had a rather exactly defined notion o...

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What is a twin and what is a “twin”?* Hitherto crystallographers and metallurgists have had a rather exactly defined notion of the concept of a twin. Details can be found in many textbooks. An essential condition for a twin assemblage was among other things that the twinned parts have the same crystal structure. Setting out from this classic concept, the present writer discussed(r) the iniluence of order/disorder in the case of mechanical twinning. In the light of this discussion it appears improbable that Fe&e (DOS-type structure) can be mechanically twinned. However, Bolling and Richman in a recent paper@) disagree with this conclusion and state that “the ~redict,ion that ordered cubic crystals should not be capable of twinning is negated for all practical purposes by the deformation of FeaBe”. B. and R. refer to the paper(l) mentioned. Now the result achieved by B. and R. in their experiments is the production of an unstable orthorhombic Fe,Be from a stable cubic Fe&e (DO,-type structure). The unstable material was produced under load by a shearing process geometrically analogous to a process leading to mechanical twins. As was to be expected such unstable material tends to resume its stable modification on unloading. Thus the process described by B. and R. (as indeed the authors themselves admit) does not produce a twin at all, and for this reason the present writer’s prediction is by no means negated. It is> in fact, substantiated. It seems most unfortunate to coin a term “metallurgical twin”, as B. and R. propose, for a structure that actually is not a twin. It is suggested, therefore, in the interest of clarity and to avoid misstatements, to drop the term twinning in connection with processes leading to a transformation into an unstable form (cp. Laves(3s4))and to use some other expression such as shear and unshear. If the words “twin” and “untwin” are actually used to designate the process discussed by B. and R. they should always be placed between inverted commas. On the other hand, a term pseudo-twin might be introduced perhaps for such processes.



~~t~t~t fiir ~~~~ta~log~a~h~~urd P~t~~ra~h,~e ~~~g~~~is~~~~heTechn~~~he Ho~h~ch~le~ Ziirich. References 1, F. LAVES, n’atu,rwiss~nschaften 39, 2. G. F.

546 (1852).

arld R. H. RICHMAN,

Attn. Met. (1965). 3. F. LAVES, iVaturwi8senschuften 39, 5461547 (1952). 4. F. LAVES,A~. Miner. 50,511(1965). BOLLING

* Received July 9, 1965.

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to “What is a twin and what is a ‘twin’?“*

The criticism offered by Professor Laves is quite proper.(i) Our reason for the unmodified wording “twin” and other similar termsc2) in describing the stress-induced shear process and its results in Fe,Be, was two-fold. Brevity was achieved, and the deformation charac~ristics were emphatically identified with the shear process in the disordered alloy. From discussion with colleagues, it had become evident that if a term such as “pseudo-twin” was used, the cursory listener (or reader) was left with the impression that the characteristics identified as CMT appeared only in a special material. That is, the information contained in the investigation of the ordered material seemed to overwhelm, by its novelty, the implications contained in the investigation of non-special materials. The original prediction by Professor Laves(s) that ordered cubic crystals should not be capable of mechanical twinning, is of course correct. Our intent in stating that the prediction is “negated for all practical purposes” was clarified, and was meant to imply that all the dynamic characteristics attributable to mechanical twinning migh t be attained by the “pseudotwinning” in Fe,Be. G. F. BOLLIN~ R. Ford



XcientiJic Laboratory



References 1. F. LAVES, Aeta Met., this issue, preceding letter. 2. G. F. BOLLING and R. H. RICHXAI=J. Acta J!let. (1965). 3. F. ~dAVES, Naturwissenschaften 39, 546 (1952).

13. 709

* Received July 19, 1965.






Balluffi and Siegel(i) have shown that the change of slope technique of determining activation energies yields values which can be interpreted in terms of a combination of the energies of motion and binding of defects only under highly restrictive conditions. In the present letter the results of an investigation of the annealing of defects in quenched nickel are reported. These results exhibited transient behavior during a change of temperature which precluded the use of the change of slope technique to determine the E, of the annealing defects. High purity Ni [(p2730K/p40K) = EOO] was quenched from 1200°C into brine at approximately [email protected]/sec initial ouenching rate. The resistivitv was measured