Remarks on the structure of martensites in Cu-Al alloys

• Published in: Acta metallurgica 1967-01, Vol.15 (8), p.1381-1396
• Main Authors: Sato, H, Toth, R.S, Honjo, G
• Format: Article
• Language: English
• ISSN: 0001-6160
• DOI: 10.1016/0001-6160(67)90013-2

The structures of martensites which appear in the β -phase region of Cu-Al alloys are investigated by electron microscopy and the results, together with other published results, are critically examined in terms of a general method of analysis of close packed structures with long period stacking order previously developed by the present authors. Particular examples are chosen to determine: the number of split spots ( L) included in a unit reciprocal layer spacing; the symmetry of the split spots ( H or B) resulting from the stacking order modulation; the manner of splitting of superlattice compared to normal lattice spots; the diffuseness and shift in location of diffraction spots resulting from stacking faults. These symmetry considerations are sufficient to uniquely determine the close packed structure in spite of considerable imperfections. Thus, it was confirmed that the β 1′, and γ′ martensites are stacking variants of a M = 1 structure (a long period superlattice with antiphase boundaries at each unit cell distance of a Cu 3Au type structure, often referred to as DO 22) with a stacking order which can be specified as 3 R and 2 H respectively. Taking into account the stacking order of the M = 1 structure, the specific stacking order of the 3 R and 2 H structures are AB′CB′CA′CA′BA′BC′BC′AC′AB′… and AB′AB′…, respectively. The 18-layer structure (3. R) then corresponds to that proposed by Nishiyama et al. A 22-layer structure with H symmetry which has been proposed for this structure can then be discredited from symmetry arguments alone. The 2-layer structure (2 H, γ′-martensite) has already been determined, but in addition, a martensite with a disordered atomic arrangement having a 3 R modulation ( β′-martensite) has been definitely confirmed. The diffuseness and shift of the diffraction spots due to the existence of stacking faults can be explained from the rhombohedral symmetry of the structure (Unterstruktur) and in fact aids in discriminating between the possible structures, although this has led to confusion in previous descriptions. On étudie par microscopie électronique la structure des martensites qui apparaissent dans la région de la phase β des alliages Cu-Al; on fait un examen critique des résultats, associés à d'autres résultats publiés, en utilisant une méthode générale, précédemment développée par les auteurs du présent article, pour l'analyse des structures compactes à longue période dans l'ordre d'empilement. On choisit des exemples part