436 SUMMARY OF CURRENT RESEARCHES RELATING TO 



1200° C. ; (3) heated to 1200° C, cooled to 600° C, and then quenched ; 

 and (4) cooled in air from 1200° C. Steels with the higher carbon and 

 molybdenum contents showed white polyhedra when quenched from 

 1200° 0. 



Elastic Limit of Alloys.* — A. Portevin points out that the only 

 method of ascertaining when the elastic limit is reached at a particular 

 point of the surface of a stressed specimen, is by microscopic observation 

 of the previously polished surface. Slip-bands appear within the crystals 

 as soon as the elastic limit is exceeded. In the author's experiments 

 pyramidal specimens of cast alloys were submitted to comjDression, one 

 polished face being under microscopic observation. In an alloy con- 

 sisting of a homogeneous solid solution slip -bands first appear in a few 

 crystals ; as the stress rises the number oi crystals affected increases 

 until they all show slip-bands. By examination of a stressed pyramidal 

 specimen, a lower elastic limit — corresponding to the stress on the cross 

 section at which slip-bands have just begun to appear — and an upper 

 elastic limit — corresponding to the stress on the largest section at which 

 all the crystals are affected — may be distinguished. In an alloy which, 

 consisting of a chemically heterogeneous single solid solution, possesses 

 a cored structure, the elastic limit of the outer layers of a crystal commonly 

 differs from that of the inner portion, and thus slip-bands may appear 

 in the central part of a crystal and not extend as far as the boundaries. 

 As the stress increases the slip-bands increase in length until the crystal 

 boundary is reached, thus demonstrating the cry stall ographic unity of a 

 chemically inhomogeneous crystal. Copper-nickel and copper- tin alloys 

 were observed to behave in this manner. In an alloy consisting of two 

 phases, such as the alloy containing 57 p.c. copper, 43 p.c. zinc, the 

 elastic limit of one (a) phase may be reached much earlier than that of 

 the other (/3) phase, and the behaviour is then still more complex. The 

 idea that the elastic limit of a metal or an alloy can be truly expressed 

 by a single value is thus seen to be illusory. 



Influence of Annealing upon the Structure of Alloys, t — A. 

 Portevin classifies systematically the structural effects of annealing, by 

 which operation is meant heating followed by slow cooling. Annealing 

 is employed (1) to remove or diminish the effects of quenching ; (2) 

 to remove the effects of cold-work ; (3) to modify the structure of cast 

 or overheated material. If the annealing is wholly within a field of 

 the equilibrium diagram, it tends to restore (1) mechanical equilibrium, 

 by removing internal stresses ; (2) physico-chemical equilibrium ; solid 

 solutions become homogeneous and unstable phases disappear ; (3) struc- 

 tural equilibrium ; stable phases become visible microscopically, under- 

 go coalescence, and become clearly separated when two or more are 

 present ; (4) crystalline equilibrium ; recry stall ization and growth of 

 crystals occur. If, during the annealing, transformation lines of the 



* Comptes Rendus, clvi. (1913) pp. 1237-40 (6 figs.), 

 t Rev. Metallurgie, x. (1913) pp. 677-721 (38 figs.). 



