562 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Limits of Elasticity, and the Hardening of Metals.* — 0. Faust 

 and (J. Tammann have submitted test pieces of various metals to tensile 

 or compressive stress. One side of the specimen, previously polished, 

 was observed microscopically during the loading ; the stress at which 

 dulling of this surface was noted, corresponding with the beginning of 

 permanent deformation, is termed the lower elastic limit, and has the 

 same value in both tension and compression. By successive compres- 

 sions with increasing loads, between which the stress was removed and 

 the face re-polished, the elastic limit was raised to a point beyond which 

 no further increase took place ; this is the upper elastic limit. The 

 hardening of metals by strain is ascribed to a diminution in the size of 

 the crystals, brought about by the formation of surfaces of slip. No 

 evidence of the existence of an amorphous phase was obtained. 



Reheating of Cold-worked Metals.f— L. Guillet has made tensile 

 tests of cold-drawn wires of hard steel, mild steel, and nickel, which had 

 been heated for 3 minutes to 100°, 250°, 300°, 400°, and other tempera- 

 tures differing by steps of 50° up to 900° C. The results showed that 

 complete annealing took place in each case between 750° and 800° C. 



Stresses in Cold-worked Metals. $ — E. Heyn and O. Bauer have 

 devised a method for the measurement of stresses existing in cold-worked 

 metals. A cold-drawn bar of nickel steel (25 p.c. Ni) was found to be 

 stressed in tension in the outer layers, and in compression in the inner 

 portion, the value of the stresses exceeding one-half of the elastic limit. 

 Such stresses disappear upon annealing. An explanation of the manner 

 in which internal stresses come into existence upon cold working is given, 

 and is illustrated with numerous examples. The specific gravity of most 

 metals is diminished by cold work, and restored to its higher value by 

 annealing. The causes of the cracking of cold-worked metals are 

 discussed. 



Simplification in Technique of Metallography^ — The preparation 

 of hard or brittle substances for microscopical examination, by the usual 

 methods, is a tedious and difficult process. Le Gris describes a method 

 of embedding minute fragments in gum lac, for polishing, etching, and 

 examination. A hole of suitable diameter (3, 6 or 10 mm.) and equal 

 depth, is bored in a small piece of brass, which is then heated and the 

 hole filled with gum lac. After cooling, the gum lac is filed level with 

 the surface of the brass ; the metal fragment is placed on the gum lac 

 and pressed into it with a heated flat metal surface to a depth of rather 

 more than half the thickness of the fragment. When cold, the prepara- 

 tion is polished by the usual methods, but as the surface of the fragment 

 is so small, the operation takes little time. In 2 or 3 minutes an em- 

 bedded fragment may be ground, polished, and etched. Filings, drillings 

 or powder may be examined in this way. Typical photomicrographs of 

 such preparations are given. 



• Zeitschr. Phys. Chem., lxxv. (1910) pp. 108-26. 



t Comptes Rendus, cli. (1910) pp. 1127-8. 



% Internat. Zeitschr. Metallographie, i. (1911) pp. 16-50 (22 fig?.). 



§ Rev. Metallurgie, viii. (1911) pp. 335-9 (6 figs.). 



