( i. roBEs 31, 1918 



NATURE 



175 



THE INFLUENCE OF PROGRESSIVE COLD 

 WORK ON PURE COPPER. 



Till, hardening effeci oi thi various Forms of cold 

 work on metals and alloys has long been known 

 and utilised in the arts, and in recent years various 

 theories have been put forward to explain the pheno- 

 mena observed. Few attempts, however, have been 

 made to test whethei anj quantitative relationship 

 exists between the amount of cold work done upon 

 a metal and the magnitude of the change in its pro- 

 perties. \ serious and well-planned attempt to obtain 

 information of this kind has been made by Mr. 

 Alkins, who presented a paper at the September 

 meeting of the Institute of Metals on the change in 

 the tensile strength of copper-wire as it is progres- 

 hardened bj cold-drawing in the ordinary way. 

 Copper was chosen as the experimental material for 

 the following reasons : — 



11) The wire used in the arts is of a high degree 

 of puritv, and seldom contains 1 part of impurity in 

 1000. 



(2) It shows the hardening by plastic deformation 

 strikingly, inasmuch as its tensile strength may 

 be doubled bj cold-drawing without any indication 

 that it is actually overdrawn. 



|) It has hitherto been accepted as a metal which 

 does not possess any allotropic transformation between 

 its freezing-point and o° C. [Prof. Cohen, however, 

 holds that there is evidence of an allotropic trans- 

 formation at 71° C] 



In Mr. Alkins's experiments a billet of copper was 

 1 ast and hot-rolled to a mean diameter of 0553 in. 

 in the ordinarv way. The rolled billet was then an- 

 nealed for four hours at about 6oo° C. in order to 

 remove stresses completely, and was allowed to cool. 

 After "pickling" in sulphuric acid to remove the 

 it was cold-drawn by light drafts (twenty-five in 

 all) down to 004 in. without any further annealing. 

 From the billet after "pickling," and from the wire 

 after each draft, a few feet were scrapped from the 

 end, and three 2-ft. lengths cut for testing. The 

 tensile strength of the wires was determined on a 

 5-ton Buckton machine. Five determinations 



were made on each sample of wire, and the 

 ngS were found to be concordant within 1 per 

 cent. The mean of the five was taken as the actual 

 breaking load. The results of the tests are shown 

 in the accompanying graph, in which the co-ordinates 

 are tensile strength in tons per sq. in. and sectional 

 area in sq. in. It will be seen that the tensile 

 strength is raised progressive^ from 15-49 tons ' n 

 the original billet to 3080 tons in the wire of the 

 smallest sectional area. It will also be seen that 

 the curve showing the variation of tenacity with 

 sectional area consists of two rectilinear portions AB, 

 CD, connected by a smooth curve BEC with a point 

 of inflection at E. Mr. Alkins's analvsis of the curve is 

 as follows : — 



The portion AB corresponds with the equation 



T=3i-6-67A, 



where T = tensile strength in tens per sq. in., and 

 A = cross-sectional area in sq. in. The curved por- 

 tion BEC agrees closely with the expression 



T=23-2- \'A i) 107. 



and the upper rectilinear branch CD corresponds with 

 the equation 



T= 3 0'83-82-66A. 



According to these equations, then, from A to B 



the tensile strength increases at the rate of 67 tons per 



sq. in. for a reduction in area of 1 sq. in., while from 



C to D the rate of increase is S266 tons per sq. in. 



NO. 2557, VOL. I02] 



per sq. in. From B to E the rate diminishes to o, and 

 increases again from E to C. This curve shows no 

 discontinuity, and at ■ here a simultaneous 



diminution in sectional area and in the tensile strength. 

 There is, however, one stage in the drawing at which 

 a reduction of area of almost 10 per cent, (from 

 010927 to 0095507 sq. in.) is unaccompanied by anv 

 change in the tensile strength. iponds with 



the point E, where the tenacity equals about 232 tons 

 per sq. in. It appears, then, that over this particular 

 range a reduction in area by cold work not accom- 

 panied by any change in the tensile strength. Of 

 this phenomenon the amorphous phase theory of 

 plastic deformation does not appear to offer anv ex- 

 planation. Assuming, as Mr. Alkins does in the 

 absence of quantitative data, that the amoun 

 work actually performed on a metal during drawing 

 is measured by the decrease in cross-sectional area, he 

 is forced to the conclusion that two distinct changes 

 occur in the hard drawing of copper, one of them 

 along the branch AE and the other along the branch 



02 04 06 03 IO /2 If 16 18 -20 22 2* 2S 

 -Sectional Area in Square Inchce. 



ED. He states that he investigated several other 

 physical properties of the metal as it was drawn 

 down — for instance, density, elongation (both general 

 and at fracture), and scleroscope hardness — and that 

 all these were found to change in a similar way to 

 the tensile strength. A full account of this and of 

 further work is promised. Meantime, as a tentative 

 explanation of the results recorded, he suggests that 

 when copper is subjected to cold work by drawing 

 through dies, the first change which occurs is allo- 

 tropic in nature, and, after this is complete, a second 

 change sets in which may be either allotropi 

 explicable on the lines of the amorphous theory. 



Another set of experiments is quoted, in which 

 were drawn down from the billet by heavy instead 

 of light drafting, the reduction in area 

 plished in thirteen operations, as against -five 



in the previous set. Here also the results yield a 

 curve of the same type. It was found that over the 

 range AE the values were identical with those ob- 

 tained in the previous set of experiments, which 



