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ISDUXTRIAL A N D ENGINEERIXG CHEMISTRY
Vol. 21, No. 6
Crystalline Changes in Copper Due to Annealing’ F. C. Howard and E. T. Dunn2 UNIVERSITY O F
ILLINOIS, URBANA, ILL.
N ORDER to make copper
Crystalline changes upon annealing a rod of coldof the atoms in primary cryssuitable for certain purworked copper have been investigated using a strong tals giving rise to “secondary poses, i t i s sometimes etching reagent. The laminated structure of etched crystals.” Primary crystals necessary to increase its tenare thosewhich have solidified crystals of copper which because of its regularity is probsile strength by cold-working. ably due to some physical property has been photoin such a way that they are Cold-working of a metal prographed a t relatively high magnification. Two recomunstrained and do not contain duces deformations and sets mendations are made for further work. twin crvstals. The Dhenomeup strains by the very nature non is” confined to various of the process. Strains are relieved by giving the metal a suit- types of secondary crystals, and therefore appears to be related able heat treatment or annealing. By annealing is understood to what may be called “atomic rearrangement.” Twinned all heating of the specimen in the equilibrium area of its exist- crystals in copper which are present in the originally colding phase. Annealing causes the grains to grow in size; worked metal are not removed either by recrystallization or consequently there is a diminution in the total number of by subsequent growth, although during the former process grains, the small grains disappearing a t the expense of the all the old crystals are replaced by new ones and during the larger grains. Annealing causes the recrystallization of the latter a large number of crystals are absorbed into a few large mass.3 This recrystallization increases with increase of tem- ones. Twin crystals develop by boundary migration and perature and time. Thus, it may readily be seen that a study growth occurs until crystals meet with sufficient resistance. of the changes in the structure of annealed copper would be Twin crystals of copper cannot be removed on heating a t of great importance. 1050” C. for several hours, though outlines change slowly The purpose of this work was to see if any more facts could and tend t o become smoother and more perfect. The capacbe brought out, by means of a strong or drastic etching re- ity for forming annealing twins appears to be closely related agent, concerning the crystalline changes occurring during to the atomic arrangement in the crystal lattice of the metal. Lucas8 says: “As the strained specimen is heated there prothe annealing of a rod of cold-worked copper. ceeds a removal of the strained condition and distorted strucHistorical ture. You get a perfect keying of the crystalline elements of one grain into the crystalline elements of an adjoining grain. So-called etching figures have been noted for a long time Twinned areas gradually develop by a reorientation process.’’ when polished metal surfaces are etched with certain chemical Rowdong states that the origin of these twinned forms is the reagents. Sauveur4 gives drawings of typical figures which rotation of the crystalline units as a consequence of the side usually take the form of squares, triangles, and angles. In of “growth.” However, this work was concerned with the the case of copper when etched with nitric acid, odd, regular, formation of twins in very thin layers of copper electrolytiand systematic etching figures are observed. Pulsifer5 gives cally deposited. the name “cells” to grain and “granules” to the laminations seen with a nitric acid etch. He further states that the conMaterial and Method stants determining the size of the laminations or the distance Only one rod of cold-worked copper was used. It was between strata or the sizes of cubic figures are a t present unknown. The granules are possibly determined by some drawn cold and annealed a t 749” C. to successively lesser physical constant and have more significance than merely the diameters by the manufacturer. However, in the final operaprogressive outline of the solution force. Some support for tion it was cold-drawn to l/z inch round in one operation and the conception that the ultimate granules are physical sub- left hard. The copper rod was held a t 700” C. in a small electric furdivisions comes from the observation that the offsets in worked or slipped stock are of the same order of magnitude as nace for a definite time. This temperature was used as the granules. There is no hint of any more minute texture higher temperatures lessen the mechanical properties.’O or smaller order of crystalline units. This, of course, means The specimen would then be allowed to cool in the air. The that the chemicals used for etching have too gross an effect end of the rod, transverse section to the direction of rolling, was polished for examination and etched. After examination to disclose any demarcation even if it should exist. Carpenter and Tamura6 grew large crystals (4 inches long) the specimen was replaced in the furnace and held there for of commercially pure copper by producing critical strain and a further definite period. Thus, for a period of 4 hours a t then giving an appropriate heat treatment. They found all 700” C. the specimen was heated three times as detailed by their crystals to contain numerous twins which were oriented the captions under the accompanying microphotographs. in as many as three directions. Later these same men’ Discussion of Results studied the formation of twinned metallic crystals. They state that metal crystals may be formed by a rearrangement Figures 1 and 2 show quite clearly the contrast in the ap1 Received January 25, 1929. pearance of the specimens etched with different reagents. 2 Present address, U. S. Tariff Commission, Washington, D. C. With concentrated nitric acid, the grain boundaries, twinning, a Guillet and Portevin, “Metallography and Macrography,” p. 71, G. etching pits, and fine grain structure are observed. With Bell and Sons, Ltd., London, 1922. 1 Sauveur, “Metallography and Heat Treatment of Iron and Steel,” ammonia and hydrogen peroxide, twinning and etching pits,
I
p
90, University Press, Cambridge, 1916. 6 Pulsifer, Trans. Am. Inst. Mining Met. Eng., 1926, No. 15243. 8 Carpenter and Tamnra, Proc. Roy. SOC.(London), 1138, 28 (1926). 9 Carpenter and T a m u r a , I b i d , l l S A , 161 (1926).
Lucas, J. Franklin Inst., 201, 177 (1926). Rowdon, Mer. Chcm. Eng., 16, 406 (1916). 10 Hoyt, “Metallography,” Part 11, p. 14, McGraw-Hill Book 8 0
CO.,
1820.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
June, 1929
Figure I-ConcentratedNlrric
Acid Etch. 150
Figure 3--OriL?inal Specimen.
x
1300 X
and in some cases grain boundaries, are observed, but in no case is there the same type of fine structure as when nitric acid was used. The succeeding specimens m r e etched with concentrated nitric acid. The microphotographs show clearly how grain growth con-
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Figure 2-Same Specimen 98 In Figure I RegaUahed end Etched with Ammonia and Hydrogen Peroxide. 150 x
Figure 4-After
3 Hours at 700° C. 1300
x
tinues with repeated heating. The structure of the original specimen is shown in Figure 3. A study of the picture gives further insight into the effectof heat treatment on the distance pits, grain between laminations, size of etchinR~. -growth, and orientation.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
552
Figure 5 - A f t u
Figure 7-After
2500
x
40 Hours a t 700" C. 1300
x
4 tloure s t 700'C.
The so-called "triangular etching pits" are shown clearly in Figures 4, 6 , and 7, and i t will be noted that one or two sides are practically parallel with laminations in the grains themselves, and in most cases at extreme magnifications they
Figure 6-After
Vol. 21, No. 6
24 H o w 8 a t 700° C. 1300
x
Figure 8-After 80 Hours at 7001 C and Ala0 a Further Treatment far b Hours at l00b' C. 1300
x
themselves show laminations.'* Figure 5 shows afine, regular, saw tooth structure observed in some of the grains. Dun% "Heat Treatment of Coid Rolled Copper? The&, University of Illinois. 1928.
I N D U S T R I A L A N D ENGINEERING CHEMIXTRY
June, 1929
The striking regularity and uniformity of the laminations must be of some r e d significance, and are probably due to some inherent physical characteristic of the grain. Figure 6 shows a number of evidences of twinning, and in one case the twinning has included one of the triangular etching pits. Figure 7 is of particular interest to users or workers of metal in that it shows that often under such drastic heat treatment grains of copper are still susceptible to chemical attack and have not returned to the original cast condition. Some of the lines are much darker than others, which leads one to speculate whether or not these may not be the former grain boundaries that have disappeared to form the new grains. After 80 hours of heat treatment a t 700” C., laminations were still visible and the result of further heat treatment of 6 hours of 1000” C. is shown in Figure 8. This photograph indicates that probably there was a slight flow of metal and that the original laminations have disappeared, but there is still an indication of a laminated structure shown by circular dark lines of a marked degree of regularity and uniformity, which might be due to the distortion of these laminations with some “healing,” or else to new strains being set up by the flow of material.
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Conclusions
1-An etching reagent has been used that brings out crystal details. 2-Twinning is observed in t,hree directions. 3-Details of laminated structure in etched crystals are brought out only a t high magnifications. &--The constantly recurring regularity and uniformity of the laminations in etched crystals may be due to some physical property of the substance. 5-If the angle a t which a crystal was oriented with respect to the microscope could be measured and the distance between the laminations determined, the crystal space lattice or some multiple might be found. 6-The complete removal of strains and distortions as shown by x-ray diagrams should be correlated with some corresponding microphotographs. Thus, by looking for some distinctive standard, as grain count, size of crystals, or regularity of outline of secondary crystals, the more piercing tests of the x-ray would be indirectly applied through microphotographs and visual microscopical examination.
Some Observations on Rubbers with Low Nitrogen Content’ A. I). Cummings and L. B. Sebrell THEGOODYEAR TIRE& RUBBERCOMPANY, AKRON,OHIO
INCE it was first discovered that rubber contained protein, many researches have been carried out to determine just what part this material played in the structure and properties of the rubber. The present work shows that the protein is of less importance than has often been supposed.
S
Rubber has been prepared protein-free, although not entirely nitrogen-free, by digesting latex with caustic soda, as recommended by Pummerer and Pahl. Rubbers with a nitrogen content of from 0.004-0.0096 per cent have been prepared by a slight modification of the original procedure. This rubber can be compounded and cured to give good quality vulcanizates, which compare very favorably with the controls. Protein-free rubber, when acetone-extracted, becomes nitrogen-free. This can still be vulcanized, although it cures slowly. The rate of cure of rubber from protein-free latex is affected very little by the pH of the coagulating medium whereas with full-nitrogen rubber rate of cure varies considerably with change in pH value at the time of coagulation. It was also found that protein-free rubber could be racked. Data on the preparation, coagulation and nitrogen content as well as the vulcanization results of protein-free rubber are presented. The observations lead to the‘conclusion that protein is not the key to the explanation of the physical properties of rubber.
Note-Some confusion exists regarding t h e terms “protein-free” and “nitrogen-free.” Most of the published articles fail t o mention what method was used for proving t h e absence of nitrogen. Consequently i t is often difficult t o tell whether t h e terms m a y be synonymous. I n this paper “nitrogen-free” will be used only with reference t o rubber containing a n amount of nitrogen which cannot be detected by sensitive colorimetric methods. Rubber containing a known amount o r a n unstated percentage above 0.001 per cent will be called “nitrogen-poor” or “protein-free.”
Many attempts have been made in this laboratory to prepare pure rubber hydrocarbon free from nitrogen or protein and to test its physical properties when vulcanized. This seemed more important than ever in view of the work reported by Dinsmore2 in which it was found that rubber coagulated from latex under varying p H values showed widely different rates of cure and physical properties. The effects noted were 1 Presented before the Division of Rubber Chemistrv a t t h e 76th .Meeting of t h e American Chemical Society, Swampscott, Mass., September 10 t o 14, 1928. 1 Dinsmore, IND. ENG. CHEM., 18, 1140 (1926).
attributed to different conditions of the rubber protein as determined by the pH value of the latex a t the time of coagulation. However, m o r e definite proof seemed desirable and it was decided to repeat in part the work mentioned above using proteinfree latex prepared according to the method of Pummerer and Pahl.3 Previous investigators have stated, as noted below, that they have prepared nitrogen-free rubber, but there are few, if any, data available in the literature as to the exact procedure used in determining the absence of nitrogen or of protein, or as to the physical properties of the vulcanizates formed from such rubbers. For these reasons it was decided to carry out the present research. Historical
Beadle and Stevens4have investigated the influence of the insoluble constituent on the physical properties of vulcanized rubber. They prepared from smoked sheet a sample of rubber poor in nitrogen and another rich in nitrogen. The nitrogenpoor rubber combined with sulfur very slowly and gave a poor vulcanizate, while the nitrogen-rich sample vulcanized more rapidly than the control and gave better physical properties. 8
4
Pummerer and Pahl, Ber., 60, 21.52 (1927). Beadle and Stevens, I n d i a Rubber J., 44, 554, 603 (1912).
