- Email: [email protected]
Surface flow due to abrasion
Short Communications
363
The authors would like to express their appreciation to Dr. B. C. Deaton for providing the x-ray diffraction patterns. This research was supported by the General Dynamics Research and Development Program. Reference
1. J. H. Ashton, et al., Solid-State Diffusion Bonding--Ill, Vacuum-Hot Press Parameter Study, General Dynamics Report FZM-12-13108, Sept., 1970.
Accepted ffanuary 25, 1972
METALLOGRAPHY 5, 363-366 (1972) Surface Flow due to Abrasion H. W. KERR
Department of Mechanical Engineering, University of Waterloo, Waterloo, Canada
It has long been known that considerable damage of the surface occurs during grinding or abrasion operations. The amount of damage during metallographic preparation, in particular, has been extensively studied by Samuels and his coworkers [1]. They have been able to demonstrate that significant deformation takes place during abrasion, even on the finest abrasive papers, in the direction normal to the macroscopic surface. Much less is known about deformation in directions parallel to the macroscopic surface. Coarse abrasive grits have been shown to change the outermost surface so much that the original structure, for example pearlite, is no longer recognizable. Samuels [1] has also shown evidence of more limited flow, however, as illustrated by the bending of pearlitic lamellae close to an abraded surface. Much of the work by Samuels and his coworkers involved taper sections of abraded or polished surfaces, followed by examination with optical microscopes. Although this technique has certainly been very informative, it has limited usefulness in determining what has occurred at or extremely close to the surface. The development of the scanning electron microscope, with its large depth of focus and high magnifications, allows more complete information about surface damage to be garnered. If a two phase material with a regular structure is abraded or polished, and the major phase is then removed, then any deviations from the Copyright © 1972 by American Elsevier Publishing Company, Inc.
364
H . W . Kerr
regular structure must be due to the metallographic technique. This approach was followed using a slowly solidified specimen of Bi-Ag eutectic alloy, in which the Ag phase occurs as laths. Figure 1 shows a typical area of Ag laths following
Fla. 1. Ag laths in Bi-Ag eutectic alloy after polishing, then etching away Bi matrix. Mag. ll00x. polishing with 3/~m and 0.05/~m alumina slurries. It was then etched in a 20~o aqueous solution of nitric acid to remove the bismuth phase for some distance back from the original surface, leaving the Ag phase standing proud. It can be seen that the laths continue straight, right to their ends at the original polished surface. The slight fuzziness at the very ends of the laths had a cotton wool appearance when viewed at 20,000 X. This may be either a slight remnant of the abrasion process or a deposit from the etching and drying process. Other specimens were observed to have cleaner lath ends. When properly polished, etched and dried, there is no apparent surface flow at 20,000 x magnification. A second specimen was completely polished as described above, but then was abraded on fresh water-washed 600 grit silicon carbide abrasive paper before etching away the Bi phase. The effect on the Ag laths is shown in Fig. 2. Bending of the
Short Communications
365
FIG. 2. Ag laths in Bi-Ag eutectic alloy after abrading on 600 grit paper, then etching away Bi matrix. Mag. 2200 x. laths below the original surface can be noted, in agreement with the damage observed by Samuels. But the more obvious feature is the very considerable flow right at the original surface, which in one case extends almost from one lath to another. The edge of this flawed region is very irregular. Some areas of the lath ends appear to gradually decrease in thickness to zero, suggesting that some material is removed by a general flow or wear process. Other areas of the lath ends appear to be bitten off, and have therefore more obviously been cut by the abrasive particles. A number of questions remain unanswered by this preliminary investigation. Single crystals of face-centred cubic materials, such as silver, have long been known to show considerable ductility. T h e smearing observed here may well not be representative of other materials, which might fracture rather than shear. It is also not clear whether even such a ductile material as Ag would always behave in this fashion. Factors such as the abrasive grit size, the imposed pressure, the abrasion rate and the surface temperature (for example whether or not the papers were water-washed) would certainly affect the maximum amount of shearing at the interface. Studies of ground and abraded surfaces of other materials are in progress to investigate these aspects. This work was supported by the National Research Council of Canada. 26
H. W. Kerr
366
Reference 1. L.E. Samuels, MetaUographicPolishingby MechanicalMethods, Pitman, London (1967).
Accepted January 28, 1972
METALLOGRAPHY 5, 366-369 (1972)
Calculation of True Volume Grain Diameter
ANTHONY W. THOMPSON Metallurgy Division, Sandia Laboratories, Livermore, California 94550
Grain size is often measured by the linear intercept method, which has been shown [1, 2] to have the optimum combination of precision and efficiency. The mean linear intercept, i, is computed as
-[= ( l / g N ) ,
(1)
where l = total line length employed, M = magnification of the photograph or screen image, and N = number of boundaries intersected by the test line. This dimension, however, does not measure the true diameter of the grain volume, since the plane of polish contains the maximum diameter of few, if any , grains. It has been pointed out, in fact, that the largest difference between l and the volume diameter, d, occurs for a uniform grain size [3]. Conversion of I to true volume diameter requires an assumption about grain shape. Deferring for the moment questions of realism, the analysis is simplest for spherical grains. Fullman showed [4] that for uniform spheres of a second phase, d = 1.5 1-; in the limit, the spheres may be regarded as in contact, and thus this result has occasionally been used for grain size computations [5, 6]. Another derivation [3] also assumed spherical grains, and was performed as follows. The mean diameter of circles sectioned from uniform spheres of diameter d can be Copyright © 1972 by American Elsevier Publishing Company, Inc.
363
The authors would like to express their appreciation to Dr. B. C. Deaton for providing the x-ray diffraction patterns. This research was supported by the General Dynamics Research and Development Program. Reference
1. J. H. Ashton, et al., Solid-State Diffusion Bonding--Ill, Vacuum-Hot Press Parameter Study, General Dynamics Report FZM-12-13108, Sept., 1970.
Accepted ffanuary 25, 1972
METALLOGRAPHY 5, 363-366 (1972) Surface Flow due to Abrasion H. W. KERR
Department of Mechanical Engineering, University of Waterloo, Waterloo, Canada
It has long been known that considerable damage of the surface occurs during grinding or abrasion operations. The amount of damage during metallographic preparation, in particular, has been extensively studied by Samuels and his coworkers [1]. They have been able to demonstrate that significant deformation takes place during abrasion, even on the finest abrasive papers, in the direction normal to the macroscopic surface. Much less is known about deformation in directions parallel to the macroscopic surface. Coarse abrasive grits have been shown to change the outermost surface so much that the original structure, for example pearlite, is no longer recognizable. Samuels [1] has also shown evidence of more limited flow, however, as illustrated by the bending of pearlitic lamellae close to an abraded surface. Much of the work by Samuels and his coworkers involved taper sections of abraded or polished surfaces, followed by examination with optical microscopes. Although this technique has certainly been very informative, it has limited usefulness in determining what has occurred at or extremely close to the surface. The development of the scanning electron microscope, with its large depth of focus and high magnifications, allows more complete information about surface damage to be garnered. If a two phase material with a regular structure is abraded or polished, and the major phase is then removed, then any deviations from the Copyright © 1972 by American Elsevier Publishing Company, Inc.
364
H . W . Kerr
regular structure must be due to the metallographic technique. This approach was followed using a slowly solidified specimen of Bi-Ag eutectic alloy, in which the Ag phase occurs as laths. Figure 1 shows a typical area of Ag laths following
Fla. 1. Ag laths in Bi-Ag eutectic alloy after polishing, then etching away Bi matrix. Mag. ll00x. polishing with 3/~m and 0.05/~m alumina slurries. It was then etched in a 20~o aqueous solution of nitric acid to remove the bismuth phase for some distance back from the original surface, leaving the Ag phase standing proud. It can be seen that the laths continue straight, right to their ends at the original polished surface. The slight fuzziness at the very ends of the laths had a cotton wool appearance when viewed at 20,000 X. This may be either a slight remnant of the abrasion process or a deposit from the etching and drying process. Other specimens were observed to have cleaner lath ends. When properly polished, etched and dried, there is no apparent surface flow at 20,000 x magnification. A second specimen was completely polished as described above, but then was abraded on fresh water-washed 600 grit silicon carbide abrasive paper before etching away the Bi phase. The effect on the Ag laths is shown in Fig. 2. Bending of the
Short Communications
365
FIG. 2. Ag laths in Bi-Ag eutectic alloy after abrading on 600 grit paper, then etching away Bi matrix. Mag. 2200 x. laths below the original surface can be noted, in agreement with the damage observed by Samuels. But the more obvious feature is the very considerable flow right at the original surface, which in one case extends almost from one lath to another. The edge of this flawed region is very irregular. Some areas of the lath ends appear to gradually decrease in thickness to zero, suggesting that some material is removed by a general flow or wear process. Other areas of the lath ends appear to be bitten off, and have therefore more obviously been cut by the abrasive particles. A number of questions remain unanswered by this preliminary investigation. Single crystals of face-centred cubic materials, such as silver, have long been known to show considerable ductility. T h e smearing observed here may well not be representative of other materials, which might fracture rather than shear. It is also not clear whether even such a ductile material as Ag would always behave in this fashion. Factors such as the abrasive grit size, the imposed pressure, the abrasion rate and the surface temperature (for example whether or not the papers were water-washed) would certainly affect the maximum amount of shearing at the interface. Studies of ground and abraded surfaces of other materials are in progress to investigate these aspects. This work was supported by the National Research Council of Canada. 26
H. W. Kerr
366
Reference 1. L.E. Samuels, MetaUographicPolishingby MechanicalMethods, Pitman, London (1967).
Accepted January 28, 1972
METALLOGRAPHY 5, 366-369 (1972)
Calculation of True Volume Grain Diameter
ANTHONY W. THOMPSON Metallurgy Division, Sandia Laboratories, Livermore, California 94550
Grain size is often measured by the linear intercept method, which has been shown [1, 2] to have the optimum combination of precision and efficiency. The mean linear intercept, i, is computed as
-[= ( l / g N ) ,
(1)
where l = total line length employed, M = magnification of the photograph or screen image, and N = number of boundaries intersected by the test line. This dimension, however, does not measure the true diameter of the grain volume, since the plane of polish contains the maximum diameter of few, if any , grains. It has been pointed out, in fact, that the largest difference between l and the volume diameter, d, occurs for a uniform grain size [3]. Conversion of I to true volume diameter requires an assumption about grain shape. Deferring for the moment questions of realism, the analysis is simplest for spherical grains. Fullman showed [4] that for uniform spheres of a second phase, d = 1.5 1-; in the limit, the spheres may be regarded as in contact, and thus this result has occasionally been used for grain size computations [5, 6]. Another derivation [3] also assumed spherical grains, and was performed as follows. The mean diameter of circles sectioned from uniform spheres of diameter d can be Copyright © 1972 by American Elsevier Publishing Company, Inc.