A REVIEW OF METALLOGRAPHIC PREPARATION
A REVIEW OF METALLOGRAPHIC PREPARATIONPROCEDURES FOR NIOBIUM AND NIOBIUM ALLOYSJames L. McCallBattelle Columbus Laboratories505 King AvenueColumbus, Ohio 43201 U.S.A.IntroductionMany investigations of niobium and niobium alloys have been conductedwith the aim to control and improve the physical properties of the materialand to achieve a better understanding of its physical metallurgy. Sincemetallographic studies are frequently a vital part of these investigations,techniques for the preparation of metallographic specimens must be available,and through the years the development of these techniques has been thesubject of study of many metallographers.A standard metallographic technique has not been universally acceptedfor niobium and niobium alloys, rather each metallographer appears to havedeveloped a specific technique which "works" well for him. The combinationof reported techniques appear to have sufficed for most of the routine andspecialized metallographic studies that are made on niobium and niobiumalloys, whether utilizing optical microscopy, scanning electron microscopy ortransmission electron microscopy.The purpose of the present paper is to review some of the varioustechniques that have been used for the metallographic preparation of niobiumand to describe these techniques in a form convenient for workers in thefield. It is recognized that the preparation of metallographic specimens isstill very much in the realm of arts and crafts, and each metallographeroften finds that his individual skills enable him to achieve the most satisfactory results with a particular technique, whereas another metallographermay achieve equally satisfactory results with a different technique. Therefore, no attempt will be made to recommend the use of specific techniques inthis paper.Techniques for preparing specimens of niobium and niobium alloys forexamination by transmission electron microscopy are included with the techniques for optical metallography because many important contributions tounderstanding the physical metallurgy of niobium and niobium alloys have beenmade through the use of transmission electron microscopy. Furthermore, pastwork has shown that transmission electron microscopy in conjunction withoptical and scanning electron microscopy is almost essential when accurateand complete interpretations of the structural features of niobium andniobium alloys are required.417
Since most of the preparation techniques that have been developed foroptical microscopy normally are amenable to examinations utilizing scanningmicroscopy (SEM) , a separate discussion on SEM metallographic techniques isnot included.Since standard metallographic sectioning and mounting procedures arenormally satisfactory for niobium and niobium alloys, and these are adequately described in several references and texts familiar to metallographers,sectioning and mounting procedures will not be described in this paper.The paper is divided into the following topics:(1)Preparation of Specimens for Optical Metallographya.b.C.d.e.(2)GrindingPolishing -- Mechanical, Vibratory, Electrolytic. ChemicalMacroetchingMicroetchingChemical, Electrolytic, Cathodic VacuumAnodic Stain Etching--Preparation of Specimens for Transmission Electron Microscopya.b.Preparation of Specimens from Bulk MaterialFinal Thinning Methods -- Electrolytic, Jet Thinning, IonBeam.It i6 the author's belief that this paper represents the first attemptto document the various preparation techniques for metallographic evaluationof niobium and niobium alloys. As such, no doubt many techniques have beenomitted that perhaps have certain advantages over those described. Theauthor encourages metallographers to make known these techniques so they willbe available for consideration to metallographers involved with niobium andniobium alloys.Preparation of Specimens for Optical MicroscopyGrindingGrinding of niobium and niobium alloy specimens for metallographicexamination can be done using almost any of the numerous conventional metallographic grinding methods. No special precautions are necessary.One technique, however, has been described and recommended specificallyfor niobium (1). This technique involves two steps:(1)Coarse grinding on wet 180-grit Sic belts(2)Fine grinding on 240-, 400-, and 600-grit wet-or-dry Sic metallographic disks.The coarse grinding step is to obtain a plane surface and to remove theeffects of sectioning. Wet grinding keeps the specimen cool and flushes thebelt of loose metal and abrasive particles. Fine grinding is necessary toeliminate any cold-working effects of the coarse grinding and to produce asmoother surface preparatory to polishing. The fine grinding disks aredressed with a stick wax to keep the embedding of abrasive particles in theground surface to a minimum and for cooling and lubricating. Water also may418
be used for cooling and lubricating purposes. The direction of grinding ischanged 90 degrees between the use of each grit size to insure the completeelimination of the previous grinding scratches. It is usually best to repeatthe grinding on the 600-grit disks two or three times before proceeding topolish.PolishingSeveral different p.olishing procedures have been reported for themetallographic preparation of niobium and niobium alloys. These proceduresare essentially based on four techniques: (1) conventional mechanicalpolishing, (2) vibratory polishing, ( 3 ) chemical polishing, and ( 4 ) electrolytic polishiing. These techniques overlay somewhat, especially sinceconventional mechanical polishing usually is required before either chemicalor electrolytic polishing. However, the four techniques are discussedindividually below.Conventional Mechanical Polishing. Polishing of niobium specimensrequires great care and much longer times than would be expected from thehardness of the metal. One particular problem is that commercial grades ofniobium frequently contain hard particles, e.g. carbides, which can causeexcessive relief during polishing.A number of mechanical polishing procedures have been reported for usewith niobium and niobium alloys. One method (2) utilizes three steps.First, rough polishing is done on a dressed wax wheel using a 15-m levigatedalumina as an abrasive. This is followed by intermediate polishing on amicrocloth-covered wheel using a l-pm alumina abrasive. Final polishing isalso done on a microcloth-covered wheel, but 0.3-10 alumina is used as theabrasive.Another method ( 3 ) that has been described as being successful involvestwo steps. Rough polishing is done on a high-speed (1750 rpm), four-inchpolishing wheel covered with a low-nap cloth. The abrasive is a slurry madeof 15g fine alumina, 35 cc H 0, and 5 cc 20 percent chromic acid. Medium to2heavy pressure is exerted upon the specimen while, with an eyedropper, thecloth is charged periodically with the aluminachromic acid slurry. Polishingin this manner is continued until the grinding scratches and any embeddedabrasive particles are removed. Alternate etching and polishing (etchpolishing) may be done at, this time to aid in the removal of any disturbedlayer at the surface of the specimen. The etchant recommended is 50 cclactic acid, 30 cc HNO and 2 ccHF* which is best applied by swabbing with a3cotton ball.Final polishing is accomplished using the same slurry as described aboveon a slow speed (250-500 rpm), 8-inch polishing wheel covered with a low-napcloth. Medium pressure is used and the cloth is charged periodically usingan eye-dropper with the alumina-water slurry. As the polishing progressesthe pressure and wheel speed are continually reduced until near the end, avery light pressure and the slowest wheel speed are used.* Hydrofluoric acid is frequently an ingredient of etchants or polishingsolutions used for niobium and niobium alloys. It is very dangerous in thatit can cause severe burns if it contacts skin. Therefore, extreme care mustbe used in using it.419
A similar two-step polishing method has been described by Dillinger (4).Initial polishing is accomplished using a slurry of 0.5-m alpha alumina andwater, to which 2 or 3 ml of 10 percent chromic acid is added. A fast wheel(1450 rpm) covered with a napless cloth, such as a "lintless cloth" or nylon,is used. In lieu of chromic acid, a few drops of 10 percent oxalic acid canbe added to the polishing wheel. Dillinger reports satisfactory finalpolishing results can be obtained in several ways: ( 1 ) using 0.05vm gammaalumina on a napless cloth with a fast-speed wheel to which small additionsof 10 percent oxalic acid have been made, or (2) using 0.0511m gamma aluminain water on a microcloth-covered fast wheel.For niobium-titanium alloys, Dillinger reports that a slurry consistingof ferric oxide in 10 percent chromic acid seems to be preferable, both forintermediate and final polishing.Finally, polishing techniques utilizing diamond abrasive have beendescribed by Nelson (5) and Petzow (6). Nelson's technique consists of twosteps. Intermediate polishing uses 6-m diamond abrasive on a duck cloth.Kerosene is used as a lubricant. Polishing is carried out only long enoughto remove the scratches and damaged layers from the grinding step. Overpolishing on this step is reported to cause relief effects which will be madeworse by the finishing step. Final polishing is done on microcloth using analumina-chromic acid slurry. A s polishing proceeds the slurry is flushedfrom the polishing wheel.Nelson points out that when anwheel is used for polishing, copperspecimen. This can be prevented byprotecting the surface of the wheelcloth.acidified slurry on a bronze polishingfrom the wheel may be deposited onto theusing a stainless steel wheel or bywith a thin plastic film placed under thePetzow's polishing technique involves the use of diamond paste on apolishing wheel with a hardwood cover. He reports that for most materialsattack-polishing with 900 ml H SO and 10 ml HF (electrolytic) is of advan2 4tage.Vibratory Polishing. Kallfass and Horz (7) investigated several polishing procedures for niobium specimens doped with nitrogen, oxygen, or carbonand concluded that vibratory polishing produced the best results. They used10 to 20 hour polishing times with fine alumina abrasive on a microcloth.With very low contents of nitrogen, oxygen, or carbon even longer times wererequired.Electrolytic Polishing. A number of electrolytic polishing methods havebeen described for niobium and niobium alloys. These methods can be carriedout using a number of conventional electropolishing devices. One such devicewhich is known to work particularly well is shown schematically in Fig. 1.In this device the electrolyte is pumped over the surface of the specimenwhich tends to produce uniform polishing.Numerous electropolishing solutions have been reported for use onniobium and niobium alloys. Several of these are listed in Table I.Chemical Polishing. Chemical polishing has been reported as a usefulmethod for preparing metallographic specimens of niobium and niobium alloys.In using this method a finely ground or rough mechanically polished specimenis prepared and then simply immersed in the chosen chemical solution. Thischemical solution attacks the surface, thereby removing the grinding and420
DrainFigure 1.Schematic diagram of commercially availableelectropolishing device(s).42 1
Table I.E l e c t r o p o l i s h i n g t e c h n i q u e s f o r niobium and niobium a l l o y s .CompositionTechniqueRemarks1 ml P e r c h l o r i c Acid99 m l Methanol-76 C. Cathode- Stainless S t e e l .Determine v o l t a g e / c u r r e n t d e n s i t ycurve f o r each material. Allowspecimen t o cool i n t h e e l e c t r o l y t e b e f o r e p o l i s h i n g . Removespecimen from e l e c t r o l y t e withc u r r e n t on.Use dry i c e / a c e t o n ec o o l i n g bath.Rinscspecimen i n e t h a n o lt h e n a i r dry ( 9 ) .90 ml H SO410 m l HJ100-200 Malcm'.5-10 minutes.o r platinum.Mix slowly i nchemical hood ( 1 0 ) .5mlHSO1.25 m? I 9 3 ml Methanol24.5- 5.4 A / c mrate. 20 C.12-2OV. 35-45 C.Cathode- graphite.Medium f a s t flow50-60V,(11).10-20 s e cp o l i s h i n g s c r a t c h e s and t h e d i s t u r b e d s u r f a c e l a y e r . An a l t e r n a t i v e t oimmersion is t o use a c o t t o n swab t o rub t h e s o l u t i o n on t h e s u r f a c e .Ing e n e r a l , i t has been found t h a t swabbing provides b e t t e r r e s u l t s on niobiumspecimens because e t c h i n g is avoided and more uniform p o l i s h i n g occurs.One chemical p o l i s h t h a t h a s been used s u c c e s s f u l l y f o r niobium s p e c i mens has been d e s c r i b e d by T i t t e r i n g t o n and Simpson (12) and c o n s i s t s of:200 m l H2S04200 m l HN03100 m l HFThey r e p o r t t h e rate of a t t a c k can be r e g u l a t e d by t h e temperature oft h e s o l u t i o n , with a " warm" s o l u t i o n p r o v i d i n g t h e b e s t r e s u l t s .Another chemical p o l i s h , r e p o r t e d by Buchheit, Brady, and Wheeler, ( 1 )consists of:50 cc l a c t i c a c i d30 cc HN032 cc HFThis s o l u t i o n i s a p p l i e d by swabbing t h e s u r f a c e of t h e specimen f o r oneminute o r more w i t h a c o t t o n b a l l soaked w i t h t h e p o l i s h i n g s o l u t i o n . T h i ss o l u t i o n a l s o has been used e f f e c t i v e l y f o r e t c h - p o l i s h i n g , i.e. a l t e r n a t echemical and mechanical p o l i s h i n g .MacroetchingMacroetching i n v o l v e s t h e development of t h e s t r u c t u r e of a specimen f o rviewing with t h e unaided e y e , o r under l e n s m a g n i f i c a t i o n up t o 35X (50X i nEurope). A method r e p o r t e d ( 1 3 ) f o r doing t h i s on niobium and niobium a l l o y sinvolves g r i n d i n g t h e specimen t o 600 g r i t on s i l i c o n c a r b i d e a b r a s i v e papersfollowed by immersion i n a s o l u t i o n c o n s i s t i n g o f :30 ml H C 115 m l HN0330 m l HFEtching is r e p o r t e d t o r e q u i r e s e v e r a l (10-20)422minutes.
Mlcroe t c h i n gA number of s a t i s f a c t o r y and reproducible etching procedures have beendeveloped t o reveal t h e microstructure of niobium and niobium a l l o y s f o rmicroscopic examination. I n general, these procedures f a l l i n t o threec a t e g o r i e s ; chemical etching, e l e c t r o l y t i c etching, and cathodic vacuume tching.Chemical Etching. Chemical etching of niobium and niobium a l l o y metallographic specimens can be performed e i t h e r by immersion or by swabbing,however, best r e s u l t s a r e generally obtained by swabbing. Table I1 l i s t ss e v e r a l chemical e t c h a n t s which reportedly have yielded s a t i s f a c t o r y r e s u l t s .I n general the etchants consist of mixtures of various a c i d s , most containinghydrofluoric acid. The many v a r i a t i o n s i n e t c h a n t s f o r niobium vividlydemonstrates the d i f f i c u l t i e s t h a t are encountered i n e s t a b l i s h i n g standardmetallographic techniques f o r a given material. This i n d i c a t e s what could be" If i t doesn't work, change something,a universal law f o r metallographers:and i f i t s t i l l doesn't work, change something else", on ad infinitum. (8)Table 11. Chemical Microetches f o r Niobium and Niobium A l l o y sEtchantRemarks30cc l a c t i c acidl0cc HNo3l0cc HFSwab with cotton about f i v e seconds andinspect f o r degree of etching. Repeat f o ri n t e r v a l s of f i v e seconds each i f heaviere t c h i n g is desired. The pressure of swabbing a f f e c t s the degree of etching. TheHF is the a t t a c k i n g ingredient and may bevaried accordingly t o give more or lessetching. Rinse i n water, then alcohol, anddry i n a i r .6g FeC13Use a t 20 C.30 ml H C 1120 m l H20two minutesReference(1)Etching time about16 ml HF30 m l H20Use at 20 C(15)30 m l HN0330 m l HC115 m l HF50 m l l a c t i c acid30 ml HN0320 m l HFEtch by swabbing20 ml HNo360 m l HFEtch approximately 10 seconds50 m l H20Requires seconds t o minutes(4)50 ml HN0350 m l HF423
Table 11.Chemical Microetches for Niobium and Niobium AlloysRemarksEtchant. ContinuedReference10 ml HF10 m l HNo330 ml lactic acid15-20 seconds. Etchant must bemade fresh each time10 m l glycerol10 m l HF10 ml HNo3Requires up to five minutes50 ml H20Requires seconds to minutes20 ml HF10 ml HNo315 ml H2S0490 m l HF5 ml H205 mlReveals general microstructureHNo330 m l HF15 ml HN0330 ml HC1Swab 3-10 seconds or immerse twominutes10 m l HF10 m l HNo3Swab 5-15 seconds20 ml glycerin5 ml HFSwab 10-30 seconds20 ml HNo350 ml acetic acid50 m l HN03Swab 3-10 seconds30g ammoniumbifluoride20 ml H20HFAmmonium fluoride30 ml H2S04Various mixed solutions can beused for etching--30 ml HT3-5 drops H202 (30%)30 ml H2036 ml HN0310 ml C3H8034 d H FFor Nb-N, Nb-0, Nb-C alloys. Etchingtimes of 0 . 5 to 3 minutes. Followingetching, specimen must be neutralizedin 25 percent NaOH solution and washedwith hot water424(6)
E l e c t r o l y t i c E t c h i n g . E l e c t r o l y t i c e t c h i n g a l s o has been used successf u l l y f o r niobium and niobium a l l o y s and two such e t c h a n t s are l i s t e d i nTable 111. These e t c h a n t s can be a p p l i e d u s i n g a simple e l e c t r o l y t i c c e l lt h a t can be c o n s t r u c t e d e a s i l y u s i n g a v a r i a b l e d.c. power s o u r c e o r batt e r i e s , o r u s i n g any one of a number of commercial e l e c t r o l y t i c e t c h i n gdevices.S c h l u t e r , Honecker, and E l s s n e r (21) have a p p l i e d t h e t e c h n i q u e ofp o t e n t i o s t a t i c e t c h i n g t o r e v e a l c e r t a i n m i c r o s t r u c t u r a l f e a t u r e s of niobiumand niobium a l l o y s . T h i s is an advanced form of e t c h i n g which is r e p o r t e d t oproduce t h e u l t i m a t e e t c h i n g c o n t r a s t through h i g h l y c o n t r o l l e d c o n d i t i o n s .The p o t e n t i a l of t h e specimen, which u s u a l l y changes with changes i n e l e c t r o l y t e c o n c e n t r a t i o n , i s maintained a t a f i x e d l e v e l through t h e use of ap o t e n t i o s t a t and s u i t a b l e r e f e r e n c e s t a n d a r d s . I n t h i s way, pronouncedc o n t r a s t u s u a l l y can be obtained.The a u t h o r s a l s o i n v e s t i g a t e d t h e e f f e c t of e l e c t r o l y t e flow rate on t h ee t c h i n g behavior of niobium and showed t h a t t h e c u r r e n t d e n s i t y v o l t a g e curvee x h i b i t s a good p l a t e a u r e g i o n a t low flow rates ( 5 cm/sec), whereas ath i g h e r flow rates (30 cm/sec) t h e curve almost f o l l o w s Ohm's l a w .Table 111.E l e c t r o l y t i c Microetches f o r Niobium and Niobium A l l o y sEtchant90 ml H2S04Remarks2Reference.60 Ma/cmTime about 1-2minutes.Use a carbon o rplatinum cathode. Use a troom temperature. Mix a c i d ss l o w l y i n a chemical hood.65 m l H2012-30 V dc.1 7 ml HN03cathodeUse a platinum17 ml HFE l e c t r o l y t e flow of 5cm/sec(21)Cathodic Vacuum Etching. Armstrong, et a l , ( 2 2 ) have d e s c r i b e d ac a t h o d i c vacuum e t c h i n g method which has been used s u c c e s s f u l l y f o r niobiumand niobium a l l o y s . Although not widely used, c a t h o d i c vacuum e t c h i n g , a l s or e f e r r e d t o as i o n e t c h i n g , can be an e f f e c t i v e method f o r d e l i n e a t i n g t h es t r u c t u r e of specimens. E t c h i n g t a k e s p l a c e by t h e s e l e c t i v e removal ofatoms from t h e s u r f a c e of t h e specimen by p o s i t i v e i o n bombardment i n a glowdischarge environment. The atoms are removed a t v a r i o u s rates, depending ont h e m i c r o s t r u c t u r a l d e t a i l s such as c r y s t a l o r i e n t a t i o n of the i n d i v i d u a lg r a i n s , g r a i n boundaries, etc. The glow d i s c h a r g e is i n i t i a t e d by a p p l y i n g ahigh D.C. v o l t a g e between t h e
Anodic Stain Etching (2) Preparation of Specimens for Transmission Electron Microscopy a. Preparation of Specimens from Bulk Material b. Final Thinning Methods -- Electrolytic, Jet Thinning, Ion Beam. It i6 the author's belief that this paper represents first attempt to document the various preparation techniques for metallographic evaluation
2. Metallographic sampling of archaeological artifacts 3. Mounting and preparation of metallographic specimens 4. Etching metallographic specimens 5. Common etchants for copper, copper alloys, iron, steel, and silver 6. Principles of the metallurgical microscope 7. Format for notes on the examination of metal objects 8.
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Introduction of Metallographic Specimen Preparation Objective: 1. Metallographic Sample Preparation. 2. To study the importance of the various steps in sample preparation. 3. To understand the need of mounting, polishing and etching. Introduction: Metallography is the study of the microstructure of metals and alloys by means of microscopy.
Metallographic preparation of stainless steel Thorough diamond polishing and final . colour etching (Beraha II), showing deformation Surface of stainless steel after 3µm polish, DIC 25x showing deformation from grinding chemical, medical and food industries, in professional kitchens, architecture and even jewelry. Metallography of stainless .
After re-etching, the fine-grained ZAXE1711 alloy reveals an equiaxed grain microstructure. to develop a single, generic and robust metallographic procedure for the preparation of currently
Grinding, Polishing, Metallographic sample prepara-tion, Ductility Method Metallography is the art of preparing metallic samples by grinding, polishing and eventual etching for sub-sequent microscopic examination. Grinding and pol-ishing is to prepare the specimen surface so as to enable the microstructure to be revealed by a suit-
incumbent upon the engineer/lab technician to consider material properties and the projected sensitivity to the metallographic process. Metallography: Important Method Parameters and Intangibles Metallography is very similar to the thermal spray in that there are many variables that define the p&XeSs. It is critical to define all the important
Korean Language (Level 1) Course Code 008.199 Class Times Mon/Wed/Thu 16:00-18:00 Classroom TBA Equivalent Year Level 2 Course Credit 2 Instructor Changdeok, Hahm Sessions 1-14 Office Bld.1, Rm. 313 Email tentiger@snu.ac.kr Instructor’s Profile Changdeok, Hahm Full-time lecturer, Korean Language, Language Education Institute, Seoul National University As a Korean language teacher, Changdeok .
