ASTM E963-95 (Reapproved 2010)

Designation: E963 − 95 (Reapproved 2010)Standard Practice forElectrolytic Extraction of Phases from Ni and Ni-Fe BaseSuperalloys Using a Hydrochloric-Methanol Electrolyte1This standard is issued under the fixed designation E963; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers a procedure for the isolation ofcarbides, borides, TCP (topologically close-packed), and GCP(geometrically close-packed) phases (Note 1) in nickel andnickel-iron base gamma prime strengthened alloys. Contami-nation of the extracted residue by coarse matrix (gamma) orgamma prime particles, or both, reflects the condition of thealloy rather than the techniques mentioned in this procedure.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. (See 3.3.2.1 and4.1.1.)NOTE 1—Ni3Ti (eta phase) has been found to be soluble in theelectrolyte for some alloys.2. Terminology2.1 Definitions:2.1.1 extraction cell—laboratory apparatus consisting of abeaker to contain the electrolyte, a dc power supply, a noblemetal sheet or screen cathode and a noble metal wire basket orwire to affix to the sample (anode).2.1.2 geometrically close-packed (GCP) phases—precipitated phases found in nickel-base alloys that have theform A3B, where B is a smaller atom than A. In superalloys,these are the common FCC Ni3(Al, Ti) or occasionally foundHCP Ni3Ti.2.1.3 topologically close-packed (TCP) phases— precipi-tated phases in nickel-base alloys, characterized as composedof close-packed layers of atoms forming in basket weave netsaligned with the octahedral planes of the FCC γ matrix. Thesegenerally detrimental phases appear as thin plates, oftennucleating on grain-boundary carbides. TCP phases commonlyfound in nickel alloys are σ, µ , and Laves.3. Significance and Use3.1 This practice can be used to extract carbides, borides,TCP and GCP phases, which can then be qualitatively orquantitatively analyzed by X-ray diffraction or microanalysis.23.2 Careful control of parameters is necessary for reproduc-ible quantitative results. Within a given laboratory, such resultscan be obtained routinely; however, caution must be exercisedwhen comparing quantitative results from different laborato-ries.33.3 Comparable qualitative results can be obtained routinelyamong different laboratories using this procedure.34. Apparatus4.1 Cell or Container for Electrolyte— A glass vessel ofabout 400-mL capacity is recommended. For the sample sizeand current density recommended later in this procedure,electrolysis can proceed up to about 4 h, and up to about4gofalloy can be dissolved in 250 mL of electrolyte withoutexceeding a metallic ion concentration of 16 g/L. Above thisconcentration, cathode plating has been observed to be morelikely to occur. A mechanism for cooling the electrolyte isrecommended. For example, an ice water bath or water-jacketed cell may be used to keep the electrolyte between 0°and 30°C.4.2 Cathode—Material must be inert during electrolysis.Tantalum and platinum sheet or mesh are known to meet thisrequirement. Use of a single wire is to be avoided, sincecathode surface area should be larger than that of sample.Distance between sample and cathode should be as great aspossible, within the size of cell chosen. For example, a samplewith a surface area of 15 cm2should have no side closer than1.2 cm to the cathode. If the cell is cylindrical, as for the caseof a beaker or the upper part of a separatory funnel, the cathodecould be curved to fit the inner cell wall to facilitate correctsample-cathode distance. The sample would then be centered1This practice is under the jurisdiction of ASTM Committee E04 on Metallog-raphy and is the direct responsibility of Subcommittee E04.11 on X-Ray andElectron Metallography.Current edition approved April 1, 2010. Published May 2010. Originallyapproved in 1983. Last previous edition approved in 2004 as E963 – 95 (2004).DOI: 10.1520/E0963-95R10.2Donachie, M. J. Jr., and Kriege, O. H., “Phase Extraction and Analysis inSuperalloys—Summary of Investigations by ASTM Committee E-4 Task Group I,”Journal of Materials , Vol 7, 1972, pp. 269–278.3Donachie, M. J. Jr., “Phase Extraction and Analysis in Superalloys—SecondSummary of Investigations by ASTM Subcommittee E04.91,” Journal of Testingand Evaluation, Vol 6, No. 3, 1978, pp. 189–195.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1within the cell at the same height as the cathode. The cathodeneed not make a complete ring around the sample nor be morethan 5 cm high.4.3 Anode—The sample must be suspended in the electro-lyte by a material that is inert during electrolysis. Anodeconnection material should be cleaned to prevent any contami-nating material from falling into the cell. Good electricalcontact should be maintained between the sample wire and thepermanent anode wire from the dc power supply. Two methodsare found to be successful. Either method is subject todisconnection of the sample due to shrinkage, which puts alimit on the electrolysis time:4.3.1 Suspend the sample by platinum or platinum-rhodiumthermocouple wire (20 gauge) wrapped around it to form abasket. To avoid a shielding problem, the ratio of sample areacovered by the wire to the exposed sample area should besmall.4.3.1.1 Mechanically attach or spot weld the platinum orplatinum-rhodium thermocouple wire to the sample.4.3.2 If the weld is not immersed, non-inert wire may besubstituted; for example, chromel, nichrome, 300 series stain-less steel, etc. Stop-off lacquer should be used below themeniscus to maintain constant electrolyte level. This alsoeliminates formation of insoluble deposits immediately abovethe meniscus and prevents arcing.4.3.2.1 Warning—Care must be taken to prevent arcingbetween anode and cathode which could ignite the methanol.4.4 Power Supply—A variable dc power supply capable ofproviding 0 to 5 V is needed to obtain currents from 0 to 1.2 Adepending on total surface area of the sample. For example, asample with total surface area of 15 cm2, electrolyzed at acurrent density of 0.1 A/cm2, requires:15 cm230.1 A/cm25 1.2 A (1)4.4.1 Current and voltage fluctuation should be no morethan 65%.A65 % current fluctuation represents a currentdensity fluctuation of about 65 % which, for samples under 15cm2total surface area, is less than or equal to one-half thecurrent density shift due to sample shrinkage over 4 h.Potentiostatic control is not necessary, but may be helpful fordetermining optimum current density when setting up proce-dures for a new alloy.4.5 Membrane Filter—Must be solvent and electrolyteresistant, with pore size of 0.4 to 0.8 µm. Filters made ofpoly(vinyl chloride) (fibrous) or polycarbonate (nonfibrous)meet these requirements and are available commercially, as aresuitable filter holder assemblies. Mass loss for these materialsin 10 % HCl-methanol is 10 %. The 2.5-cm diameter size isuseful for preparing the residue for the X-ray diffractometer,which is commonly used for phase analysis of the residue.Otherwise, filter diameter is not critical. Filters should behandled with blunt tweezers.4.6 Centrifuge—Centrifuging for residue collection can beperformed as an alternate to microfiltration.4.7 Balance—If quantitative analysis is desired, a balancesensitive to 0.0001 g is required.5. Reagents5.1 Electrolyte—Add and mix 1 part of 12 N hydrochloricacid (sp gr 1.19) to 9 parts of absolute methyl alcohol byvolume to make a 10 % HCl-methanol solution. For alloyscontaining W, Nb, Ta, or Hf, add one part by weight tartaric orcitric acid to 100 parts by volume HCl-methanol to make anapproximately 1 % tartaric or citric acid solution. All reagentsshould be of at least ACS reagent grade quality.5.1.1 Warning—Add hydrochloric acid to absolute methylalcohol slowly and with constant stirring; otherwise sufficientheat is generated to cause a hazardous condition. Mixing mustbe done in an exhaust hood, because the fumes are toxic.5.2 Sample and Residue Rinse—Absolute methyl alcohol isto be used.6. Procedure6.1 Sample Size and Geometry—A cube, cylinder, or rect-angular prism is preferred. Ideally, constant density should bemaintained during electrolysis. Flattened samples, especiallythin sheet, will experience considerable shrinkage due to edgeeffects and current density increase as the electrolysis pro-ceeds. A cube approximately 1.6 cm on a side will have a totalsurface area of approximately 15 cm2. Smaller samples havelarger increases in current density during constant currentelectrolysis due to shrinkage. Larger samples may require morethan 250 mL of electrolyte and a power supply capable ofFIG. 1 Schematic Diagram of Extraction CellE963 − 95 (2010)2delivering more than 1.2 A. Samples requiring higher totalcurrent may cause a cathode plating problem due to the highervoltage required, and may make a cooling mechanism abso-lutely necessary.6.2 Sample Preparation—The sample must be free of allsurface contamination that could be mistakenly identified asincluded material extracted from the bulk alloy. Two methodsknown to be useful are as follows: (1) Grind all surfaces to 120grit. This method is not recommended for porous sampleswhich may become imbedded with grit material. An advantageof the method is the removal of surface cracks and irregulari-ties; or (2) Perform a light etch cleaning which does notsubstantially alter the surface. A short electroetch with thesame electrolyte and current density as used for the actualextraction is suitable.6.2.1 Corners, if sharp, may become areas of localizedhigh-current density and therefore must be smoothed. Aftersurface preparation, the sample may be ultrasonically cleanedto remove any adhering particles. A final rinse is done withmethanol. Air drying is sufficient.6.3 Determine Current Density to be Applied—Measure thedimensions of each face of the sample and calculate the totalsurface area in square centimetres (correct for any surface notsubmerged). Current density is in the range from 0.05 to 0.1A/cm2for most nickel and nickel-iron base alloys. The specificcurrent density required for optimum electrolytic dissolution isa function of both alloy composition and heat treatment. Theoptimum current density is the highest current density at whichno matrix contamination occurs. This can be monitored poten-tiostatically if such equipment is available.4Multiply thechosen current density by total surface area to obtain therequired total current.6.4 Attach Anode Wire—Methods are described in Section4.Alength of wire at least 2 in. should project from the sample.This is needed for clamping to or looping to the permanentanode wire.6.5 Weigh Sample—Only if quantitative analysis isperformed, weigh the sample (with wire, if welded) then thefilter pad or centrifuge tube to the nearest 0.0001 g. Note thatfor samples over 10 g, a weighing error of 60.001 g may beconsidered negligible relative to an error of 60.0001 g in themass of the residue.6.6 Anode Connection—Suspend the sample by its wire inthe cell. Center the sample with respect to the cathode.6.7 Add Electrolyte—If the anode is prepared as in 4.3.1 or4.3.1.1 completely cover the sample and cathode with about250 mL of electrolyte. If the anode is prepared as in 4.3.2, thenthe weld must remain above the liquid, and the depth of sampleimmersion must agree with that used in the surface areacalculation. At this point the cooling mechanism, if used,should be started.6.8 Electrolyze—Set power supply to the predeterminedcurrent. Allow electrolysis to proceed, usually for a period of 4h. If the power supply will not automatically maintain constantcurrent, monitor the current at 15-min intervals, correcting forany current drift. Record the voltage for future reference. Addfresh electrolyte as required to maintain original volume. Thisis extremely important for non-totally immersed specimens.Depleted hydrogen ion is replaced by adding 3 mL of concen-trated HCl/A-h of electrolysis.6.9 Remove Sample—When power is turned off, suppressthe cooling mechanism. Raise the sample above the liquid leveland rinse with methanol. Disconnect the anode wire from thepower source and remove sample and its attached wire fromthe cell. If the sample has detached from the wire and falleninto the cell, retrieve it with stainless steel tweezers and rinsewith methanol into the cell.6.9.1 If a heavy coating is adhering to the sample, place thesample in a 100-mL beaker, cover with methanol, and placebeaker in ultrasonic cleaner for about 10 s. Remove samplewith tweezers and rinse with methanol, collecting the rinsingsin the 100-mL beaker. Set the sample aside to air dry. Cover thebeaker. If the sample does not require ultrasonic cleaning, setit aside to dry after removal from the cell.6.10 Sample Weighing—Only if quantitative analysis is tobe performed, weigh the sample with or without wire as donein 5.5 and calculate the loss in mass of the sample.6.11 Residue Collection—Follow method in 6.11.1 or6.11.2.6.11.1 Microfiltration—If the cell is not a beaker, transferthe electrolyte to a beaker, rinsing the cell and cathode withmethanol and collecting the rinsings into the electrolyte.Normally a 400-mL beaker is of sufficient size. Pour theelectrolyte through the tared-membrane filter. Rinse the beaker,pouring the rinsings through the filter.6.11.1.1 If residue was collected in a 100-mL beaker fromultrasonic cleaning, filter the beaker contents with the samefilter used for the electrolyte. Rinse the 100-mL beaker withmethanol, pouring the rinsings through the filter.6.11.1.2 Normally a 500-mL filter flask is sufficient tocontain the original electrolyte plus the methanol used forultrasonic cleaning, plus all rinsings. A water aspirator or filterpump should be used to speed the filtration process.6.11.1.3 Wash the residue three times with methanol. Re-move the filter with residue from the filter support. Place it ona clean surface to dry in air where it is protected from airbornecontamination and any other disturbance.6.11.1.4 If quantitative analysis is being performed, blankone filter pad from the same lot of filter pads, using 100 mL of10 % HCl-methanol. Remove the blank filter and weigh to thenearest 0.1 mg. Calculate the mass lost to the acid by the blankfilter. Weigh the filter with the residue to the nearest 0.1 mg.Calculate the mass of residue collected. Add to this thecorrection for the mass loss of the blank filter to obtain thecorrected mass of residue. Calculate the mass % residue, R,asfollows:R 5@Mr/~Mi2 Mf!#