[ATM] Dense

[Anthony Stillman <atmer@flash.net>]

  Two thirds copper, one third tin.  What is it?
   
  Some time ago in some personal communication the topic of speculum's density arouse.  Two years of making ingots and I still hadn't gotten to that.  I put a measuring cup on the dinning room table and with some tap water and half kilograms of as marked alloy, I arrived at a value startlingly close to Herschel’s, 8.9 grams per cubic centimeter.  Then I worked out the disappointing uncertainty, +-0.4  
    
  Next I embarked on an extended effort to weigh on a triple beam balance many individual hundred gram billets in air and while buoyed by water.  I made a LOT of measurements.  The calculated densities ranged from 8.77 to 8.95+-0.04 g/cm^3.  The rounded average, 8.9.
   
  Way back when I began this metal mirror making exercise, I wanted to see what methods I might devise on my own.  I have abandoned this approach.  What follows are a few highlights of my investigation:
   
  There is renewed industrial interest in the Cu-Sn system.  This due in part to the demand for ruggedized lead free electronics and the need to understand and control the brittle intermetallics(1) that form where copper meets solder.  
   
  In the 45 year old text "Tin and Its Alloys," bulk speculum is identified as Cu3Sn, the epsilon phase in the copper tin system.  In that same book, electroplated speculum, used as a hard decorative tarnish resistant coating up to the 1960s, is identified as Cu6Sn5, the eta phase. Eta and epsilon are both intermetallic compounds.
   
  NIST has recently determined the bulk properties of Cu3Sn and Cu6Sn5.  As casting could not create sufficiently pure intermetallics samples, they devised a means to create pure powders and then heated and compressed them into billets of maximum density.  Their results show these species have densities of 8.9 and 8.3. (2)  
   
  Two other compositions of interest in speculum are the intermetallic delta phase(3) and alpha bronze.  Alpha bronze is a solid solution(1), not an intermetallic.  It occurs in great quantity in alloy containing less than 14 percent tin.  Bronze(4) used in casting sculptures is almost entirely the alpha phase.
   
  Speculum mirror makers from 300 odd years ago note the difficulties associated with surface defects (pits) encountered during polishing.  My personal experience confirms the pit nightmare.
   
  Porosity lowers density, leaves pits, and weakens castings.  Porosity in bronze occurs by four means.  Gas entrapment, gas evolution, coring, and condensation of Kirkendall voids.  
   
  Coring(5) and Kirkendall voids(6) are of little consequence in cast bronze containing more than 25% tin.
   
  Gas entrapment occurs during melting and pouring.  The latter particularly so if it is turbulent. 
   
  Gas evolution may result from contamination in the melt and/or from the mold dressing.  Of principal interest is water or water vapor in contact with the liquid metal.  The water disassociates into its constituents which either dissolve into, react with, or escape the melt.  This is called the inverse steam reaction.  When bronze solidifies, dissolved hydrogen is rejected.  This reacts with dissolved oxygen and oxides to form steam.  The steam then creates pressure voids.
   
  Stop casting porosity.
   
  Hydrogen may be removed from liquid metal by oxidizing it.  This is called degassing and may be done using an oxidizing flux.  Flux is used to cover the melt and more may be mixed in prior to casting.(7)
   
  After degassing the melt must be deoxidized.  Historically, this was done by stirring in a small amount of zinc just prior to casting.  Aluminum will also work.  However, both these metal's oxides are solid and become contaminates.  Alternatively, phosphorus forms oxides which are fluid and float on the molten metal.  The down side of using phosphorus,(8). 
   
  Early telescope mirror makers added one half to two percent of arsenic after melting the speculum ingots.  The resulting cast metal was described as denser and "whiter".  Later workers omitted the arsenic.  
   
  The addition of arsenic produces a marked increase in the amount of alpha+delta constituent.  This hardens bronze and in chill cast degassed deoxidized speculum, lowers the density.  Arsenic concentrations in excess of one percent do not significantly increase these effects.  Arsenic, like zinc and aluminum, though not nearly so effectively, will function as a deoxident.
   
  Porosity is not the only source of pitting in speculum.  Arsenic is not the only adulterant capable of increasing the amount of delta intermetallic present in a casting. 
   
  Speculum is not a pure intermetallic, but a combination of delta, epsilon, and eta along with alpha solid solution.  How much of each and where it is found determines the structure and properties of the metal.  This depends somewhat on the constituent ratios and greatly on the thermal history.
   
  At this point I've done a fair amount of research and am under the illusion that I know enough to give a talk.  I've offered to do so at this year's RTMC.  I'm still waiting to learn if I will be given the opportunity.  If anyone would like to lobby for the talk, please write the RTMC program director. 
   
  Anthony
   
  alea iacta est
  Literally: The die is cast.  (That’s die as in the singular of dice.)
   
   
  Footnotes:
  1) A solid solution is a single phase solid formed from a liquid with the same average composition.  One metal has dissolved into the other and together they have solidified.  Supersaturation occurs when there is more metal in solution than there can be for that temperature.  This results in the precipitation of intermetallic compounds.  Solid solutions generally have the same crystalline structure as the element present in the greatest amount.  An intermetallic compound has a crystalline structure different from its constituents.  Intermetallics form when the bonding strength of unlike atoms is greater than that of like atoms.
   
  2) The uncertainty in the NIST measurements is +-0.02g/cm^3.  It is reported without a value for uncertainty that Hershel measured a density of 8.8 for "ideal" speculum.  The meaningfulness of that number is blurred by unknown porosity and trace adulterants.
   
  3) Until about 1930 the delta phase was thought to be Cu4Sn.  Strong and others believed this to be speculum.  Subsequently delta was determined to be Cu31Sn8.  This identification persists in published work to this day.  However, currently NIST identifies delta as Cu41Sn11.  Delta to some degree is present in all cast bronze, often at the grain boundaries.  Delta is metastable below 348C.  To exist in bulk at room temperature this intermetallic must be chill cast down to about 200C.  Though extremely slow, measurable changes in the composition of high tin bronze continue to occur even below 200C.
   
  4) Much modern bronze is in fact not bronze at all but brass, a mixture of copper and zinc.  Bronze is a difficult metal to cast, work, polish, and generally everything else.  Brass can be made to look like bronze and is comparatively simple to manipulate.
   
  5) Coring occurs as a result of non-equilibrium conditions which exist at the freeze boundary.  The principle is used to good effect in a process known as zone refining. 
   
  6) Cast bronze bearings (high tin bronze) subject to prolong exposure to temperatures above 250C will develop Kirkendall voids.
   
  7) One such flux contains equal parts of silica (dry sand), borax, and CuO.  Flux is expensive and production casting can consume large quantities.  A vacuum or electric furnace will keep the hydrogen content of the melt low.  A draft, oil or gas furnace are successively less desirable.
   
  8) Care must be taken to insure thorough mixing.  Residual un-oxidized phosphorus exceeding 0.02 percent can significantly alter the properties of bronze.  Elemental phosphorus is particularly difficult to handle.  Red phosphorus powder compounded with zinc powder is an alternate de-oxidant.
   
  References:
  "The History of The Telescopes"  King
  "Reflecting Telescope Optics I"  Wilson
  "Reflecting Telescope Optics II"  Wilson
  "Amateur Telescope Making - Book One"  Ingalls
  "Amateur Telescope Making - Advanced"  Ingalls
  "Prism and Lens Making"  Twyman
  "Procedures in Experimental Physics"  Strong
  "Tin and Its Alloys"  Hedges
  "Chill Cast Bronze"  Hanson and Pell-Walpole
  "Structure of Metals through Optical Microscopy"  Tomer
  "Principles of Solidification"  Chalmers
  "Intermetallics"  Sauthoff
  "CRC Handbook of Chemistry and Physics"  Weast
  http://www.metallurgy.nist.gov/mechanical_properties/solder_paper.html

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