Institute of Metals Division - Nucleation Catalysis by Carbon Additions to Magnesium Alloys

Grain refinement of Mg-Al melts by carbonaceous additions has been attributed to nucleation by aluminum carbide. The effects of process and alloy variables are interpreted and predicted in terms of the dispersion and chemistry of this phase. The grain coarsening action of Be, Zr, Ti, R.E., chlorination, temperature extremes, and prolonged holding times is described. Measures necessary to insure an adequate dispersion of the catalyst are discussed. CARBON inoculation treatments have become fairly well known and used for grain refinement of magnesium alloys containing Al. Although there is general agreement that a nucleation process occurs, the process is not understood and the inoculants are used in a rather empirical fashion. The treatment is applied to the class of alloys containing 3 to 10 pct Al, i.e., AZ31A to AM100A. Typical methods involve melting, alloying, and adjusting the temperature to 1400° to 1450°F. Then 0.01 to 0.5 pct C as CaC2, C6C16, or lampblack is added by any convenient means, and the melt poured within 10 to 30 min. Investigators generally have been impressed by an assumed similarity of this refinement process to superheat grain refinement, which depends on heating approximately the same alloys to a temperature in the range of 1550" to 1650°F, then pouring promptly after the melt is cooled to the pouring temperature. Various predictions have been made that carbon refinement would replace superheating in commercial practice due to reduced process costs, but this replacement has not fully taken place because of production difficulties and conflicting observations. Davis, Eastwood, and DeHaven1 agree with Nelson2 and wood3 in suggesting that an excess of inoculant may be harmful. Wood however says that overtreat-ment is not a problem in production use of hexa-chlorobenzene inoculation, and Hultgren and Mitchell4 claim no evidence of harm from excess additions. Various grain coarsening reactions are known to occur, including the possibility of overtreatment mentioned above. Trace amounts of Be,2 Zr, and Ti may prevent refinement by either a carbon treatment or a superheat. Occasionally treatment with cl25 may cause coarsening, although the Battelle refinement process' uses a CC14-C12 blend. Grain coarsening also tends to occur on holding at temperatures below 1350°to 1400°F, especially after a superheat treatment, and for this reason Nelson2 stresses the desirability of a refinement method useful at lower temperatures for open pot melting practice. Since a carbon treatment can be made to work at temperatures below 1400°F, it seems desirable to investigate the mechanism of the refinement and the mechanisms of the coarsening reactions in order to establish control conditions for use in commercial production. The identity of the nucleating phase must first be established and then the factors affecting its chemistry and physical dispersion must be determined. THE IDENTITY OF THE NUCLEATING PHASE Davis, Eastwood, and DeHaven suggested that the nucleating phase in this system is Al4c3,1 but Mahoney, Tarr, and LeGrand8 disagree, largely because they found no evidence of the compound in alloys after carbon treatment and because there is no indication that aluminum carbide should be unstable over the temperature range used in the superheat treatment. This latter objection is based on the assumption that both the carbon treatment and the superheat treatment introduce the same nuclei. Electron diffraction studies have been made to identify the nucleating phase. Samples of grain refined A292 have been selectively etched SO that clean surfaces are obtained and so that secondary phases are in relief. Electron diffraction patterns from these surfaces have established that the carbon treatment of A292 introduces into the metal a large number of small, plate-like particles with a structure very similar to Al4C3. In most cases, the plate-like nature of the particles prevented positive identification but in the cases where the identification could be made the particles proved to be AIN A14C3. However, enough variation in lattice constants was observed so that all compositions from pure A14C3 to the 50:50 solid solution A1N.Al4C3 were probably present.14 In A14C3 and especially AlN.Al4C3 the A1 atoms occur in layers within which they have the same hexagonal symmetry and spacing as the Mg atoms in a single basal plane of a magnesium crystal. The solid solution spacing lies between the 3.16 of AIN and the 3.3? for Al4C3, in satisfactory agree-