Thermodynamic assessment of the Me-Co-C systems (Me=Ti, Ta, or Nb)
- PDF / 143,697 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 99 Downloads / 157 Views
INTRODUCTION
II. DTA EXPERIMENTS
elements Co, C, Ti, Ta, and Nb are important in cemented carbides. A recognized tool in alloy development is phase equilibria calculations. Consequently, there is a need for complete and consistent thermodynamic descriptions of the ternary systems Ti-Co-C, Ta-Co-C, and Nb-Co-C. The purpose of the present work is to evaluate the thermodynamic properties of the three ternary systems using the CALPHAD method,[1,2] which aims at modeling the Gibbs energy of each alloyed phase in the system. The models contain parameters that are evaluated to obtain the best fit to the available experimental information using a computerized optimization procedure. Some of the parameters for the Me-Co-C systems (Me ⫽ Ti, Ta, or Nb) represent quantities already determined when modeling the lower order systems. Other parameters represent purely ternary quantities. The present work was based on previous descriptions of the binary systems CoC,[3] C-Nb,[4] C-Ta,[5] Co-Nb,[6] and Co-Ta[7] and on recent evaluations of the C-Ti[8] and Co-Ti[9] systems. There is a ternary phase, M6C, in both Ta-Co-C and Nb-Co-C for which a set of ternary parameters has now been optimized. In the present article, the differential thermal analysis (DTA) experiments carried out for the Co-MeC sections are presented. The thermodynamic models for the Gibbs energy of various phases of the systems are presented and the assessment of the parameters for the Me-Co-C systems is described. Finally, the properties calculated using the optimum model parameters are compared with the experimental data.
The DTA experiments have been performed to determine the temperature of the invariant equilibria in the Co-MeC pseudobinaries (Me ⫽ Ti, Ta, or Nb) at carbon activity unity. The samples were prepared by dry blending 10 g lots of raw material powders (Co, cubic carbide, and carbon black) utilizing a Glen Mills TURBULA shaker-mixer (Glen Mills, Clifton, NJ). The composition was chosen such that graphite should never be completely dissolved during the temperature cycling. A Setaram 92-16.18 instrument equipped with an alumina furnace tube and a differential scanning calometry (DSC) 1500 rod was utilized. Approximately 100 mg of the sample mixture was placed in a graphite crucible in order to achieve a carbon activity of unity. An empty graphite crucible was used as reference. The phase transformations were studied by ramping up and down in temperature at rates of 5 ⬚C/min and 2 ⬚C/min in vacuum, P ⬍ 10 Pa. The temperature readings were calibrated by comparing the eutectic temperature for the Co-C system with the assessed value of Ferna´ndez Guillermet.[3] This calibrating procedure resulted in an adjustment of the experimental values by ⫹4 ⬚C. Most experiments were performed for compositions such that the cubic carbide was stable during the temperature cycling, and therefore, only the eutectic temperature was detected. For the Co-TiC system, a series of TiC compositions was investigated. For the lower TiC compositions, the liquidus temperature also c
Data Loading...
