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Phase Diagrams

Cobalt-Chromium (Co-Cr) Phase Diagram

Due to their excellent magnetic, wear, corrosion, and high-temperature properties, the cobalt-chromium (Co-Cr) alloys have been of great industrial and biomedical interest (e.g., magnets, sputtering thin-film high-coercivity high-density magnetic recording media in hard disks, surgical and dental implants, turbine disks in early jet engines, etc).

According to Thermo-Calc (coupled with PBIN thermodynamic database), the following five phases are present in the cobalt-chromium binary system above 800 °C (see Figure 1):

Liquid phase
• Cobalt-rich face-centered cubic (FCC) phase
• Cobalt-rich hexagonal close-packed (HCP) phase
Sigma phase and
• Chromium-rich body-centered cubic (BCC) phase.

The melting point of pure cobalt and pure chromium is 1495 °C (2723 °F) and 1906 °C (3463 °F), respectively.

Cobalt-Chromium (Co-Cr) Phase Diagram

Figure 1. Co-Cr phase diagram shows which phases are to be expected at equilibrium for different combinations of chromium content and temperature (in °C) in Co-Cr binary alloys. The Co-Cr phase diagram was calculated with Thermo-Calc, coupled with PBIN thermodynamic database.

The FCC phase is the high-temperature modification and the HCP phase is the low-temperature modification of the cobalt-rich terminal solid solution. The sigma phase is a brittle intermetallic compound of cobalt and chromium with a composition corresponding approximately to the ratio Co2Cr3.

According to PBIN thermodynamic database, the Co-Cr binary system has the following four reactions:

Eutectic reaction Liquid —> FCC + BCC at 1397 °C (2547 °F) and 42.2 % Cr

Congruent reaction BCC —> Sigma at 1281 °C (2338 °F) and 58.3 % Cr

Eutectoid reaction BCC —> FCC + Sigma at 1269 °C (2316 °F) and 52.5 % Cr and

Peritectoid reaction FCC + Sigma —> HCP at 967 °C (1773 °F) and 37.6 % Cr.

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• The phase diagram of Cobalt-Chromium binary system

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