High Pressure and Magnetic Study of Fe-Cr Alloys

We investigated the high pressure characteristics of Fe-Cr alloys using energy dispersive x-ray diffraction on beamline X17C of the National Synchrotron Light Source. The effect of alloying is to increase the compressibility (decrease the bulk modulus) in the BCC phase. The high pressure phase of the Fe-rich alloys is determined to be DHCP, rather than HCP as found in elemental Fe.

At ambient temperature, pure Fe is known to transform to the HCP structure at ~13 GPa, while Cr maintains the BCC structure up to at least 2 Mbar. At elevated temperature and pressure pure Fe transforms to the dHCP structure. The HCP structure is characterized by a stacking sequence of ABABAB, while the dHCP structure has an ABACABAC stacking sequence.

High pressure energy dispersive x-ray diffraction (EDXRD) was performed at beamline X-17C of the National Synchrotron Light Source. A liquid nitrogen-cooled Ge detector was placed at a fixed two-theta of 12 degrees. The sample was loaded in a Merrill-Bassett DAC of the TAU design with diamonds having 500 micron culets, silicone oil as the pressure medium, and 301 stainless stell as a gasket material.

The results are shown here as the detected number of counts per energy. The higher the energy, the smaller the d-spacing. Curves are offset to show differences in the pressure.

The Fe-Cr T-x phase diagram has three solid phases: BCC (α), FCC (γ), and Sigma (σ). There is a miscibility gap associated with the bcc phase. Our samples were quenched from high temperature to avoid the miscibility gap. Spinodal decomposition is a phase separation into Fe-rich and Cr-rich phases. The Sigma phase is brittle and was avoided for the purpose of this

experiment.

Mössbauer, EDAX, BPT, and X-ray techniques where used to clarify the Fe-Cr system.