Observations of stress-strain curves of hcp-Fe at high pressures and temperatures

We performed in-situ X-ray diffraction and radiography experiments of hcp-Fe within its stability field to observe some independent stress-strain curves of this material. Deformation experiments were carried out at the GSECARS 13-BM-D beamline (APS) using a deformation-DIA with a monochromatic X-ray diffraction and a radiographic imaging system. We used four tungsten carbide and two sintered diamond (SD) anvils with truncated edge length of 2 mm. Pressure medium was a mixture of boron and epoxy and a cylindrical graphite furnace was employed. The starting material is a fragment of pure bcc-Fe wire (0.5 mm in diameter and 0.5 mm long) and two deformation pistons made of alumina were situated above and below the sample. First, the cell assembly was compressed uniformly at room temperature up to about 13 GPa. After that, the sample was heated to about 700 K and we observed transformation of the sample to hcp-Fe. Several deformation cycles were repeated at high pressures and temperatures after the transformation. The incident beam was directed through an anvil-gap and impinged the sample. The diffracted X-rays went through the SD anvils, and thus we were able to observe diffraction Debye rings over the entire 360 degrees detector azimuth range. Two-dimensional diffraction patterns were collected using an X-ray CCD detector. Using distortion of the Debye rings from the true circle and single-crystal elastic moduli of hcp-Fe, differential stresses can be calculated. The sample length was measured by radiography using a wide X-ray beam. Using the radiographic data, axial strains of the sample can be determined. We observed ten independent stress-strain curves with pressures of about 12 GPa, axial strains in excess of 15 percent, three different temperatures, and some strain rates. These stress-strain curves indicate that hcp-Fe deforms elastically at the beginning of deformation. In some of these, we observed saturation of the sample stresses, which means that the deformation reaches steady state. The differential stresses at the steady state flow tend to decrease with increasing temperature and with decreasing strain rate.