Microstructure revealing molten history

Figure-1. Microstructure signifying melting. (a) Cross-sectional schematic showing the Mo sample. Grain growth is promoted in the central region when temperature is below T<sub>M</sub>; fine-grained microstructure emerges from the quenched molten region. (b) Cut-out view of the experimental design; the sample is irradiated by laser beams, concentric with the x-ray probe; x-ray diffraction from the heated volume of the sample is observed; external pressure-generating mechanism is not shown.
Figure-1. Microstructure signifying melting. (a) Cross-sectional schematic showing the Mo sample. Grain growth is promoted in the central region when temperature is below TM; fine-grained microstructure emerges from the quenched molten region. (b) Cut-out view of the experimental design; the sample is irradiated by laser beams, concentric with the x-ray probe; x-ray diffraction from the heated volume of the sample is observed; external pressure-generating mechanism is not shown.

High-pressure melting anchors the phase diagram of a material, revealing the effect of pressure on the breakdown of the ordering of atoms in the solid. An important case is molybdenum (Mo), which has long been speculated to undergo an exceptionally steep increase in melting temperature when compressed. On the other hand, previous experiments lacking reliable melting criteria, showed a nearly constant melting temperature as a function of pressure, a large discrepancy with theoretical expectations. Using microstructure to define a material’s molten history, a group in recent HPCAT experiments found a high-slope melting curve in Mo. Synchrotron X-ray diffraction was used to analyze the crystalline microstructures, generated by heating and subsequently rapidly quenching samples in a laser-heated diamond anvil cell. Distinct microstructural changes, observed at pressures up to 130 gigapascals, appear exclusively after melting (T>TM), thus offering a reliable melting criterion. Starting with coarse-grained samples of Mo, when the laser pulse power is adjusted to above melting temperatures followed by fast quenching, continuous Debye diffraction rings were observed arising from newly crystallized fine grains of the sample. In addition, a detailed analysis of the X-ray diffraction images revealed an unexpected new kind of microstructure transition at high pressures and at temperatures of approximately 400–500o below melting, yielding highly textured fine grains of body-centered cubic Mo. The newly established method may be broadly applied for studies of high pressure melting of other materials. More details in Hrubiak, et al.  Nat. Comm. 8, 14562.