Determining the Mechanical Strength of Ultra-Fine-Grained Metals

The grained particle allow us to track the change of different stress and determine the strength of ASFA metal, which overcome the deficiency of traditional techniques. This technique can be used to statistically examine the mechanical property of even sub 10 nanometer grain size metal with a reproducible and reliable results To prepare captain supporting gaskets, use laser drilling machine to cut the inner circle, followed by the outer rectangular part. The rectangular dimension is eight by 1.4 millimeters.

Next, prepare boron epoxy gaskets from a 10 millimeter diameter boron disc by manually polishing the raw discs with sandpaper to a thickness of 60 to 100 micrometers. Then cut the inner circles and outer circle with a laser drilling machine. Repeat and stop the procedure immediately when the right sized and center drilled gasket comes off.

Next, to assemble the gaskets, place a captain supporting gasket on a glass slide and place a drilled boron gasket on the inner hole of the captain gasket, ensuring that the larger end of the boron gasket is at the top. Then put another clean glass slide on the top, hold it firmly and press until the boron gasket is firmly inserted in the hole of the captain gasket. Store the fabricated gasket assemblies between two clean glass slides and wrap them with glue tape for future use.

To mount the gasket assembly, mark a dot locating the center of the diamond on the computer monitor connected to the optical microscope, then mount the boron epoxy gasket and mark the center of the gasket hole. Next, use a glass slide to press down the gasket assembly such that the gasket firmly sets on the diamond of the piston. For cleaning and compacting the gasket setup, load samples with a chunk size smaller than the gasket hole, such that there is no overflow of materials on the gasket surface.

After loading a new piece of sample, close the cell to achieve compactness. Use a monochromatic synchrotron x-ray to conduct diffraction experiments. Focus the x-ray beam to approximately a 30 by 30 square micrometer surface area on the sample.

Collect the x-ray diffraction patterns at pressure intervals of one to two gigapascals by a two dimension image plate with a resolution of 100 micrometers per pixel. Under hydrostatic compression, unrolled x-ray dIffraction lines should be straight not curved. Under non hydrostatic pressure, the curvature significantly increases with the decreasing grain size at similar pressures, suggesting continuous mechanical strengthening.

At similar pressures, the differential stress of the three nanometer sized nickel is the highest. In transmission electron microscopy images of representative nickel, quenched from around 40 gigapascals, a high content of dislocations is seen in the course grained sample as expected. In contrast, nano twins are well captured in the high pressure recovered nano crystalline nickel accompanied by some stacking faults.

In short, twining induced by stacking faults observed in these measurements originates from the nucleation and motion of partial dislocations. We need to load the sample properly to ensure the chamber is full of powder and the gasket would not crack at high pressure. We can perform TM limit under recovery sample and then examine the microstructure and the deformation defect for determining the deformation mechanism.

We have also used this technique to explore as a research topic such as probing green rotation at a nano scale energy reading, the ductility of the nano ceramics.