Strong Plastic Deformation (SPD) refines crystal grains to the nanometer level by causing the material to undergo strong plastic deformation. Such as: equal channel corner extrusion, high-pressure torsion, stacking and rolling composite technology, etc. The preparation of submicron and nanomaterials by strong plastic deformation has aroused wide interest from material research scholars, and the microstructure and grain refinement mechanism caused by different strong plastic deformation methods have been thoroughly studied. However, the grain size prepared by the SPD method is larger than 100 nm.
Surface mechanical grinding treatment (SMAT) is a newly developed technology to refine the original coarse-grained grains to the nanometer level through strong plastic deformation, and the minimum average grain size can reach 10nm. It is different from other strong plastic deformation methods and ordinary shot peening technology, and has its unique characteristics. At present, surface nano-milling has been achieved in some metals and alloys using surface mechanical grinding technology. Researchers took the densely packed hexagonal metal industrial pure titanium (TA2) with high-level fault energy as a sample to study and analyze the microstructure changes of industrial pure titanium after SMAT strong plastic deformation.
The working principle of SMAT technology is: place a large number of spherical projectiles in a closed container, the upper part of the container is fixed with the sample, and the lower part is connected with the vibration generating device. During operation, the projectile makes high-speed vibration movement inside the container, and the test is conducted in random directions. The sample collides, and the surface of the material is nanometerized through strong plastic deformation. During processing, the size of the projectile impacting the surface of the material can be adjusted by changing the projectile size and vibration frequency. SMAT technology can accumulate huge microscopic deformation on the surface of the material when the macroscopic deformation of the material is very small.
Experimental results show that SMAT technology can make the smallest grain size reach 10nm. The grain size obtained by SMAT and other strong plastic deformation methods differs by an order of magnitude, the fundamental reason is the strain rate and load direction; the difference in grain refinement mechanism between the two deformation methods is due to the increase in high strain rate at room temperature The critical shear stress of dislocation slip makes twinning in the SMAT mode more likely to occur; dynamic recrystallization plays an important role in the SMAT grain refinement process.
Surface Modification of Titanium