Mechanical Properties and Microstructures of Severely Plastic Deformed Pure Titanium by Mechanical Milling and Spark Plasma Sintering

Severe plastic deformation (SPD) was applied to pure titanium powder by mechanical milling (MM) process with stearic acid, added as a process control agent (PCA), by using a vibrational ball mill, and MMed powders possessing large strains were subsequently consolidated into bulk materials by spark plasma sintering (SPS) in order to enhance the hardness and strength of pure titanium. Changes in the hardness and constituent phases of the MMed powders have been examined by microhardness measurements and X-ray diffraction. The hardness and microstructure of the SPS materials have also been studied by hardness measurements and optical microscopy. The Vickers microhardness of the pure titanium powders with PCA 0.25 g increased sharply from 189 HV to 513 HV after 8 h of the mechanical milling (MM) process. Formation of TiH2 as the solid-state reaction product occurred in the MMed powder during 4 and 8 h of the MM process. The near full density was obtained for the SPS materials under the condition of an applied pressure at 49 MPa with a sintering temperature at 1073 K for 0.5 h. The Vickers hardness of the SPS material fabricated from 8 h MMed powder with PCA 0.50 g exhibited a maximum value of 1253 HV.