High-strength Ti-6Al-4V with ultrafine-grained structure fabricated by high energy ball milling and spark plasma sintering

Ultrafine-grained Ti-6Al-4V alloys were fabricated by high energy ball milling and spark plasma sintering. The effect of ball milling time and interstitial content on the microstructure and properties of sintered compacts was investigated and discussed. The sintered compacts consisted of equiaxed α+β matrixes with average grain sizes of 0.51-0.89 µm and 2-8% micrometer-sized α grains. When the ball milling time increased from 10 to 50 h, the volume fraction of coarse grains was reduced. The improvement of thermal stability may be attributed to the pinning of grain boundaries by nanostructured TiO2 particles and solute drag of interstitial atoms. The sintered compacts with ultrafine-grained structures exhibited 80-120% higher compressive yield strength than that of the coarse-grained alloy. The contributions of grain refinement strengthening and solid-solution/oxide dispersion strengthening via interstitial elements were evaluated by a modified Hall-Petch equation: σcy=393+0.46d−1/2+519Oeq1/2. When the ball milling time was 10 h, a balance of high strength (compressive yield strength=1260 MPa, ultimate compressive strength=1663 MPa) and sufficient plasticity (plastic strain to failure=20%) could be achieved.