High Cycle Fatigue Properties of a Minor Boron-Modified Ti-6Al-4V Alloy

摘要

Ti-6Al-4V alloys modified with minor amounts of boron (B) were prepared, and two types of microstructures, a full lamellar microstructure and an equiaxed microstructure, were generated through combinations of hot-deformation and heat treatments. The beneficial effect of adding a minor amount of B in refining microstructures was confirmed in as-cast ingots and a full lamellar microstructure. For example, a refined prior β grain size of about 100 μm in diameter was obtained for the 0.1 mass percent B-modified alloy with a full lamellar microstructure: accordingly, the size of each colony within the grains was reduced. Contrary to this, equiaxed microstructures with α grain sizes of about 8 μm were obtained for both B-free and B-modified alloys. The room temperature high cycle fatigue (HCF) strength of the B-modified alloys increased compared to the B-free alloy for both microstructures. For example, HCF strength at 10~7 cycles for the alloy with an equiaxed microstructure increased to 750 MPa by the addition of 0.1 % B from 650 MPa for B-free alloy. The fatigue crack was found to originate neither from the TiB/matrix interface nor from the TiB itself but rather from the shear fractures across microstructural units such as colonies or spherical α phases. The reduced colony size and the retarding effect of TiB against the movement of the fatigue initiation area were thought to be responsible for the improved HCF properties of Ti-6Al-4V with lamellar and equiaxed microstructures, respectively.