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

Abstract

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 10⁷ 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.