Fatigue Crack Initiation and Facet Formation in Ti-6Al-4V Wires

In Ti-6Al-4V, fatigue cracks can nucleate internally instead of at the surface. Internal or subsurface cracks in titanium alloys are found to initiate at faceted features, which can be seen on the fracture surface after failure. These facets are fractured primary α (hcp) grains, which have broken in a very planar manner. The main purpose of this work is to study internal fracture in drawn Ti-6Al-4V wires, which has not been done before, since all published studies use forged or rolled samples. Fatigue tests with load ratio R=0.1 and a frequency of 60 Hz have been performed on wires with average α grain sizes of approximately 1, 2, 5 and 10 µm. In total, 50 samples were successfully tested. Four samples broke due to an internally initiated crack after testing at a maximum stress of 750 MPa: three samples with average alpha grain size 5 μm, which failed after 2.6 x 10^7, 5.7 x 10^7 and 9.6 x 10^7 cycles, and one sample with average alpha grain size 10 μm, which failed after 7.6 x 10^6 cycles. In general, an increase in grain size resulted in lower fatigue lives. In the four samples with internal crack initiation, a cluster of facets was observed at the initiation site on the fracture surface. The projected area size of the facet-containing area was not related to the fatigue life, and the estimated threshold stress intensity factor range was between 6 and 8 MPa√m. The actual surface area, taking into account the roughness, was correlated to the fatigue life. The facets were not smooth, but showed some nano-roughness with linear markings. As a result of the crystallographic texture, the facets were highly inclined, at angles mostly between 50° and 70°. Nearly all facets were parallel to a prismatic lattice plane. The linear markings on these facets were parallel to the direction of the prismatic slip system, which means that facet formation most likely occurred by a prismatic slip band mechanism. One anomalous facet was observed in the sample with an average grain size of 10 µm. This facet coincided with a near-basal plane, and displayed a fan-shaped pattern instead of linear markings, which indicates that a cleavage mechanism took place. These observations suggest that both a slip-based and a cleavage-based mechanism are possible, depending on the grain size, local texture and possibly other parameters. A local comparison of Taylor factors showed that the Taylor factor of each faceted grain had a tendency towards being higher than the Taylor factors of its corresponding neighbouring grains. It is suggested that the Taylor factor may be used to distinguish between plastically hard and soft grain orientations, and therefore could be capable of explaining the stress distribution between grains, which is the root cause for facet formation. Fracture surface cross-sections revealed secondary facets, which in some cases extended through more than one grain. Short crack growth through a grain boundary occurred when two compatible slip systems were present in these grains. This compatibility could be expressed by calculating misalignment factors. Additionally, a limited number of dwell fatigue tests have been performed, with a hold time of 30 or 120 s at maximum load. Introducing a hold time at maximum load drastically reduced the amount of cycles to failure by two to three orders of magnitude compared to regular fatigue tests, and promoted surface crack initiation. This was caused by strain accumulation due to cold creep, which occurred during each maximum load hold. A recovery process took place during each minimum load hold, which caused an increased initial strain rate during each following maximum load hold.

Jaar van publicatie:2017

Toegankelijkheid:Open