THE INFLUENCE OF HYDROGEN ON THE FATIGUE BEHAVIOUR OF THE BETA‐TITANIUM ALLOY Ti‐3Al‐8V‐6Cr‐4Mo‐4Zr

Abstract

In order to characterize the influence of hydrogen on the mechanical properties of B-titanium alloys, monotonic tensile and straincontrolled fatigue tests were performed on samples of the metastable alloy Ti-3A1-8V-6Cr-4Mo-4Zr in uncharged (0.5 at.% hydrogen) and hydrogen-charged (3-4 at.% hydrogen) conditions. The hydrogen was introduced into the material during the last 8 h of an ageing treatment (28 h at 482°C) from the gas phase, whereas the reference (uncharged) specimens were annealed completely in vacuum. The results of the mechanical tests indicate that hydrogen slightly increases the strength of the alloy in monotonic as well as in cyclic loading. Under tensile loading the fracture strain decreases as a result of hydrogen. Under cyclic loading both charged and uncharged conditions show initial softening followed by a saturation state. The cyclic lifetime at a constant total strain amplitude, however, is not reduced by the hydrogen charging. The effect of hydrogen on the mechanical behaviour can be interpreted and understood on the basis of microstructural observations that reveal a hydrogeninduced change in the precipitation state. This indirect influence of hydrogen on the microstructure, which leads to a reduction of the mean size of the a-precipitates, in combination with a slight decrease on the volume fraction of the a-phase, seems to dominate over any direct intrinsic hydrogen effect, NOMENCLATURE C, = atomic hydrogen concentration E = Young's modulus E, = Young's modulus at zero stress k = elastic constant of the quadratic extension of Hooke's law N = number of cycles Np = number of cycles to fracture A E /~ = total strain amplitude Aa/2 =stress amplitude E = total strain E* =elastic strain E, , , =plastic strain a = stress

Stress corrosion cracking (SCC) and hydrogen-assisted cracking in titanium alloys

“…This is also observed for the metastable b alloy Ti-3Al-8V-4Mo-4Zr (Christ et al, 1996), which showed loss in ductility on hydrogen charging, but no effect of hydrogen on the fatigue life. This apparent inertness to hydrogen effects in cyclic loading is baffl ing at fi rst glance.…”

“…A reasonable explanation for this phenomenon was not forthcoming. For a two-phase alloy the FCGRs for hydrogen and argon atmospheres were compared (Christ et al, 1996). No differences in the FCGRs were observed during elevated temperature testing, although signifi cant embrittlement due to hydrogen was observed at RT through tensile tests.…”

Effect of α-phase morphology on low-cycle fatigue behavior of TC21 alloy

“…Furthermore, dislocations are accelerated to overcome the barrier from other dislocations which results in cyclic softening. Similar cyclic softening behavior was also observed in Ti-3Al-8V-6Cr-4Mo-4Zr induced by the high density of dislocation in the a p particles [34]. The authors suggest that the homogeneous plastic deformation with sufficient stress or strain amplitude enables a rearrangement of dislocations and a relaxation of internal stress, and finally causes cyclic softening.…”

Influence of α‐Precipitate Orientation and Distribution on the Deformation Behavior of the Additively Manufactured Metastable β‐Titanium Alloy Ti‐5553 Assessed by Cyclic Nanoindentation

“…Reports on the behavior of conventionally processed Ti-5553 on the macroscale differ from our observation, and they are not consistent: only cyclic softening, cyclic softening followed by saturation, or cyclic hardening followed by softening were observed. [12,42,43] Microstructural and compositional differences are one likely reason for the differences. All the reports refer to conventionally processed metastable β-alloys, with different compositions and heat treatments, compared to each other and our alloy.…”