Effects of microtexture and Ti3Al (α2) precipitates on stress-corrosion cracking properties of a Ti-8Al-1Mo-1V alloy
Graphical abstract
Proposed facet formation mechanism involves Hydrogen Enhanced Localized Plasticity (HELP) in aqueous NaCl SCC for Ti-8Al-1Mo-1V. The black arrow in (b) indicates crack propagation direction.
Introduction
For aircraft, extra weight leads to an increase in fuel consumption [1], [2], [3], therefore the cost penalty of extra weight is approximately €1000 per kg [4]. Weight reduction can be achieved by using materials having low density and good mechanical properties, such as high elastic modulus (stiffness) and strength [4], [5], [6], [7]. Compared to the widely used Ti-6Al-4V (Ti-64) alloy, Ti-8Al-1Mo-1V (Ti-811) is characterized by a lower density and a higher stiffness, but unfortunately it suffers Stress-Corrosion Cracking (SCC) [8], [9], [10], [11], [12].
The excellent corrosion resistance of titanium (Ti) alloys in oxidizing environments is due to the native protective passive Ti oxide film. For Ti-811, spontaneous passivation occurs in 3.5% sodium chloride (NaCl) solution [10], [13], [14], [15]. However, the protective film will be broken if load applied is beyond the yield point of Ti oxide, and corrosive media will then contact with the underlying fresh metal, and initiate SCC [11], [16].
In Ti alloys, aluminium (Al) and oxygen (O) additions increase SCC susceptibility and change the slip mode to planar slip [17], [18]. This in part is ascribed to the high Al content (>6 wt.%) promoting ordered Ti3Al (α2) precipitate formation at appropriate aging temperatures. The α2 precipitate has an ordered D019 structure, in which Ti and Al atoms occupy specific positions in the Hexagonal Close Packed (HCP) structure [6], [8], [10], [19], [20]. In Ti-811, SCC is attributed to localized planar slip due to the presence of ordering in the α phase. An increase in ordering will contribute to an increasingly coarse and planar slip [18]. In a previous study on rolled Ti-811 plate, the threshold stress intensity factor of SCC (KIscc) was found to decrease, and the SCC crack velocity was found to increase by 4 times in a sample containing α2 phase compared to one in the precipitate-free condition [17]. It should be noted that the study [17] was conducted on the samples from a rolled sheet, which had a pronounced crystallographic texture.
In addition to ordered precipitates, crystallographic texture has also been found influence SCC susceptibility. The effect of texture on SCC has been widely studied in Ti-811 and Ti-64 alloys, and a correlation between SCC susceptibility and orientation of (0001)α texture relative to loading axis has been established [8], [9], [18], [21], [22], [23], [24], [25], [26]. However, all previous studies of the texture effects have been performed at a macro-scale (macro-texture). On the other hand, microtexture or macro zones, region of grains with a similar crystallographic orientation, have been found to affect the fatigue and dwell fatigue properties of Ti alloys [27], [28], [29], [30], [31], [32], [33], [34]. To the best of our knowledge, no investigation has been carried out into the effects of microtexture (or macro zone) on SCC for Ti alloys.
Electron Backscatter Diffraction (EBSD) analysis was employed to characterize microtexture in this work. Hot Isostatic Pressed (HIPped) Ti-811 with a random texture [35], [36], [37], [38], was compared with wrought Ti-811 containing an intrinsic microtexture in this study. The present research investigated the independent effect of α2 precipitation and microtexture on the SCC susceptibility of Ti-811 by using appropriate heat-treatments and hot isostatic pressing (HIPping) schemes with the aim of identifying possible means to reduce the SCC susceptibility. A constant displacement SCC test was used to characterize SCC susceptibility. The effects of α2 precipitates and microtexture on SCC susceptibility were studied by using TEM and EBSD respectively. The mechanism of SCC propagation was investigated by fractography fracture surfaces and metallography of cross section through fracture path using SEM and EBSD.
Section snippets
Materials and methods
A wrought Ti-811 bar with a diameter of 80 mm was obtained from Timet UK Limited, and pre-alloyed Ti-811 powders produced by electrode induction melting gas atomization were provided by the Institute of Metal Research, Chinese Academy of Sciences. The pre-alloyed powders had a wide particle size distribution from 5 μm to 832 μm, with 90% having a diameter less than 307 μm. The pre-alloyed powders were encapsulated in mild steel cans and HIPped by an Avure QIH-9 hot isostatic press at 100 MPa and 990
Microstructure and mechanical properties
Microstructures of wrought Ti-811, wrought Ti-811 + HT860, HIPped Ti-811, and HIPped Ti-811 + HT860 are shown in Fig. 2. The size of globular α grains and transformed β grains varied over a range from 20 to 30 μm for wrought Ti-811 (Fig. 2a). HIPped Ti-811 (Fig. 2c) consisted of recrystallized equiaxed α grains (along the red dashed lines) with a size range from 5 to 10 μm, and α laths with a thickness of 2 μm. After a 30 min post heat-treatment at 860 °C followed by WQ, both wrought Ti-811 + HT860 (Fig. 2
Effect of the presence of microtextured regions on SCC susceptibility
In the present study, wrought Ti-811 and wrought Ti-811 + HT860 samples had the same DCB sample orientation when sectioned from the bar material, but the only difference was the presence of α2 precipitates. It is important to note that there was minor grain growth after the post heat-treatment. As the SCC crack propagation is related to the effective slip length of dislocations, the grain size influences the effective slip length. However, there are microtextured regions with a size scale of a
Conclusions
- i
Tear ridges and slip bands on SCC propagation facets indicate a heavily localized plastic deformation in the microstructure adjacent and below the fracture surface, which aligns well with the concept of HELP as the SCC mechanism. The SCC propagation is the result of joining the primary with the secondary SCC cracks formed by HELP in α grains ahead of the primary crack-tip. The hydrogen source is mainly provided through externally absorbed hydrogen from the environment at the crack-tip.
- ii
The
Conflict of interest
The authors declare no competing financial interest.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Acknowledgements
The authors appreciate the discussions with Prof. Rodney Boyer, Dr. Xiaobo Chen, and Dr. Steven Knight, and Prof. James Williams. S.C also acknowledges both the Australian Postgraduate Awards (APA) and International Postgraduate Research Scholarship (IPRS) for his PhD sponsorship. The authors would also like to thank Dr. Yichao Zou on HR-TEM assistance, Dr. Xi-Ya Fang on large area EBSD mapping assistance and Mr. Xigen Zhou for the assistance on sample preparation. Finally supports from Monash
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