High cycle fatigue and fatigue crack propagation behaviors of β-annealed Ti-6Al-4V alloy
© The Author(s). 2017
Received: 5 August 2016
Accepted: 12 December 2016
Published: 4 January 2017
The effect of β annealing on high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of Ti64 alloy were examined, and the results were compared to those of mill-annealed counterpart.
The tensile tests, stress-controlled HCF tests, and FCP tests were conducted, and the fractographic and micrographic analyses were performed before and after the tests.
The β-annealed Ti64 specimen showed inferior HCF properties as compared to the mill-annealed counterpart, as a result of lower yield strength. On the other hand the resistance to FCP of β-annealed Ti64 specimen was higher than that of mill-annealed counterpart in low and intermediate ΔK regime.
Relatively easy fatigue crack initiation at the colony boundaries of β-annealed Ti64 specimen reduce the resistance to HCF. The resistance to FCP of β-annealed Ti64 specimen increased significantly particularly in low ΔK regime along with severe crack branching and deflection.
Today’s high performance aircrafts, including engine power to weight ratios, airframe strength, aircraft speed and range, are largely attributable to the use of titanium alloys with high specific strength and fracture toughness (Ezugwu and Wang 1997; Andrade et al. 2010). The use of titanium alloys in airframe application has recently been growing as the use of CFRP (carbon fiber reinforced polymer) increases (Inagaki et al. 2014; Mrazova 2013). It is because designing joints between heterogeneous materials in an airframe must consider the prevention of galvanic corrosion and the elimination of strain caused by the difference in thermal expansion coefficients (Kaminaka et al. 2014; Donachie 2000).
A number of literatures have demonstrated that different heat treatment routes can produce a variety of microstructures in α + β Ti-6Al-4 V (Ti64) alloys, which strongly affects the static and dynamic properties (Venkatesh et al. 2009; Morita et al. 2005; Semiatin et al. 2003; Ivasishin et al. 2002; Chandler 1996). Typical heat treatment processes of Ti64 alloy are conducted in α + β region below β transus, including duplex annealing, solution heat treatment and aging (STA), recrystallization annealing, and mill annealing. Unlike the α + β processing, the β annealing of α + β Ti64 alloy is a less common type of hot working (Chandler 1996; Campbell 2011). The purpose of β annealing is to induce the microstructure of Widmanstatten or acicular α phase to obtain enhanced resistance to fracture, fatigue, and creep, albeit moderately sacrificing strength and ductility as compared to the microstructure of equiaxed α phase (Campbell 2011; Welsch et al. 1993; Wanhill and Barter 2011; Yoder et al. 1976). The β annealing is done at a temperature only slightly higher than the β transus to prevent excessive grain growth (Ivasishin et al. 2002). Annealing time depends on section thickness and should be long enough to permit complete transformation to β phase, while it should also be held to a minimum to suppress the grain growth of β phase (Donachie 2000; Ivasishin et al. 2002; Davis 1995; Borisova et al. 1975). The β annealing can be followed by passive air cool, although larger sections may need to be fan cooled or even water quenched to prevent the formation of detrimental layer of α phase at grain boundaries (Donachie 2000). In α + β titanium alloys, thermal instability is a function of β phase transformation since, cooling from the annealing temperature, the β phase can transform to the undesirable (brittle) intermediate ω phase (Donachie 2000; Froes 2015; Rajan et al. 2011). To prevent the formation of ω phase, a stabilization anneal is generally given for the α + β titanium alloys (Donachie 2000; Froes 2015). This annealing treatment produces a stable β phase capable of resisting further transformation when exposed to elevated temperatures in service. The α + β Ti64 alloy that is lean in β can be air cooled from the annealing temperature without impairing their stability (Donachie 2000; Campbell 2008).
Despite the advantages of β-annealed Ti64 alloys, notably in the thick sections required for large primary aircraft structure, little is known in detail about fatigue properties. In this study, the high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of β-annealed Ti64 alloy were investigated, and the results were compared to those of mill-annealed Ti64 alloy. The effects of tensile properties and crack nucleation mechanism on the HCF behavior of β-annealed Ti64 was discussed based on the detailed fractographic and micrographic observation. The mechanisms associated with the microstructure-sensitive FCP behavior of β-annealed Ti64 alloy was also discussed with a particular emphasis placed on the morphology of crack path.
Results and discussion
The tensile properties of β-annealed and mill-annealed Ti64 specimens
Tensile Elongation (%)
The crack nucleation under fatigue loading largely determines the resistance to HCF of Ti alloys (Nicholas 2006). Since yield strength is the indicative of the resistance to slip deformation inducing crack nucleation on smooth surface, it is therefore one of the most important parameters affecting the HCF resistance (Jung et al. 2014; Jeong et al. 2013). As in the case of tensile behavior, the microstructure can have a significant influence on the HCF resistance of Ti64 alloy. For the mill-annealed Ti64 specimen, smaller primary α grain tends to improve the HCF resistance since yield strength increases with decreasing primary α grain size. Higher applied stress is then required to initiate a crack in primary α grain by slip band cracking (Lucas and Konieczny 1971; Bowen and Stubbington 1973). The smaller primary α grain also induces higher density of α/α grain boundaries, which may increase the resistance to HCF by retarding the crack growth with the grain boundaries acting as microstructural barriers (Demulsant and Mendez 1995). For the β-annealed Ti64 specimen, the microstructural factors affecting the HCF behavior include the sizes of colony, α platelet and prior β grain. Smaller sizes of colony and prior β grain in the β-annealed Ti64 specimen increase the resistance to HCF by reducing effective slip length, as in the mill-annealed counterpart (Lütjering 1998; Ziaja et al. 2001; Lucas and Konieczny 1971; Bowen and Stubbington 1973; Demulsant and Mendez 1995). The narrower aligned α platelets within colonies can further suppress the crack initiation by slip band cracking, since the crack needs to cross through more of the β ribs in the aligned α platelets (Eylon and Pierce 1976; Bania et al. 1982; Ruppen et al. 1979). As in the mill-annealed Ti64 specimen, colony boundaries and prior β grain boundaries in the β-annealed specimen can act as microstructural barriers for crack to grow, improving the resistance to HCF.
The β-annealed Ti64 specimen showed inferior tensile properties as compared to the mill-annealed counterpart. The decrease in tensile strength and ductility with β-annealing was related to the increase in the effective slip length as colony boundaries, as well as prior β grain boundaries, acted as microstructural barriers for dislocation movement.
Along with the reduced yield strength with β-annealing, the resistance to HCF of β-annealed Ti64 specimen was also lower than that of mill-annealed counterpart. Relatively easy fatigue crack initiation at the colony boundaries of β-annealed Ti64 specimen, as compared to the initiation either at α/β interface or within α grain for the mill-annealed counterpart, further reduce the resistance to HCF.
Due to the extrinsic effect of crack branching and deflection, the resistance to FCP of β-annealed Ti64 specimen was higher than that of mill-annealed counterpart in low and intermediate ΔK regime at both R ratios of 0.1 and 0.7. At high ΔK regime, the crack branching and deflection was no longer observed, and the FCP rates of β-annealed Ti64 specimen became similar to those of mill-annealed counterpart.
This work has been supported by the Engineering Research Center (ERC) Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0030801). This work was also supported by the Industrial Technology Innovation Program (10050561, Forming, Post-treatment and Assembly Manufacturing Technology for Nozzle Fairing of 17700 lbs Supersonic Engine) funded By the Ministry of Trade, Industry and Energy (MI, Korea), and the Fundamental Research Program of the Korea Institute of Materials Science (KIMS).
DH conducted the experiments, collected the data, and wrote the paper. YN, M and SS guided and supported this work and contributed with their expertise and advices to write a paper. All authors read and approved the final manuscript.
TThe authors declare that they have no competing interests.
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