Recent progress in the high-cycle fatigue behaviour of γ-TiAl alloys

Figures & data

Figure 1. Schematic of FCG curves. Adapted and redrawn from [9] (reproduced with permission).

Table 1. Characteristics of the three regimes of FCG, adapted from [10].

	Stage I: Near-threshold	Stage II: Paris regime	Stage III: High-growth rate
Microscopic failure mode	Single shear mechanism	Duplex slip, eventually striations	Various static modes
Crack closure levels	High	Low	–
Load ratio effects	Large	Small	Large
Microstructural effects	Large	Small	Large
Environmental effects	Large	Variable	Small
Near-tip plasticity

and refer to the cyclic plastic zone size and the grain size, respectively. Owing to their high Paris slope, n, γ-TiAl alloys are commonly operated in Stage I, where environmental and microstructural effects on the FCG rate are high. for plane stress loading.

Figure 2. FCG curves for various TiAl alloy microstructures; all have high gradients in Stage II. From [13] (reproduced with permission).

Figure 3. Gigacycle fatigue S-N of Ti–45Al–10Nb. Redrawn from [32] (reproduced with permission).

Figure 4. Mechanisms for FCG in intermetallics, compared with those present in metals and ceramics. Extrinsic toughening in intermetallics may be achieved by grains bridging cracks, such as bridging lamellae in lamellar γ-TiAl alloys. In ceramics and intermetallic composites, crack bridging may additionally involve strong and stiff fibres, or elongated metallic grains that undergo ductile rupture [16]. Ahead of the crack in γ-TiAl intermetallics, crack growth may occur by cleavage fracture along fatigue cycled slip planes and by coalescence of microcracks ahead of the crack tip; static fracture modes also occur. Microcrack/microvoid toughening occurs in the proximity of cracks. Adapted from [16] (reproduced with permission).

Figure 5. Small crack growth rates in TiAl alloy TNM-B1, with the minimum in FCG rate at the first microstructural barrier arrowed in red. Adapted from [46] (reproduced with permission).

Figure 6. Kitagawa–Takahashi diagram for EBM-processed TiAl alloys. Redrawn and adapted from [47] (reproduced with permission).

Figure 7. Effect of temperature on FCG behaviour: (a) from [39] and (b) from [61] (reproduced with permission).

Figure 8. Schematic of a cuboidal PST (single colony) lamellar TiAl specimen with the angle Φ of the lamellar planes to the vertical loading axis indicated.

Figure 9. Effect of the equiaxed γ content on the measured fatigue threshold. From [83] (reproduced with permission).

Figure 10. Stress-life testing of PST crystals. From [70] (reproduced with permission).

Figure 11. Potential microstructural variety in directionally solidified crystals with the same Φ angle: (a) polysynthetically twinned crystal, PST, (b) columnar grains, approximately co-planar lamellar interfaces, (c) columnar grains, non-coplanar lamellar interfaces, but same Φ angle in every grain. In the present example, . Adapted from [107] (reproduced with permission).

Figure 12. (a) Yield stress of PST specimens as a function of the angle Φ at which the compression axis is inclined to the lamellar planes, from [9], using data from Fujiwara et al. [119] (black circles) and Nomura et al. [120] (white circles). (b) Dependence of the fatigue threshold on the lamellar orientation, Φ, of the colony where the crack is propagating, from [118] (reproduced with permission). Note that both the yield stress and the fatigue threshold present a U-shaped curve against Φ, suggesting that there may be a same mechanistic cause to both.

Figure 13. SEM images of similar deformation features near lamellar interfaces, interpreted as (a) ledges at lamellar interfaces [123], i.e. interfacial sliding, (b) interlamellar cracking [122], i.e. debonding of the lamellar interface, and (c) longitudinal slip in the γ-TiAl lamellae, near the lamellar interface [124]. Above (b): AFM linescan across the feature reported as interlamellar cracking; above (c): scanning transmission electron microscopy (STEM) images of a slip step near an /γ interface. White arrows indicate the features of interest at the lamellar interfaces; black arrows are illustrative of AFM scan or cross-sectional imaging slice directions (reproduced with permission).

Figure 14. A selection of possible loading strategies for measuring the FCG rate as a function of , and hence determining the fatigue threshold where  m cycle⁻¹ (growth/no-growth transition). The FCG specimen is loaded at for an extended period (as per ASTM E647 [11]), followed by successive steps , and so on, until an FCG rate of  m cycle⁻¹ is either subceeded or exceeded, depending on whether is being decreased (a, b) or increased (c, d), respectively. Constant and constant versions of each exist. Further, prior cycling at a large stress intensity range for a short period (e–h) may serve to generate a large plastic zone ahead of the crack tip within which the near-threshold crack must then grow. Finally, the large cycling may be applied between each step in , , (i–l) to remove history effects of the previous loading step by forcing the near-threshold crack to grow in the same sized large plastic zone at each step. Extended from [125] (reproduced with permission).

Figure 15. High-resolution digital image correlation strain map for a lamellar grain captured from a sample cyclically loaded (ex situ) in compression at a maximum nominal stress of 390 MPa and R=0.05. Axial (vertical here) strain accumulates inside the defined lamellar platelets which are composed by the γ-TiAl phase. From [136] (reproduced with permission).

Figure 16. Atom probe tomography reconstruction of the /γ interface in Ti–43Al–4Nb–1Mo–0.1B–0.75C. Note the peak in C, arrowed in red, near the interface; variations in the C content at the interface may change the cohesion strength of the /γ interface. Adapted from [147] (reproduced with permission).

Table 2. Summary of the microstructural properties to optimise for improved HCF behaviour of fully or nearly lamellar γ-TiAl alloys as a function of lamellar orientation, Φ.

Microstructure	Polysynthetically twinned			Directionally solidified (columnar)
Φ	0	45	90	0	45	90	Polycrystalline atextured
Optimal lamellar orientation	+			+			N/A
Optimal colony size	N/A			Unknown			50–100 µm
Optimal lamellar thickness	Refined (<1 µm, at least)
Plasticity (mainly longitudinal)	+	−	+	+	−	+	Unknown
Interlamellar debonding	+	+	−	+	+	−	−

+: Mechanism to be promoted; −: mechanism to be inhibited or avoided.

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