PSI - Issue 14

M S Nandana et al. / Procedia Structural Integrity 14 (2019) 314–321 M S Nandana/ Structural Integrity Procedia 00 (2018) 000–000

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RRA treated alloy. The fracture toughness of RRA treated alloy is also reduced by a unit value compared to T6 as reported in our earlier findings by Nandana et al. (2018).

Fig. 6. da/dN vs ΔK (log-log plot) for different heat treatment conditions: a) FCGR curve for T6 and RRA b) Near-threshold FCGR curve for T6, RRA

Table 2. Paris constants C, m and FCGR at various ΔK levels in RRA and T6 conditions

da/dN = C(ΔK) m mm/cycle

Alloy

C

m

ΔK=12 MPa√m

ΔK=17 MPa√m

ΔK=22 MPa√m

3.99 x 10 -12

6.45 x 10 -5

7.35 x 10 -4

4.21 x 10 -3

RRA

6.7

4.02 x 10 -10

1.33 x 10 -4

1.22 x 10 -3

5.23 x 10 -3

T6

5.05

The microstructure of the alloy plays a vital role in reduction of FCGR in the RRA treated alloy sample. It is observed from TEM micrographs that the fine scale matrix precipitates are predominantly observed in the T6 treated sample. The edge to edge inter-particle space between the precipitates stood at 3.3 nm in T6 and 6.1 nm in RRA treated condition. It is well known that the retrogression leads to the partial dissolution of the GP zones and η´ precipitates in the matrix. The undissolved η´ precipitates will get coarsened during re-ageing stage of RRA, which lead to the increase in precipitate spacing in the alloy matrix. It was reported in the literature by Desmukh et al. (2006) and Xia et al. (2016) that various microstructures such as grain size, grain boundary and precipitates affect the FCGR behavior in near-threshold regime. The near threshold fracture surfaces of both RRA and T6 treated sample exhibited failure by shear deformation (Fig. 7a,d). A large facets of shear deformation in RRA appear to be visible in the near-threshold fracture region. The size of the faceted area depends on the strain accumulation during cyclic plastic deformation as described by Gupta and Agnew (2011). The fractured surfaces observed in Paris regime and high ΔK regime of FCG are shown in Fig. 7. The grain refinement and recrystallization during heat treatment is prevented by addition of the Zr that forms Al 3 Zr dispersoids to pin the grain boundaries. The PFZ width forms an important factor in the nature of crack propagation.