PSI - Issue 14

M S Nandana/ Structur

al Integrity Procedia 00 (2018) 000–000

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

317

(a)

(b)

Fig. 3. (a) Standard tensile test sample (b) Standard CT sample geometry for FCGR tests

(All dimensions in mm)

3. Results and Discussion

3.1. Microstructure

Bright-field TEM micrographs of the T6 and RRA treated alloy samples are shown in Fig. 4(a,b). The T6 microstructure consists of continuous grain boundary precipitate (GBPs) whereas the RRA discontinuous GBPs similar to the over aged condition The matrix of the T6 treated sample consists of GP zones and fine scale η The RRA treated sample exhibits a range of precipitates. Majority of them are fine η Along with them are elongated disc shaped precipitates of η retrogression step of RRA heat treatment, which was detailed earlier GBPs present in T6 treated condition get disintegrated and rearranged discontinuously during retrogression stage. The precipitate free zone (PFZ) in RRA treated samples vary from 30 condition. The inter-particle spacing of grain interior precipitates was observed to increase from 3.3 nm in T6 treated condition to 6.1 nm in RRA treated condition. microstructure exhibits , similar observations were made by Desmukh et al. (2006) ´ (MgZn 2 ) precipitates of about 3-5 nm. ´ precipitates of about 4-5 nm. ´ of about 8-13 nm. These are formed during by Nandana et al. (2018). The continuous -35 nm compared to 25-30 nm in T6 treated . The stress vs strain plot for both RRA and T6 treated samples are presented in Fig. 5. The UTS of RRA treated sample is 586 MPa which is similar to that of the T6 treated sample et al. (2008). From Fig. 5 it is inferred that RRA treated alloy sample is stiffer when compared to the T6 treated alloy and the observed plastic strain to failure is 11.3 % in RRA alloy when compared to 14.4 % in T6 treated sample. The average elastic modulus of the T6 and RRA treated alloys are 75 GPa and 90 GPa respectively. During retrogression stage, the fine precipitates that are present in the grain interior are partially dissolved due to high temperature of retrogression (200˚ C - 20 min ). These dissolved precipitates are re , a similar observations were also made by R -nucleated in the matrix during eda 3.2. Tensile properties