PSI - Issue 14

Amit Singh et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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Amit Singh et al. / Procedia Structural Integrity 14 (2019) 78–88

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The double logarithmic plot of strain hardening rate with flow stress as per the modified C-J equation is shown in Fig. 3(c) and Fig.4. The parameters of modified J-C equation are listed in Table 5. It can be observed that as ST temperature decreases, the strain hardening exponent ( ) of stage I increases. This is similar to the trend observed for the strain hardening exponent for other constitutive equations as tabulated in Table 4. The stage I strain hardening can be explained using the colony size i.e. effective slip length. As explained earlier, decrease in ST temperature leads to a decrease in transformed β grain size (i.e. colony size) as well as lamellae thickness and increase in α p phase. During early stages of the deformation, α phase deforms though dislocation glide, which result in stage I strain hardening in modified C-J plot. From Fig. 4 and Table 5 it can be seen that strain hardening exponent ( ) for stage I strain hardening is low compared to stage II strain hardening. As deformation progresses more number of dislocations are generated which results in more dislocation – dislocation interaction, dislocations pileup at the grain boundaries, lamellae interfaces, and dislocations – precipitate (Ti 3 Al) interaction happening. This results in a higher strain hardening exponent during stage II which can be seen from Table 5. The work of Lutjering and Williams (2007) shows fine distribution of Ti 3 Al ( α 2, aluminide phase) precipitate inside α grain and (Ti, Zr) 6 Si 3 (S 2, silicide phase) precipitate at interface of lamellae, which is shown in Fig. 5. Since, the aluminide precipitate is present inside α phase, they contribute more to stain hardening compared to the silicide precipitate. The result of it is shown in Fig. 4, Fig. 3c and in the strain hardening exponent ( ) values tabulated in Table 5 which shows a higher strain hardening in stage II. It can also be observed from Table 5 that (strain hardening exponent) for stage II is relatively same for two ST temperature viz., 1318 K and 1303 K with a α p phase fraction of 14.39 and 9.10%. While, for 1288 K (i.e. α p ~22.32%) the value of for stage II is lower. This can be explained by solute partitioning effect, which is a dominant factor at low ST temperature. The value are very low for stage III strain hardening for all ST temperature, which can be explain by the saturation of strain hardening and leads to instability of strain hardening. 5. Conclusions  Decreasing the solution treatment temperature from β transus temperature (1333K) to 1288K, increases α p phase from 0.045% to 22% . This results in improved yield strength, tensile strength, and ductility up to 14.5% α p. However, with further increase in α p phase (i.e. ST done at 1288 K) deteriorates the aforesaid tensile properties.  The improvement in properties is attributed to decrease in colony size/lamellae size while deterioration of tensile property is governed by solute element partitioning effect.  The improvements in strain hardening is noticed in all microstructure with increasing α p from 0.045% to 22% α p . It is attributed to decrease in the lamellae and colony size with decreasing the ST temperature.

Acknowledgements

The authors are thankful to Dr. Vikas Kumar, Director, DMRL for encouraging us to publish this work. One of the author (AS) is very grateful to Dr. Manasij Yadava, Research Scientist, IIT Kanpur for insightful discussions on strain hardening.

References

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