PSI - Issue 14

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

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

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then decreases with further increase in α p . It can be noticed that the maximum in tensile properties was achieved when α p phase fraction is about ~14.5%. Hence, this much α p is critical to achieve optimum tensile properties. By changing ST temperature with constant moderate cooling rate two kinds of microstructures formed one is bi modal and another is lamellar microstructure. These microstructures have certain key features/parameters which will govern the mechanical re sponse to the imposed deformation. These microstructural parameters are grain size of α p , transformed beta, colony size and solute element partitioning effect. It is understood from Fig.2 that for moderate cooling (i.e. air cooling) the transformed β grain size is equal to the colony size. Extensive study by Lutjering (1998), Lutjering and Williams (2007) on the effect of aforesaid microstructural parameters report that, colony size determines the effective slip length of lamellar microstructure and as the colony size decreases the yield strength, tensile strength and ductility increases. This agree well with the results shown in Fig. 3a and Table 2 up to ~ 14.5 % α p from 0.045% α p (i.e. ST temperature from 1333 K – 1303 K). While, further decrease in the yield strength, tensile strength and moderate decrease in ductility ( as shown in Fig.3a and Table 2) is attributed to solute elements partitioning effect for more than ~ 14.5% α p in bi-modal microstructure. This solute element partitioning effect leads to lower basic strength within the lamellar regions of bi-modal microstructure as compared to fully lamellar microstructure and it has only a very small effect on ductility as well as on propagation behavior of micro and macro cracks. The dependence of yield s trength on α p phase is a synergistic combination of contribution from colony size and solute element partitioning. Hence, it can be reported that for small α p phase fraction (i.e. 0.045% α p - 14.5% α p ), colony size is dominating while for more than ~ 14.5% α p phase, solute elements partitioning is dominating. Hence, it can be seen that there is a competition between dominating parameters to achieve optimum mechanical properties. From Fig. 3a and Table 2 it may be seen that the ductility of the sample ST at 1333 K (i.e. ~0.045% α p ) is very less compared to other ST temperature sample. This may be understood in term of interaction of mobile dislocations with coherent Ti 3 Al ( α 2 aluminide) precipitates. The α 2 phase can be easily sheared by moving dislocation which result in the formation of intense, planar slip bands leading to easy crack nucleation and also to fast crack propagation within these slip bands. Lutjering and Williams (2007), reported that the crack nucleation and crack propagation depends on the slip length. Since, as per earlier discussion the slip length is related to colony size (i.e. equivalent to β grain size) , so this effect is very pronounced for coarse lamellar microstructure and less pronounced for bi-modal microstructure. The above reasoning concurs well with the work of Srinadh et al. (2007).

Table 1. The statistical data shows role of ST temperature on microstructure features Samples [ST + Aged (973/2hrs)] α p (area %) Lamellae thickness (µm)

Size of α p (µm)

Transformed beta grain size (µm)

ST (1288 K) ST (1303 K) ST (1318 K) ST (1333 K)

22.32 14.39 9.10

1.84 2.49 2.79 3.27

21.7 19.7 17.9 1.54

129 479 534 795

0.045

Table 2. The average value of primary α (α p ) and tensile properties Samples [ST + Aged (973/2hrs)]

σ UTS / σ 0.2%

α p (area %) Yield Strength (MPa)

Tensile Strength (MPa)

Elongation (%)

ST (1288 K) ST (1303 K) ST (1318 K) ST (1333 K)

22.32 14.39 9.10

728.053

877.614 1087.032 1051.401 940.778

9.712 11.23 9.10 4.91

1.208 1.174 1.163 1.099

925.370 901.818 853.796

0.045