PSI - Issue 14

Amit Singh et al. / Procedia Structural Integrity 14 (2019) 78–88 Amit Singh et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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the solution treated and aged material, blanks of required dimensions were extracted through electro discharge machining (EDM). ASTM E8 standard cylindrical tensile samples with 6 mm diameter and 30 mm gage length as shown in Fig.1 were prepared from the extracted blanks and tested at room temperature using a computer controlled 200KN servo-hydraulic testing machine custom built by M/s BISS, Bangalore with a cross head speed of 1 mm/min.

4. Results and discussions

4.1. Effect of solution treatment temperature on microstructure

The optical microstructures obtained after solution treatment at various temperatures and ageing are shown in Fig.2. It can be seen that solution treatment at different temperatures has resulted in two types of microstructures viz., bimodal and lamellar. The bimodal structure with primary alpha and transformed beta is obtained when the material is solution treated till 1318K (i.e. up to ~9% α p ) while solution treatment at 1333K has resulted in a lamellar structure with ~ 0.045% α p as listed in Table.1. From Table 1 and Fig.2 it can be understood that with increasing ST temperature, the volume fraction and size of α p phase decreases and completely vanishes at 1333 K (Fig.2d). At higher temperature, the α p phase is found only at triple point of transformed β grain boundaries while at low temperature, it forms a complete network along the trans formed β grain boundaries (Fig.2 a, and 2c). It is expected and can be seen that, the trans formed β grain size decreases with decrease in solution treatment temperature (Fig.2 and Table 1). It can be observed that the colony size increases with increasing solution treatment temperature. The observation here agrees well with that reported in literature, Lutjering and Williams (2007), Balasundar et al. (2014).

Fig. 2. Microstructure of IMI 834 alloy obtained by air cooling after solution treatment at (a) 1288 K, (b) 1303 K, (c) 1318 K and (d) 1333 K for 2Hrs. followed by aging at 973 K/ 2Hrs. followed by air cooling.

4.2. Effect of so lution treatment temperature (i.e.α p phase fraction) on tensile behavior

The effect of ST temperature (i.e. effect of α p phase) on tensile properties is shown in Fig. 3a and data is summarized in Table 2. It can be seen that by decreasing ST temperature from β transus temperature (1333 K) where α p is ~0.045%, the yield strength, tensile strength and ductility (% elongation) increases and reaches a maximum and