PSI - Issue 52

Long Jin et al. / Procedia Structural Integrity 52 (2024) 12–19

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Author name / Structural Integrity Procedia 00 (2019) 000–000

3. Results and discussion 3.1. Tensile and microhardness test

The tensile stress-strain curves and detail properties of as-received and after aging at 450 ° C for 1000h specimens can be seen in Fig. 4a. Both specimens share the similar yield stress and ultimate tensile strength, while the elongation of aged specimens drop from 27% to 24% and the section shrinkage declines from 76% to 74% after aging. The changes of Vickers hardness after aging are shown in Fig. 4b. It can be seen that the hardness slight decreases around 3.4%, from 191.6 to 185. This means the material softens slightly after 1000h aging at 450 ° C.

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(b)

Fig. 4. (a) Tensile properties and (b) micro hardness of as-received and aged specimens

3.2. Cyclic stress response curves Fig. 5 presents the relationship between the stress amplitudes and the number of cycles of as-received and aged specimens. Both states of the materials undergo cyclic hardening in the first 10-30 cycles under different strain amplitudes, and then a trend of cyclic softening with the increase of the cycle, until the stress amplitudes experience a sharp fall and final fracture failure. In addition, the initial stage of hardening is more pronounced at large strain amplitudes ( Δ ε t /2=0.65% and Δ ε t /2=0.8%). The ratios of the tensile strength to yield stress ( σ b / σ s ) in Fig. 5 are 1.33 for as-received specimen and 1.35 for aged specimen, respectively. Smith et al. (1963) proposed that the cyclic hardening of the material occurs when σ b / σ s ＞ 1.4, cyclic softening occurs when σ b / σ s < 1.2 and either hardening or softening or both may occur when 1.2 < σ b / σ s < 1.4 . The behaviour of as-received and aged specimens exhibits both cyclic hardening and softening in this study and agree with the results of Smith. Fig. 6 shows the maximum cyclic hardening ratio H max =( Δ σ max - Δ σ 0 )/ Δ σ 0 under a variety of strain amplitudes. The hardening ratio H max rises as the stress amplitude increases. When the strain amplitude is 0.003, the value of H max is almost 1, indicating a solely cyclic softening process at this strain amplitude for both materials. It is noticed that the aging samples have a greater hardening ratio H max . This can be probably explained by the morphology of distributed carbides in Fig. 2. The distributed precipitations after aging hinders the movement of dislocations in deformation, which increases the hardening rate of the material (Silva et al., 2018).