Low-cycle fatigue life prediction of a polycrystalline nickel-base superalloy using crystal plasticity modelling approach

A crystal plasticity model is developed to predict the cyclic plasticity during the low-cycle fatigue of GH4169 superalloy.Accumulated plastic slip and energy dissipation as fatigue indicator parameters(FIPs)are used to predict fatigue crack initiation and the fatigue life until failure.Results show that fatigue damage is most likely to initiate at triple points and grain boundaries where severe plastic slip and energy dissipation are present.The predicted fatigue life until failure is within the scatter band of factor 2 when compared with experimental data for the total strain amplitudes ranging from 0.8%to 2.4%.Microscopically,the adjacent grain arrangements and their interactions account for the stress concentration.In addition,different sets of grain orientations with the same total grain numbers of 150 were generated using the present model.Results show that different sets have significant influence on the distribution of stresses between each individual grain at the meso-scale,although little effect is found on the macroscopic length-scale.

Grain-refining and strengthening mechanisms of bulk ultrafine grained CP-Ti processed by L-ECAP and MDF

The microstructural evolution and mechanical properties of ultrafine-grained(UFG)CP-Ti after an innovative large-volume equal channel angular pressing(L-ECAP)and multi-directional forging(MDF)were systematically examined using monotonic tensile tests combined with transmission electron microscope(TEM)and electron backscatter diffraction(EBSD)techniques.Substantially refined and homogeneous microstructures were achieved after L-ECAP(8-pass and 12-pass)and MDF(2-cycle and 3-cycle),respectively,where the grain size distribution conformed to lognormal distribution.The grain refinement of450℃L-ECAP is dominated by dynamic recrystallization(DRX)and dynamic recovery(DRV),while that of MDF is dominated by DRX.The iron impurities promote recrystallization by pinning-induced dislocation accumulation so that DRX is prone to occur at iron segregation regions during L-ECAP.The monotonic tensile results show that the strain hardening rate of CP-Ti increases with the decrease of grain size,while the continuous strain hardening ability decreases.The relationship between the average grain size and yield strength is in accordance with Hall-Petch relationship.Meanwhile,the individual strengthening mechanisms were quantitatively examined by the modified model.The results indicate that the strengthening contribution of dislocation accumulation to yield strength is greater than that of grain refinement.