Papers by Author: Abbas Najafizadeh

Abstract: The effect of grain size on the flow stress in TWinning Induced Plasticity (TWIP) steel was investigated via the X-ray diffraction (XRD) measurements of dislocation density. The results indicated that the hardening behavior of fine grained samples (mean grain sizes in the range of 2.1-3.8μm) can be described as typical dislocation interactions. However in coarse grained samples (mean grain sizes in the range of 4.7-38.5μm) where extensive mechanical twinning occurs, another strengthening mechanism is required. Consequently, the effect of grain size on the flow stress parameters of the proposed equation was considered and it was found that in the fine grained samples, the Holloman analysis can describe the hardening behavior. However, in coarse grained samples, a second hardening term due to the strengthening effect of mechanical twin boundaries needs to be added to the Holloman equation.

Abstract: The kinetics of interpass softening and the strain (ε*) at which they become strain-independent were determined by means of double-hit hot compression tests. For this purpose, interrupted compression tests were conducted at strain rates of 0.01 and 0.1 s-1 to initial strains ranging from that corresponding to the initiation of DRX (εc) to the onset of steady state flow (εs.). Test temperatures between 1000 and 1100 °C (inclusive) were employed. Interpass times were varied from 0.3 to 1000 seconds. The fractional softening was determined using the 0.2% offset method. It is clear from the results that there is a transition strain (ε*) that separates the straindependent range of post-dynamic softening from the strain-independent range. The value of ε* obtained in this work was ε* = 4/3 εp. It was also found that the strain hardening rate was identical at all the critical strains (ε*) and took the value -22 MPa.

Abstract: Experiments were carried out in which the dependence of the fractional softening on temperature, time and strain rate was determined in a 304H stainless steel. Three prestrain ranges were identified pertaining to three different post-deformation softening behaviors: 1) prestraining to below the DRX critical strain: strongly strain dependent softening by SRX alone with softening kinetics controlled by growth rate of the nuclei; 2) prestraining to above the DRX critical strain: SRX + MDRX softening with weaker strain dependence of the kinetics but still controlled by grain growth; 3) at a prestrain of ε* and beyond: nucleation-controlled MDRX softening with the full inhibition of SRX. The transition prestrain ε* can exceed the peak strain if the DRX grain refinement ratio g = D0/DDRX > 4. The transition to MDRX-dominated softening can be attributed to a constant value of the normalized strain hardening rate independent of the preloading temperature and strain rate. The softening data from the compression tests show that at ε*, the time for half softening t50 exhibits a minimum. These data differ somewhat from observations obtained in the torsion testing of solid bars, in which no strain dependence of t50 was detected at ε* and beyond. Whether or not the strain dependence of t50 vanishes in the MDRX range is sensitive to the test method employed to study the post-deformation softening.