Study On Strain Behavior And Fracture Mechanism Of Fe-20Mn-3Cu-1.3C TWIP Steels

The effect of pre-strain,strain rate and temperature on tensile strain behavior and fracture mechanism of Fe-20Mn-3Cu-1.3C TWIP steels was studied. OM, SEM and TEM techniques were used to determine the contribution of dynamic strain aging(DSA) and twinning induced plasticity(TWIP) on mechanical properties in Fe-20Mn-3Cu-1.3C TWIP steels during tensile tests. And fracture mechanism was investigated as well.The strengthening behavior of Fe-20Mn-3Cu-1.3C TWIP steels after different pre-strain ranging from 0 to 0.81 was analyzed by tensile tests. The results indicate that yield strength, tensile strength and total elongation increased with the pre-strain. It is proposed that, besides the dislocations strengthening, DSA plays an important role in the enhanced mechanical properties of the alloys, because pre-strain induces a large amount of voids in the crystal lattice, and the voids allow the solute atmosphere diffuse rapidly to a sufficient density degree for pinning-depinning-repinning dislocations.The tensile strength of alloys first decreased and then stayed unchanged in the tensile tests at strain rate ranging from 2.5×10-4 s-1 to 2.5×10-2 s-1. However, the elongation exhibited parabola-like tendency in the strain rate range, touching bottom at the strain rate of 2.5×10-3 s-1,The OM observation show that the deformation twin density reaches minimum at this strain rate. A great number of deformation twin lamellas as well as dislocations distributed among the lamellas were found in the matrix of the alloys by TEM technique. The average value of the spatial distance between twin lamellas hits bottom at the strain rate of 2.5×10-3 s-1, which further illustrates that the influence of TWIP on the alloys is minimum at this strain rate.The strain behavior and fractography of Fe-20Mn-3Cu-1.3C alloys in the temperature range of -100～200℃ were investigated, the results show that in lower temperature stage, the tensile strength increases with the decrease of temperature. While above 0℃, the tensile strength increases with an increase of temperature up to 100℃ and reaches a maximum value of 1201.7MPa at 100℃. Then the tensile strength decreases as the temperature increases from 100℃ to 200℃. The ductility of the alloys in the tested temperature range first increase and then decrease, the elongation reached a peak value of 85.4% at 100℃. The product of tensile strength and ductility at 100℃ is 106217MPa·%, which suggests record high value among the temperature range and declares the optimum mechanical properties of Fe-20Mn-3Cu-1.3C alloys. SEM fractography reveals that the dimple fracture at lower temperature is tiny and shallow. While at higher temperature, the dimples widen and deepen, dimples with the maximum area and depth was observed in the alloy deformed at 100℃.Tensile tests of TWIP steels at different strain rates were performed at 200℃, the strain-stress curves reveal that the PL effect of the alloys almost disappeared at this temperature. This result can be attributed to a weakened DSA effect, at this temperature, the solute diffuse much more rapidly and as a result in the alloy it is impossible for the solute atmosphere to reach the critical density required for pinning and depinning the moving dislocations. The fractography deformed at this temperature reveals typical dimple-like fractures. The deforming microstructure evolution at this temperature becomes much more complicated, especially because thermal effect caused by surrounding environment as well as internal friction, so the change of the strength and ductility becomes more complex.The fracture mechanism of Fe-20Mn-3Cu-1.3C TWIP alloys can be explained as follows:during deformation process, the moving dislocations is blocked by twin boundary, micro-voids nucleate at boundary of deformation twins by the interaction between deformation twins and dislocation. The alloys deformed to fracture as micro-voids experience propagation and aggregation process.