Study Of Twinning Induced Plasticity With Addition Of Nitrogen Or Niobium

The combination of high strength and good ductility is a seeking objective of the structural material design. Recently, the new developed high manganese steels containing 15 to 25 mass% Mn and additions of silicon and aluminum of about 2 to 4 mass% exhibit the large elongation (60-95%) and high tensile strength (600-1100MPa) due to twinning induced plasticity (TWIP) effect or transformation induced plasticity (TRIP) effect via multiple martensitic transformations. Although the effects of Mn, Si and Al on the microstructure and mechanical properties of TWIP steel had been investigated, the effect of N, Nb on those of TWIP steel are not reported, and the mechanism of TWIP effect, TRIP effect are not revealed sufficiently. Several TWIP steels with different content of N, Al or with addition of Nb were designed, and the effects of N, Al and Nb on stacking fault probability (SFP, reverse proportional to stacking fault energy) and mechanical properties were studied by means of tensile tests and characterization through optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). And the mechanisms of TWIP effect andεhcp martensite formation as well as the approaches of martensitic transformation were systematically investigated, and the main conclusions are described as follows.The SFP of TWIP steels with different addition of N, Al were determined, and tensile tests were performed at different temperature. The results indicate that both N and Al increase stacking fault energy (SFE) of TWIP steel, suppress martensitic transformation and favour the TWIP effect. The XRD spectra of TWIP steels with different composition were analyzed and the results show thatαmartensitic transformation is intensively suppressed in the TWIP steel with addition of N. When FCC austensite or HCPεmartensite transforms to BCC structureαmartensite in these steels with addition of N, the biggest interstice decreased from 0.1044nm in FCC or HCP structure to 0.0733nm in BCC, and the lattice distortion energy of BCC martensite were greatly enlarged by N situated in the interstices, leading to the suppression of theαmartensitic transformation .The tensile tests were carried out and the SFP of TWIP steels with different addition of Nb were determined at different temperature. The results show that the addition of Nb remarkably decreases SFP, or increases SFE, and lead to superior ductility by favouring the formation of deformation twinning and TWIP effect. The microstructure characterization of TWIP steel containing Nb show that the addtion of Nb greatly reduces the size of austensite grain, and thus improves the strength of steel. The comprehensive effects of Nb on grain refinement and SFE give rise to excellent mechanical properties, the product of tensile strength and elongation(PSE) can be as high as 70000MPa% and is 20000MPa% higher than the TWIP steel without addition of Nb; the average PSE is as high as 56150.5MPa% at low temperature range from 0 to -30℃and is also higher than the 44729 MPa% of the later.During the observation of stacking faults by TEM, the directional shifts of some diffraction spots were found in the local area with high density of parallel stacking faults, the relationship between the shift of diffraction spot and SFP in FCC structure is deduced and the method of determining SFP was established. Based on the results of local SFP measured by electron diffraction, the mechanism ofγfcc→εhcp martensitic transformation was proposed, i.e. deformation provides the energy for the dissociation of perfect dislocation, and thus a large number of partial dislocations accompanying stacking faults are formed during deformation, and the stress field of partial dislocations can induce the formation of other perfect dislocations and their dissociation, resulting in the localization of stacking faults, then the high density of local stacking faults evolute from stacking disorder to stacking order and formedεmartensite eventually. When the stacking fault probabilityα=1, the perfectεmartensite is formed, otherwise, whereasα→1, theεmartensite with stacking fault is formed. Based on the mechanism ofεmartensite formation it is clear that both of the nucleation and growth ofε,αmartensites will consume a large number of stacking faults and make the number of stacking faults decrease sharply, which caused that the SFP measured by XRD is not reverse proportional to SFE after martentic transformations in TWIP steels.Tensile test and XRD analysis of several TWIP steels at different temperature indicate that stacking fault energy is proportional to temperature, i.e. stacking fault energy decreases with the lowering temperature. The close relationship between stacking fault energy and temperature result in different mechanical properties of the alloy at different temperatures. At higher temperature, the SFE is higher and is favoured to twinning during deformation, and TWIP effect is induced; at lower temperature, TRIP effect is favoured. The deformation temperature range in which TWIP or TRIP effect takes place is different for steels with different SFE. With the increasing of SFE, the peaks in an elongation-temprature curve caused by TWIP or TRIP effect move to lower temperature.The morphologies of stacking faults and twins in FCC TWIP steels were predicted by crystallographic analysis and were verified by means of OM, SEM and TEM. The mechanisms of inducing plasticity (sliding induced plasticity, twinning induced plasticity or transformation induced plasticity) in TWIP steels was suggested based on both the observation of morphology of twins at different deformation conditions and fundamental principle of martensitic transformation, namely, the start temperature (Tf) of deformation twinning and the start temperature (Ms) of therma-induced martensite determine the deformation mechanisms. Above the Tf, the ductility of TWIP steel is realized only by sliding; between Tf and Ms, the predominant mechanism is deformation twinning accompanying TWIP effect; and below Ms, it is TRIP effect.The results of tensile tests at different deformation rate show that, the mechanical properties of TWIP steel is affected by deformation rate. The yield strength is slowly enhanced with increasing deformation rate; while the ultimate strength lowly decreases with the increase of deformation rate. Both of the total elongation and uniform elongation decrease firstly and then increase with increasing deformation rate. Generally, the effect of deformation rate on elongation is more powerful than on yield strength and ultimate strength.The mechanical behavior of TWIP steel at different temperature show that, at higher temperature, martensitic transformation experiences in two ways:γ_fcc→ε_hcp,γ_fcc→ε_hcp→α_bcc; at lower temperature, there is the other way of martensitic transformation:γ_fcc→α_bcc, i.e. austensite can be directly transformed toαmartensite when TRIP effect became a predominant mechanism. The high elongation of TWIP steel result in the formation and tangle of a large number of dislocations inγaustensite during deformation, and such tangle dislocations are not favourable for the formation ofεmartensite, and cause the transformation of dislocation-type martensite from austensite directly.