Strain hardening in Fe–16Mn–10Al–0.86C–5Ni high specific strength steel

Abstract

Keywords:High specific strength steel Dual-phase nanostructure Strain hardening Ductility In situ high-energy X-ray diffraction a b s t r a c tWe report a detailed study of the strain hardening behavior of a Fee16Mne10Ale0.86Ce5Ni (weight percent) high specific strength (i.e. yield strength-to-mass density ratio) steel (HSSS) during uniaxial tensile deformation. The dual-phase (g-austenite and B2 intermetallic compound) HSSS possesses high yield strength of 1.2e1.4 GPa and uniform elongation of 18e34%. The tensile deformation of the HSSS exhibits an initial yield-peak, followed by a transient characterized by an up-turn of the strain hardening rate. Using synchrotron based high-energy in situ X-ray diffraction, the evolution of lattice strains in both the g and B2 phases was monitored, which has disclosed an explicit elasto-plastic transition through load transfer and strain partitioning between the two phases followed by co-deformation. The unloadingreloading tests revealed the Bauschinger effect: during unloading yield in g occurs even when the applied load is still in tension. The extraordinary strain hardening rate can be attributed to the high back stresses that arise from the strain incompatibility caused by the microstructural heterogeneity in the HSSS.

Shock and spall behaviors of a high specific strength steel: Effects of impact stress and microstructure

“…After cold rolling, two phases are visible, of which one is the fcc c-austenite with an elongated shape and the other is the lamellar B2 (FeAl intermetallic compound) phase with a large aspect ratio, as indicated in our previous paper. 18 After annealing at 900 C for 15 min, the c-austenite is the equiaxed recrystallized grains, while both granular and lamellar B2 phase precipitates are observed, as shown in Fig. 2(a).…”

“…The quasi-static uniaxial tensile tests were performed using an MTS Landmark testing machine under a strain rate of 5 Â 10 À4 s À1 at room temperature, and the other procedures and the details about the tensile tests can also be found in our previous paper. 18 …”

“…Similar to the procedures in our previous research, 18 an Fe-16.4Mn-9.9Al-0.86C-4.8Ni-0.008P-0.004S (wt.%) HSSS was produced using arc melting in a high frequency induction furnace under a pure argon atmosphere and then cast into a cylindrical ingot with a diameter of 130 mm and a length of 200 mm. The ingot was first homogenized at 1180 C for 2 h and then hot forged into slabs with a thickness of 14 mm in between 1150 C and 900 C. With a starting temperature of 1100 C, the hot forged slabs were hot-rolled into strips with a thickness 9.2 mm.…”

“…The details of the sample preparations and the operation procedures for obtaining OM, SEM, and TEM images can be found in our previous paper. 18 In the present study, two microstructures (annealed state 1 and annealed state 2) were considered in order to study the effects of the microstructure on the shock and spall behaviors. Before plate impact experiments, the quasi-static uniaxial tensile tests are also carried out to obtain the yield strength, the strain hardening behaviors, and the uniform elongation under quasi-static uniaxial stress conditions for two different microstructures, and these properties will then be used to qualitatively reveal the effects of the microstructure on the shock and spall behaviors.…”

“…The back-stress-induced strain hardening has been reported to contribute to the large uniform elongation in TWIP steels, 1,14 TRIP steels, 15 and DP steels. 16,17 In our previous research, 18 we have also shown that the HSSS developed by Kim et al 13 is better understood as a dual-phase microstructure since the B2 phase is deformable. Moreover, the stress/strain partitioning and the back-stress-induced strain hardening have been proven to play an important role during the plastic deformation for this HSSS based on in situ high energy X-ray diffraction data, scanning electron microscopy (SEM) images of samples before and after tensile tests, and the back stress measurement from tensile load-unloadreload (LUR) tests.…”