Investigation On Unidirectional And Cyclic Deformation Behavior Of Commercially Pure Iron And Fe-Cr Alloy Single Crystal

For better understanding the mechanical properties of body-centre cubic (BCC) metals, commercially pure iron (CP iron) and Fe-35wt.%Cr single crystal were selected to investigate their mechanical properties under unidirectional and cyclic loading, focusing on the effects of micro-voids pre-existing on grain boundaries (GBs) in CP iron and fine Cr-rich precipitates in Fe-Cr single crystals on their deformation and damage characteristics. The following major conclusions can be drawn.The plastic deformation and damage behavior of coarse-grained commercially pure iron containing grain boundary micro-voids were investigated at room temperature with different strain rates ranging from1.0×10-5s-1to1.0×10-2s-1under uniaxial tension. It is found that, with increasing strain rate, the yield stress and ultimate tensile strength increase normally, and the elongation increases abnormally as well. By comparison with the similar cases of other experimental materials, the present phenomenon of anomalous strain rate effect on elongation is thought to be related to the pre-existence of GB micro-voids in the raw CP iron material. The extending deformation of GB micro-voids towards the tensile direction would contribute more to the total elongation, as the strain rate increases; this should be the most possible reason for the anomalous strain-rate dependence of elongation.Fatigue deformation behaviour of coarse-grained CP iron containing GB voids was examined under different total strain amplitudes Δεl/2. The coarse-grained CP iron exhibits different degrees of cyclic hardening, and no stress saturation stage appears. The relationship of plastic strain amplitude Δεpl/2versus fatigue life meets the Coffin-Manson law. The cyclic deformation surface characteristics are somewhat dependent upon the applied Δεt/2. As Δεt/2increases, the slip formation and thus-induced extrusions and intrusions become more serious, and slip cracking is more significant; more and more micro-voids at GBs are distorted and/or evolved into long cracks along GBs, even leading to GB cracking. All the fatigue fracture surface features at different Δεt/2are composed of the fatigue source zone, the crack growth zone and the final rapid fracture area, and distinctive features exhibit in the individual zone. The fatigue crack source zone is mainly featured by brittle fatigue striations, plenty of second cracks and cleavage steps or small cleavage planes. In the fatigue crack growth zone, regular fatigue striations form, and there are various secondary cracks between fatigue striations. The spacing between fatigue striations in the different regions (relevant to different grains) and the direction of crack propagation are not exactly the same. The final rapid fracture areas of all the fracture surfaces are characterized by the formation of dimples. Persistent slip band (PSB) ladder-like dislocation structures are observed just at a low total strain amplitude of Δεt/2=1.0×10-3. With increasing Δεt/2applied, dislocation cells are developed as the major microstructural features, and the average size of them decreases.The plastic deformation behaviors of the [149] and [014] Fe-35wt.%Cr alloy single crystals containing fine Cr-rich precipitates were investigated under uniaxial compression. The compressive flow behavior is slightly sensitive to the crystallographic orientation. These two oriented crystals exhibit a clear yield plateau in their compressive stress-strain curves, but the yield plateau of the [014] crystal is somewhat shorter than that of the [149] crystal. As the compressive strain is larger than a certain critical value, e.g.,～13%and～18%for the [014] and [149] crystals, respectively, the work hardening rate for both two orientations decreases obviously, but the decrease in work hardening rate is more remarkable for the [014] crystal rather than the [149] crystal. These phenomena are discussed to be all related to the interactions between moving dislocations and fine Cr-rich precipitates, and the interaction intensity depends strongly on the orientation. Careful observations of slip deformation characteristics and dislocation structures well provide supports for the explanations to the macroscopic compressive plastic flow behavior.Fatigue deformation features of [149] single-slip-oriented Fe-35wt.%Cr alloy single crystal containing Cr-rich precipitates were investigated under constant plastic strain amplitude control. When Δεpl/2≥2.5×10-3, the Cr-rich precipitates can be sheared readily by the moving dislocations during deformation, leading to an obvious stress softening phenomenon at the tensile loading stage of the first cycle, and subsequently to a slight cyclic softening phenomenon at a very early stage of cycling. In addition, the tension-compression stress asymmetry is found during cyclic deformation of the crystals, and this enhanced stress asymmetry should be related to the deformation instability of Cr-rich precipitates. The slip deformation features are mainly manifested by the formation of coarse slip bands comprising a quantity of fine slip lines and also by the formation of the kink band at high Δεpl/2(e.g.,5.0×10-3). The primary crack develops roughly along the slip plane (101) and the crystal finally cracks along this plane, accompanied with some secondary cracks having various morphologies forming on the crystal surface. Microstructural observation indicates that PSB ladder-like structures can be found at a low Δεpl/2of5.0×10-4, and the volume fraction of them increases with increasing Δεpl/2. As Δεpl/2is raised to5.0×10-3, the microstructural features are primarily characterized by the formation of dislocation cells.