Takahiro Sawaguchi

Friction stir welding (FSW) was applied to a 10 mm-thick plate for the Fe-15Mn-10Cr-8Ni-4Si seismic damping alloy. A sound FSW joint was obtained successfully without macro-defects such as groove-like defects and tunnel holes. However, small pores with diameters of 1–5 μm were formed owing to the wear of the FSW tool during the FSW. The decrease in the heat input suppressed the tool wear. Consequently, the distribution of small pores was limited to the border of the stir zone at the advancing side under smaller heat input conditions. The stir zone of the FSW specimen produced at 125 rpm showed a higher tensile strength of 759 MPa owing to the grain refinement and the high elongation of 50% compared with the base metal. In addition, the stir zone exhibited a remarkable fatigue life of 9,723 cycles. This was higher than that of the base metal (8,908 cycles). Grain refinement occurred by discontinuous dynamic recrystallization (DDRX) via high-angle boundary bulging and direct nucleation in the high-dislocation area. The increase in the heat input suppressed the DDRX owing to the promotion of dynamic recovery.

The γ-ε martensitic transformation in austenitic steels generally enhances work hardening capacity and fatigue resistance. However, it also induces brittle-like fracture and compromises resistance to hydrogen embrittlement. Understanding the evolution mechanisms of plasticity, stress, and damage during deformation involving γ-ε martensitic transformation is essential to mitigate these adverse effects. To facilitate this understanding, this study reviews the mechanisms of stress concentration and accommodation, identifies cracking and crack arrest sites, and analyzes fracture behaviors associated with γ-ε martensitic transformation under monotonic tensile deformation and cyclic loading conditions. Furthermore, potential strategies for enhancing damage tolerance are introduced.

A brace-type seismic damper made of an Fe-15Mn-11Cr-7.5Ni-4Si alloy solidified in the ferrite-austenite (FA) mode and SN490B steel, which can be constructed via welding, was proposed. To realize the proposed seismic damper, gas metal arc welding was applied to produce similar FMS/FMS fillet welds and dissimilar FMS/SN490B fillet weld joints. Based on the Schaeffler diagram, similar and dissimilar welding consumables were designed such that the fillet weld metal solidified in the FA mode without solidification cracking. Sound similar fillet welded joints were obtained using two types of welding consumables with different Cr/Ni equivalent ratios although both the similar fillet weld metals had a coarse columnar austenite grain structure. These displayed higher tensile strengths (716–736 MPa) and marginally lower elongations (67–70%) than the FMS alloy. Moreover, a similar fillet weld metal with a chemical composition almost identical to that of the FMS alloy exhibited a remarkable low-cycle fatigue life (5740 cycles). This was shorter than that of the FMS alloy (9351 cycles) owing to the easier formation of α'-martensite. A dissimilar fillet welded joint with a chemical composition within the austenite region was produced without solidification cracking. The dissimilar fillet weld metal showed high tensile strength (867 MPa) and total elongation (61%). These were comparable to those of similar fillet weld metals.

Previous research has shown that Fe–Mn–Cr–Ni–Si alloys offer excellent low-cycle fatigue resistance via reversible bidirectional transformation between face-centered cubic (FCC) γ-austenite and hexagonal closed-packed (HCP) ε-martensite. The alloy shows superior low-cycle fatigue life and is used for seismic damping applications, but there have been concerns over their resistance to highly corrosive environments. In this study, Fe–15Mn–aCr–bNi–4Si alloys were prepared with different Cr and Ni concentrations to evaluate the effects on the fatigue and corrosion resistances: Z1 with (a, b) = (14, 10.1), Z2 with (a, b) = (12.5, 8.8), Z3 with (a, b) = (11, 7.5), Z4 with (a, b) = (9.5, 6.1), and Z5 with (a, b) = (8, 4.8). Z2 had the longest fatigue life. The alloy showed Gibbs free energy difference between γ-austenite and ε-martensite phases close to the ideal of zero and the α′-martensitic transformation was suppressed well, which agreed with the design criteria for achieving bidirectional transformation-induced plasticity. The developed alloys showed superior corrosion resistance in seawater. Local pitting corrosion was observed that was attributed to the high Mn concentration of the alloys, although this was greatly mitigated by adjusting the Cr and Ni concentrations, especially with Z1 and Z2.

The effect of stability of austenite (γ) phase on Lüders-like deformation behavior of cold-rolled Fe-Ni-Al-C alloys was investigated by digital image correlation (DIC) analysis, magnetic measurements and electron backscatter diffraction (EBSD) analysis. In this study, cold-rolled Fe-23Ni-5.0Al-0.5C (mass%) alloy (23Ni alloy) and Fe-25Ni-5.0Al-0.5C (mass%) alloy (25Ni alloy) were used. The tensile strength increased and the total elongation decreased with decreasing Ni content. Lüders-like deformation after yielding was observed up to a nominal strain of approximately 0.04 for the 23Ni alloy and approximately 1.0 for the 25Ni alloy. The transformation from γ to martensite (α') with a specific crystal orientation relationship was observed during tensile deformation, and it was found that the volume fraction of α' during tensile deformation increased more easily in 23Ni alloy than in 25Ni alloy. In addition, the increment of α' phase in 25Ni alloy was higher during Lüders-band propagation than after the band propagation. These results suggested that change of the Lüders-like deformation behavior was due to the change in γ stability.

We investigate the effects of the annealing microstructure on the low-cycle fatigue (LCF) life (N_f), cyclic stress behaviour and fatigue microstructure of a Fe–15Mn–10Cr–8Ni–4Si alloy that exhibits a deformation-induced transformation of austenite (γ-phase) into ε-martensite (ε-phase). The alloy rolled at 800°C was annealed at 600°C, 700°C, 800°C and 900°C to vary the grain size, the fraction of recrystallised grains and the texture intensity. Fully reversed axial strain-controlled LCF tests were conducted at total strain amplitudes, Δε_t/2, ranging from 0.007 to 0.02. The alloy showed a higher N_f than common steels and ferrous high-Mn alloys in this strain range. This type of annealing microstructure was found to impact the N_f, fatigue behaviour and deformation-induced ε-martensitic transformation (ε-MT) in the studied alloy. The fully recrystallised and weakly textured austenite formed at T ≥ 800°C facilitated the uniform development of ε-martensite under cyclic deformation and led to an increased N_f. The partially recrystallised and textured austenite-containing substructure with high dislocation density formed at T ≤ 700°C suppressed the ε-MT, retarded reversible dislocation motions in the un-recrystallised regions and moderately decreased N_f. Moreover, N_f and the deformation-induced ε-MT were observed to be less sensitive to variations in grain size.