•Compressive residual stress reduced by 90% of the as-built value following 1100 °C for 5 min.•Pitting potential decreased by around 180 mV on stress-relieving specimens.•Repassivation potential ...increased with decreasing compressive stress for all specimens.•Donor density in the passive film increased with decreasing compressive stress.
Selective Laser Melting (SLM) and various subsequent stress-relieving treatments were used to obtain 316 L specimens with compressive residual stresses, varying from 15 to 250 MPa. This enabled a study on the effect of residual stress on corrosion of 316 L using electrochemical methods, which is relevant for durability of additively manufactured materials. Overall, compressive stresses in SLM 316 L result in a measurable increase in the pitting potential, accompanied by a decrease in the passive film currents and donor densities. It is proposed that compressive stresses lower the film growth and repassivation kinetics but slightly enhances the pitting resistance of SLM 316 L.
The effect of the relative contents of Al and Ti on the equilibrium phase formation in a novel Ni51Co18Fe5Cr10Al16-XTiX high entropy superalloy (HESA) system was analysed using Thermo-Calc software ...(TCHEA-3 database). The proposed HESAs showed a broad γ+γ′ equilibrium phase field over an extended temperature range (from 660 °C to a γ′-dissolution temperature >1184 °C) without the presence of brittle intermetallic phases. The effect of Al/Ti ratio on the microstructure, mechanical properties and precipitation strengthening of the HESAs was determined. The proposed alloys showed a high volume fraction (∼60%) of a (Ni, Co)3(Al, Ti)-type ordered (γ′) phase in a disordered face-centered cubic matrix phase (γ). The microstructure and the thermal properties of the HESAs showed excellent correlation to the theoretical thermodynamic predictions. A lower Al/Ti ratio improved the mechanical properties of the HESA system due to the high lattice misfit between the γ and γ′ phases. The homogenised and aged Ni51Co18Fe5Cr10Al8Ti8 HESA exhibited a γ′-volume fraction of 63%, γ′-dissolution temperature of 1200 °C and yield strengths of 1056 MPa and 852 MPa at room-temperature and 800 °C, respectively. The present work shows the potential to design Al–Co–Cr–Fe–Ni–Ti HESAs with a high volume fraction of γ′-phase and excellent mechanical properties.
•Ni51Co18Fe5Cr10Al16-XTiX showed a broad γ+γ′ only equilibrium phase field over an extended temperature range (660 °C to a γ′-dissolution temperature >1184 °C).•Yield strength of Ni51Co18Fe5Cr10Al16-XTiX alloys increased with a decrease in Al/Ti ratio due to the increased the lattice misfit between γ and γ′ phases.•Ni51Co18Fe5Cr10Al16-XTiX alloys with a γ′-volume fraction of 63% and γ′-dissolution temperature of 1200 °C.•Homogenised and aged Ni51Co18Fe5Cr10Al8Ti8 HESA exhibited yield strengths of 1056 MPa and 852 MPa at room-temperature and 800 °C.
In order to improve the understanding of the dynamic and post-dynamic recrystallization behaviours of AISI 304 austenitic stainless steel, a series of hot torsion test have been performed under a ...range of deformation conditions. The mechanical and microstructural features of dynamic recrystallization (DRX) were characterized to compare and contrast them with those of the post-dynamic recrystallization. A necklace type of dynamically recrystallized microstructure was observed during hot deformation at 900
°C and at a strain rate of 0.01
s
−1. Following deformation, the dependency of time for 50% recrystallization,
t
50, changed from “strain dependent” to “strain independent” at a transition strain (
ɛ
*), which is significantly beyond the peak. This transition strain was clearly linked to the strain for 50% dynamic recrystallization during deformation. The interrelations between the fraction of dynamically recrystallized microstructure, the evolution of post-dynamically recrystallized microstructure and the final grain size have been established. The results also showed an important role of grain growth on softening of deformed austenite.
In this study, the microstructure dependence of impact toughness was studied for a 2205 duplex stainless steel in the temperature range of −196 to 25 °C. Three markedly different austenite ...morphologies (i.e., rolled (R), equiaxed (E) and Widmanstätten (W)) were produced through different thermomechanical routes. It was found that while the room temperature impact toughness of all microstructures were quite similar, the microstructure dependence of impact toughness significantly increased with decreasing testing temperature. At cryogenic temperatures, microstructure R showed significantly higher toughness compared to microstructures E and W. Considering a 40 J criterion, the ductile to brittle transition temperature was estimated to be ∼ −80 °C for microstructures W and E, while microstructure R showed impact toughness values higher than 40 J even at −196 °C. The lamellar character of microstructure R and the termination of ferrite phase on the (100) plane orientation in this microstructure were found to have a positive effect on the toughness. The occurrence of deformation twinning within ferrite at low temperatures, facilitated by significantly coarser grain sizes in microstructures E and W compared to R, appeared to be the main reason behind the observed deterioration of the impact toughness of the former microstructures at these temperatures.
The microstructure evolution within the shear localization areas formed in commercial titanium subjected to cold rolling is systematically investigated. Sheared micro-regions are first initiated ...followed by the formation of distinct microscopic shear bands, which gradually grow and coalesce to form a macroscopic shear band. The latter contains thin lath structures in the boundary regions, fine elongated subgrains in the outer areas and roughly equiaxed (sub)grains with a mean size of 70
nm in the centre region. The early stage of shear localization involves the formation of twin/matrix lamellae aligned along the shear direction. The lamellae subsequently undergo longitudinal splitting into thin laths, which are in turn subjected to transverse breakdown, giving rise to fine elongated subgrains. The continuing thermally assisted lath breakdown, in conjunction with lateral sliding and lattice rotations, ultimately leads to the formation of roughly equiaxed, nanosized (sub)grains in the macroscopic shear band centre at large strains.
Most of the applications planned for high entropy alloys (HEAs) entail thermomechanical processing. In the current study, the hot deformation and dynamic restoration mechanisms of two duplex high ...entropy alloys (Al0.6CoCrFeNi and Al0.9CoCrFeNi) are revealed through a detailed electron back-scattered diffraction and transmission electron microscopy study. A standard homogenization of the cast microstructures results in face-centred cubic/body-centred cubic (FCC/BCC) duplex structures for both alloys. Al0.6CoCrFeNi has an FCC matrix containing dispersed BCC particles surrounded by interdendritic BCC. Al0.9CoCrFeNi has a BCC matrix with allotriomorphic and intragranularly dispersed FCC particles. Hot deformation at 1030 °C under the strain rate of 0.1s−1 is mainly accommodated through the matrix phase in both alloys. In Al0.6CoCrFeNi alloy, the FCC matrix undergoes discontinuous dynamic recrystallization mechanism (DDRX) while there are also some local observations of subgrain coalescence. DDRX mainly occurs at the interphase boundary mantle regions through the formation of Σ3 boundaries and the propagation of multiple twinning chains. Dislocations within the hot deformed FCC matrix have a large screw component. In the Al0.9CoCrFeNi alloy, the BCC matrix softens through continuous dynamic recrystallization (CDRX), characterised by the progressive conversion of low-misoriented subgrains into (sub)grains delineated partly by low-angle and partly by high-angle boundaries. Dislocations within the BCC matrix have a mixture of edge and screw character. The dispersed second phases in both alloys are mechanically fragmented during hot deformation and are quite evenly distributed throughout the microstructure at a strain of 1.5.
•Softening mechanisms in Al0.6CoCrFeNi and Al0.9CoCrFeNi alloys were studied.•The FCC matrix undergoes discontinues dynamic recrystallization in the Al0.6 alloy.•DDRX occurs at the interphase boundary mantle regions through the formation of Σ3 boundaries.•In the Al0.9 alloy, the BCC matrix softens through continuous dynamic recrystallization mechanism.•The dispersed second phases in both alloys become mechanically fragmented during hot deformation.
The effect of austenite grain size on the kinetics of the isothermal bainitic transformation in a high-carbon super-bainitic steel was investigated. Experimental results showed that the ...transformation of super bainite was accelerated by a coarse austenite grain size. This is because while coarse austenite grains provide less nucleation sites, it is beneficial for bainite sheaf growth. Meanwhile, there is a critical austenite grain size below which there is a distinct grain size effect and above which it is not evident.
•The transformation of super bainite was accelerated by a coarse austenite grain size.•The growth played a more important role than nucleation for the acceleration of super bainitic steels.•There is a critical austenite grain size below which there is a distinct grain size effect and above which it is not evident.
Two Fe-0.2C-1.55Mn-1.5Si (in wt pet) steels, with and without the addition of 0.039Nb (in wt pet), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. ...The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite. PUBLICATION ABSTRACT
The hot deformation behavior of a 304 austenitic stainless steel was investigated to characterize the evolution of the dynamically recrystallized structure as a starting point for studies of the ...postdeformation recrystallization behavior. The effect of different deformation parameters such as strain, strain rate, and temperature were investigated. The flow curves showed typical signs of dynamic recrystallization (DRX) over a wide range of temperatures and strain rates (
i.e.
, different Zener–Hollomon (
Z
) values). However, under very high or very low
Z
values, the flow curves’ shapes changed toward those of the dynamic recovery and multiple peaks, respectively. The results showed that while DRX starts at a strain as low as 60 pct of the peak strain, a fully DRX microstructure needs a high strain of almost 4.5 times the initiation strain. The DRX average grain size showed power-law functions with both the Zener–Hollomon parameter and the peak stress, although power-law breakdown was observed at high
Z
values.
The wear behaviour of as-cast Al1.0CoCrFeNi with a body-centred cubic (BCC) structure and Al0.3CoCrFeNi with a face-centred cubic (FCC) structure in the as-cast, homogenized and recrystallized ...conditions was studied by determining the intrinsic response to abrasive scratch testing. For the FCC material, the wear mechanism was qualitatively the same for the as-cast, homogenized and recrystallized conditions. The wear rate increased with increasing load, and the fraction of material removed from the groove (f) fell in the range of 0.2–0.7. The wear rate for the as-cast BCC material was significantly lower, which was ascribed to its high hardness. The f value for the BCC material lay in the range of 0.7–0.9, which implies dominance of the cutting wear mechanism. A wear map was constructed to enable comparison of the wear performance of materials under different loading conditions. It was found that the importance of hardness in wear is dependent on load. At high loads the wear performance of alloys highly depends on the scratch hardness. For low loads, however, the combination of scratch hardness and scratch ductility dictates total material loss.
•The scratch behaviour of Al1.0CoCrFeNi (BCC) and Al0.3CoCrFeNi (FCC) was studied•For the FCC materials, the fraction of material removed from the groove (f) fell in the range of 0.2–0.7•The f value for the BCC material lied in the range of 0.7–0.9 implying dominance of cutting mechanism.•A wear map was constructed which allows for alloy design and development for different wear conditions•At high loads hardness plays a major role in wear performance.•For low loads, a combination of hardness and scratch ductility dictates the material loss.
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