Materials Science Forum Vol. 1175

Abstract: Thermomechanical tension tests were conducted on an Al-Mg-Si alloy at temperatures of 250°C, 350°C, and 450°C, with displacement rates of 2, 20, and 200 mm/s. The tests utilized an axisymmetric specimen with a diameter of 6 mm and a parallel length of 91 mm. Net axial stress versus logarithmic strain curves beyond diffuse necking were generated from the force measurements combined with edge tracing using synchronized images captured by a digital camera. Transmission electron microscopy (TEM) analyses were conducted on selected samples extracted near the fracture surface after testing and cooling to room temperature. After testing at 450°C, overaged needle precipitates were nucleated in bulk and on dispersoids. The dislocation density was low, but sub-grains of one to a few micrometres were observed. In contrast, after testing at 250°C, fine needle precipitates were nucleated in the bulk and on dislocations. A high dislocation density was found, with no subgrain formation. A combined precipitation, yield strength, and work hardening model for Al-Mg-Si alloys, known as NaMo, was finally employed to simulate the evolution of the precipitate structure and the stress-strain behaviour in the thermomechanical tension tests. In the model, variables representing the instantaneous state of the precipitate structure are predicted and used to calculate the corresponding yield strength and work hardening rate incorporating the effects of strain rate and temperature. A comparison between the calculated and measured yield stresses for small plastic strains showed good agreement.

Abstract: The microstructural evolution of the WZ73 magnesium alloy (Mg-7.4Y-3.8Zn-0.4Zr) was systematically investigated during finishing heat-treatments following twin-roll casting and hot rolling at equivalent strain rates of 17 s⁻¹ and 50 s⁻¹. Hot rolling at 500 °C was performed to achieve a logarithmic strain of 0.7 (thickness reduction from 5.3 mm to 3 mm). Higher strain rates during hot rolling enhanced dynamic recrystallization (DRX), resulting in refined microstructures, whereas lower strain rates promoted the formation of lamellar long period stacking ordered (LPSO) phases. Subsequent heat-treatments at 200 °C to 550 °C for up to 24 hours revealed temperature-dependent microstructural transformations. At 500 °C, complete recrystallization occurred with minimal grain growth, while 550 °C caused grain coarsening, partial grain boundary melting, and morphological changes of the LPSO phase from lamellar to spherical and rod-like structures. Notably, at temperatures above 500 °C, prior hot rolling had limited influence on microstructure. The microstructure and phase evolution were characterized using optical and scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD).

Abstract: We recently proposed a new mechanism to simultaneously improve the strength and ductility of multiphase alloys, named “Anisotropic mechanical property-induced ductilization (AMID)”. In this paper, the variations in tensile deformation behavior of Mg/LPSO extruded alloys depending on the volume fraction of the LPSO phase were examined, to deepen the understanding on AMID. As expected, the work-hardening rate of the Mg/LPSO two-phase extruded alloy increased as increasing in the volume fraction of LPSO phase. This demonstrates the validity of the AMID mechanism. However, the increase in the volume fraction of the LPSO phase decreased the elongation, even though the work-hardening rate was increased in them. The present study revealed that an appropriate amount of Mg grains is necessary to obtain the effect of AMID in improving the uniform elongation of the alloy, by the suppressing the development of microcracks formed in the LPSO phase grains into macroscopic fracture.

Abstract: The manufacturing of high temperature heat exchangers and precoolers within the aerospace industry often requires the joining of thin-walled components through brazing. Existing commercially available brazing alloys are often relatively hard with brittle failure modes despite ductility and strength being desirable properties of a brazed joint. High entropy alloys have been demonstrated to have desirable material properties such as high ductility and strength. Previous work on CoCrCuFeNi has demonstrated that the addition of melting point depressants such as Boron, are able to beneficially reduce the brazing temperatures sufficiently to allow brazing of steel components while maintaining a strong and ductile joint. The current work has focussed on finding new, non-equimolar, HEA compositions with a lower targeted melting point for a wider range of brazed substrate materials. Alloy compositions were down-selected through empirical thermodynamic classification and CALPHAD simulations. Identified potential compositions were synthesised using induction casting, and solidus and liquidus measured using DSC. Phases were confirmed using a combination of microscopy, hardness and XRD analysis. The best alloy candidates were then modified with the addition of Boron to further reduce the melting point to meet the required manufacturing temperatures of the joint. Finally, shear strength measurements were carried out on the samples which met the brazing temperature requirements.

Abstract: Recently, high strength at high temperature can be achieved by inducing kink bands in alloys having aligned lamellar microstructure. However, the kink-bands formation has been confirmed only in alloys with lamellar microstructures, where slip plane is limited to the plane parallel to the lamellar interface, and not confirmed in alloys with rod-like or Chinese script microstructures. In this study, we clarified the contribution of rod-like Si phases in Al-Si alloy on the mechanical properties and focused on the feasibility of introduction of kink bands in the alloys without lamellar structure. The results showed that in Al-Si eutectic alloys, the non-lamellar second phase, i.e., the Si phase, is aligned by directional solidification, and refined by rolling. The directionally-solidified sample showed high yield strength with long and aligned Si phase, while the rolled samples showed high ductility with refined microstructure. The rolled samples were uniformly deformed in all the samples with variety of reduction ratios, and wedge-shaped deformation bands were observed after the compression test, especially in the 5-10% rolled specimens. Crystallographic orientation analysis indicated that these deformation bands were not kink bands but were localized slip bands.

Abstract: Ultra-fine-grained (UFG) Al alloys have excellent mechanical properties such as high tensile strength without remarkable loss of elongation. Severe plastic deformation (SPD) process is an effective method for obtaining UFG microstructure. SPD-processed Al alloys has extremely high strength than the extrapolated from Hall-Petch relationship due to their microstructure with residual excess strain after dynamic recrystallization. Especially, on account of Al alloys have high stacking fault energy, the dislocation rearrangement in the dynamically recrystallized grain is difficult to form high angle grain boundary. As a result, there are substantial dislocation wall and low-angle grain boundary after SPD processing. These dislocations remain in the grain after recrystallization and partially form low-angle grain boundaries and subgrain boundaries. Consequently, the strength increases from Hall-Petch relationship, which is the degree of extra-hardening, was measured up to 200 MPa in as-SPD processed Al-3%Mg alloy. The authors previously reported that the low-angle grain boundaries distributed in the microstructure after the repetitive equal-channel angular extrusion processing. The strength difference calculated by Bailey-Hirsch equations was not in accord with measured extra-hardened strength. In this study, the effect of grain boundary distributions on the extra-hardening was investigated by changing SPD-processing and subsequent annealing conditions.

Abstract: In recent years, AlCoCrFeNi High-Entropy Alloy (HEA) has attracted attention because it is expected to be a next-generation aerospace engine material because it has specific strength, excellent corrosion and wear resistance. However, the influence of microstructure of the material synthesized via spark plasma sintering (SPS), a powder metallurgy technique, on high temperature mechanical properties, in particular, creep features, has rarely been reported. In the study, to investigate the abovementioned issue, the HEAs using powder mean size of 14.6, 41.9 and 82.4 μm synthesized via SPS at 1273 K and 1373 K were prepared. Creep tests were conducted at 973 K. The obtained results indicated that HEAs SPSed at 1373 K exhibited higher creep strength than those of synthesized at relatively low temperature, because the microstructure of the former is different from those of the latter. In addition, FCC/B2 phase boundary fracture was observed for HEA synthesized at 1373 K. By contrast, powder boundary fracture was observed for the remaining HEAs. Moreover, the Monkman-Grant relation can be employed to predict creep rupture time for all types of HEAs on one master curve.

Abstract: The Harmonic Structure (HS) design was implemented in a high-entropy CrMnFeCoNi alloy compact to study its deformation characteristics at elevated temperatures, with particular emphasis on comparison with the homogeneous (Homo) compacts. The HS compact was prepared by powder metallurgy, employing a mechanical grinding process with a planetary ball mill in an argon atmosphere. The rotational speed was set at 150 rpm, and the milling time was either 180 or 360 ks. The resulting powders were then exposed to spark plasma sintering at 1223 K for 1.8 ks under 50 MPa. Subsequently, the compacts were subjected to high-temperature compression tests at 1073 K or 1173 K, at varying initial strain rates over a range of temperatures. These tests were conducted after the sintering process was completed. Homo exhibited a work hardening at the initial stage of deformation, followed by a slight decrease in flow stress, which then remained nearly constant. In contrast, HS exhibited a distinctive softening in flow stress following initial work hardening. A thorough examination of the microstructure during the softening process revealed that adjacent Shell/Core units caused grain boundary sliding in the Shell region. Furthermore, each Core exhibited a rotation of approximately 2.3 degrees and a lateral displacement of 1.5 μm. Observation of the softening phenomenon during high-temperature deformation was confirmed through TEM analysis, revealing that this softening resulted from dynamic recrystallization within the Shell region. Consequently, dynamic recrystallization in the Shell was postulated, followed by rotation of the Shell-Core unit through grain boundary sliding of the UFG structure.

Abstract: The shear band observed during tensile tests of AA5083 aluminum alloys with different grain sizes were visualized using a two-dimensional electronic speckle pattern interferometer. The effect of grain boundaries on shear band formation was investigated by extracting displacement and strain fields from interference fringe patterns and stress-strain curves. The intensity of stress oscillations and the strain level at which shear bands appeared were dependent on grain size. Small-grained specimens exhibited regular shear band formation with clear serrations, while large-grained specimens showed delayed and irregular bands with reduced stress oscillations. The formation and propagation of shear bands across grain boundaries were further analyzed from the perspective of wave dynamics.