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Volume 1, September
 
 

Alloys, Volume 1, Issue 1 (June 2022) – 7 articles

Cover Story (view full-size image): Discontinuous precipitation (DP) is frequently observed in magnesium–aluminium alloys and is considered deleterious to properties. It has been demonstrated experimentally that prior deformation can suppress DP and promote continuous precipitation (CP). This work develops a model to predict this interaction. It is shown that the main effect suppressing DP is the accelerated nucleation of CP on dislocations. However, even with deformation, DP is not usually completely prevented. This results from the heterogenous strain distribution in deformed magnesium alloys, with strain gradients present at grain boundaries that promote DP. Crystal plasticity modelling has been used to demonstrate this effect. View this paper
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Article
Kinetic Model of Isothermal Bainitic Transformation of Low Carbon Steels under Ausforming Conditions
Alloys 2022, 1(1), 93-115; https://doi.org/10.3390/alloys1010007 - 13 Jun 2022
Cited by 1 | Viewed by 945
Abstract
Carbide-free bainitic steels show attractive mechanical properties but are difficult to process because of the sluggish phase transformation kinetics. A macroscopic model based on the classical nucleation theory in conjunction with the modified Koistinen–Marburger relationship is proposed in this study to simulate the [...] Read more.
Carbide-free bainitic steels show attractive mechanical properties but are difficult to process because of the sluggish phase transformation kinetics. A macroscopic model based on the classical nucleation theory in conjunction with the modified Koistinen–Marburger relationship is proposed in this study to simulate the kinetics of incomplete bainitic and martensitic phase transformations with and without austenite deformation. A 0.26C-1Si-1.5Mn-1Cr-1Ni-0.003B-0.03Ti steel and a 0.18C-1Si-2.5Mn-0.2Cr-0.2Ni-0.02B-0.03Ti steel were investigated with different levels of ausforming. The concept of ausforming is expected to accelerate the onset of the bainitic transformation and to enhance the thermodynamic stability of austenite by increased dislocation density. The phase transformation kinetics of both steels is quantitatively analyzed in the study by dilatometry and X-ray diffraction so that the carbon concentration in the retained austenite and bainitic ferrite, as well as their volume fractions, is determined. A critical comparison of the numerical and experimental data demonstrates that the isothermal kinetics of bainite formation and the variation of driving energy can be satisfactorily described by the developed model. This model captures the incompleteness of the bainite phase transformation and the carbon enrichment in the austenite well. A fitting parameter can be used to elucidate the initial energy barrier caused by the ausforming. An increase in austenite stability can be described by the nucleation reaction and the thermodynamic energies associated with the change of dislocation density. The proposed model provides an in-depth understanding of the effect of ausforming on the transformation kinetics under different low-carbon steels and is a potential tool for the future design of heat treatment processes and alloys. Full article
(This article belongs to the Topic Numerical Modeling on Metallic Materials)
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Article
Modification of Cantor High Entropy Alloy by the Addition of Mo and Nb: Microstructure Evaluation, Nanoindentation-Based Mechanical Properties, and Sliding Wear Response Assessment
Alloys 2022, 1(1), 70-92; https://doi.org/10.3390/alloys1010006 - 10 May 2022
Cited by 2 | Viewed by 1290
Abstract
The classic Cantor (FeCoCrMnNi) isoatomic high entropy alloy was modified by separate additions of Mo and Nb in an effort to optimize its mechanical properties and sliding wear response. It was found that the introduction of Mo and Nb modified the single phase [...] Read more.
The classic Cantor (FeCoCrMnNi) isoatomic high entropy alloy was modified by separate additions of Mo and Nb in an effort to optimize its mechanical properties and sliding wear response. It was found that the introduction of Mo and Nb modified the single phase FCC solid solution structure of the original alloy and led to the formation of new phases such as the BCC solid solution, σ-phase, and Laves, along with the possible existence of intermetallic phases. The overall phase formation sequence was approached by parametric model assessment and solidification considerations. Nanoindentation-based mechanical property evaluation showed that due to the introduction of Mo and Nb; the modulus of elasticity and microhardness were increased. Creep nanoindentation assessment revealed the beneficial action of Mo and Nb in increasing the creep resistance based on the stress sensitivity exponent, strain rate sensitivity, and critical volume for the dislocation nucleation considerations. The power law and power law breakdown were identified as the main creep deformation mechanisms. Finally, the sliding wear response was increased by the addition of Mo and Nb with this behavior obeying Archard’s law. A correlation between microstructure, wear track morphologies, and debris characteristics was also attempted. Full article
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Article
Modelling the Effect of Deformation on Discontinuous Precipitation in Magnesium—Aluminium Alloy
Alloys 2022, 1(1), 54-69; https://doi.org/10.3390/alloys1010005 - 11 Apr 2022
Viewed by 1227
Abstract
Magnesium–Aluminium alloys can decompose from a supersaturated solid solution by either continuous or discontinuous precipitation. Deformation prior to precipitation has been shown to strongly suppress the discontinuous precipitation mode and promote continuous precipitation. In this work, a model is used to explore the [...] Read more.
Magnesium–Aluminium alloys can decompose from a supersaturated solid solution by either continuous or discontinuous precipitation. Deformation prior to precipitation has been shown to strongly suppress the discontinuous precipitation mode and promote continuous precipitation. In this work, a model is used to explore the interaction between deformation and precipitation in the Mg–Al system. It has been shown that accelerated nucleation of continuous precipitates on dislocations is predicted to have the dominant effect in suppressing discontinuous precipitation by reducing the solute supersaturation. A secondary effect is the direct role played by twins in the deformed structure, which act as impenetrable barriers to discontinuous precipitate growth. However, even in the deformed case, small regions of discontinuous precipitation are still observed. It is proposed that this is due to the high level of strain concentration expected in the grain boundary regions, which provides a locally enhanced driving force for the migration of grain boundaries such that limited discontinuous precipitation occurs before continuous precipitation becomes established. Full article
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Article
Oxide Modified Iron in Electron Beam Powder Bed Fusion—From Processability to Corrosion Properties
Alloys 2022, 1(1), 31-53; https://doi.org/10.3390/alloys1010004 - 07 Mar 2022
Cited by 1 | Viewed by 1578
Abstract
Additive manufacturing (AM) processes are not solely used where maximum design freedom meets low lot sizes. Direct microstructure design and topology optimization can be realized concomitantly during processing by adjusting the geometry, the material composition, and the solidification behavior of the material considered. [...] Read more.
Additive manufacturing (AM) processes are not solely used where maximum design freedom meets low lot sizes. Direct microstructure design and topology optimization can be realized concomitantly during processing by adjusting the geometry, the material composition, and the solidification behavior of the material considered. However, when complex specific requirements have to be met, a targeted part design is highly challenging. In the field of biodegradable implant surgery, a cytocompatible material of an application-adapted shape has to be characterized by a specific degradation behavior and reliably predictable mechanical properties. For instance, small amounts of oxides can have a significant effect on microstructural development, thus likewise affecting the strength and corrosion behavior of the processed material. In the present study, biocompatible pure Fe was processed using electron powder bed fusion (E-PBF). Two different modifications of the Fe were processed by incorporating Fe oxide and Ce oxide in different proportions in order to assess their impact on the microstructural evolution, the mechanical response and the corrosion behavior. The quasistatic mechanical and chemical properties were analyzed and correlated with the final microstructural appearance. Full article
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Article
The Energetics and Topology of Grain Boundaries in Magnesium: An Ab Initio Study
Alloys 2022, 1(1), 15-30; https://doi.org/10.3390/alloys1010003 - 16 Feb 2022
Viewed by 1460
Abstract
First principles calculations were carried out on six different grain boundaries with complex, non-symmetrical, crystallography’s. Solute species (Gd and Zn) were placed in multiple locations to investigate their effect on the boundary energetics. The grain boundaries were found to have an intrinsic grain [...] Read more.
First principles calculations were carried out on six different grain boundaries with complex, non-symmetrical, crystallography’s. Solute species (Gd and Zn) were placed in multiple locations to investigate their effect on the boundary energetics. The grain boundaries were found to have an intrinsic grain boundary energy, and this energy was not markedly affected by the solute concentration at the boundary. However, the work of separation (WSEP) was very sensitive to grain boundary chemistry. Boundaries of higher disorder were found to be more sensitive to boundary chemistry and showed higher values of WSEP and in the case of Gd, were more sensitive to solute concentration at the boundary. No correlation between the boundary behaviour and crystallography could be found, apart from the over-riding conclusion that all six boundaries showed markedly different behaviours, and the effect of solute on each were unique. Full article
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Article
Cu-Ni-Based Alloys from Nanopowders as Potent Thermoelectric Materials for High-Power Output Applications
Alloys 2022, 1(1), 3-14; https://doi.org/10.3390/alloys1010002 - 11 Jan 2022
Viewed by 2147
Abstract
A new approach for the development of thermoelectric materials, which focuses on a high-power factor instead of a large figure of merit zT, has drawn attention in recent years. In this context, the thermoelectric properties of Cu-Ni-based alloys with a very high [...] Read more.
A new approach for the development of thermoelectric materials, which focuses on a high-power factor instead of a large figure of merit zT, has drawn attention in recent years. In this context, the thermoelectric properties of Cu-Ni-based alloys with a very high electrical conductivity, a moderate Seebeck coefficient, and therefore a high power factor are presented as promising low-cost alternative materials for applications aiming to have a high electrical power output. The Cu-Ni-based alloys are prepared via an arc melting process of metallic nanopowders. The heavy elements tin and tungsten are chosen for alloying to further improve the power factor while simultaneously reducing the high thermal conductivity of the resulting metal alloy, which also has a positive effect on the zT value. Overall, the samples prepared with low amounts of Sn and W show an increase in the power factor and figure of merit zT compared to the pure Cu-Ni alloy. These results demonstrate the potential of these often overlooked metal alloys and the utilization of nanopowders for thermoelectric energy conversion. Full article
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Editorial
Introducing Alloys: A Journal for Fundamental and Applied Research
Alloys 2022, 1(1), 1-2; https://doi.org/10.3390/alloys1010001 - 12 Nov 2021
Cited by 1 | Viewed by 1923
Abstract
Humans have made alloys for thousands of years [...] Full article
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