•A new candidate for applications at high temperature is proposed.•The calculated melting point of the alloy is 1700°C.•The alloy possesses a simple microstructure.•The alloy exhibits perspectives in ...terms of mechanical properties and oxidation resistance.
A new refractory high-entropy alloy system Mo–W–Al–Cr–x is proposed as a family of candidate materials for structural applications at high temperatures. Thermodynamic assessment was used to set the chemical composition of the first alloy as 20Mo–20W–20Al–20Cr–20Ti (at.%) with a calculated melting temperature of about 1700°C. A single disordered BCC phase should be stable at high temperatures between 1077°C and 1700°C. Microstructural examination and XRD results clearly show that the alloy in the as-cast condition exhibits a non-homogeneous microstructure with pronounced dendritic and interdendritic regions. Heat treatment processes, however, reveal a strong tendency of the alloy 20Mo–20W–20Al–20Cr–20Ti to homogenize. While possessing a high hardness of around 800HV, the crack-free indents allow the assumption that the alloy studied may be intrinsically ductile at room temperature. Despite the fact that the alloy possesses 40at.% of refractory elements, high temperature oxidation tests show a surprisingly good oxidation resistance. Strategies to enhance the long-term stability of the disordered BCC phase aiming at achieving the required mechanical properties as well as optimizing the alloy’s chemical composition in terms of high temperature oxidation resistance are discussed.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In the present investigation, we provide results on the casting, homogenization, and deformation behavior of a new Al-containing refractory high-entropy alloy, namely the equiatomic Nb-Mo-Cr-Ti-Al. ...The alloy shows a dendritic microstructure after arc melting. The dendrites completely dissolve due to a heat treatment at 1300 °C for 20 h. Besides a major phase in the form of a solid solution of W prototype structure, identified by X-ray diffraction (XRD) measurements as well as electron backscatter diffraction (EBSD), additional phases of small volume fraction within the grains and at the grain boundaries were observed. Quasistatic compression tests, performed between room temperature and 1200 °C, reveal sustaining and high yield strength up to 800 °C and an increasing ductility with increasing test temperature. The dominant deformation mechanism for quasistatic compression loading between 800 °C and 1200 °C is the 〈111〉 pencil glide of dislocations within the solid solution which was proven by the according fiber texture components, evolving during deformation.
Display omitted
•The as-cast microstructure of equimolar Nb-Mo-Cr-Ti-Al can be homogenized @ 1300 °C.•After homogenization, bcc solid solution with minor secondary phases are observed.•Compression tests reveal a maximum strength of ≈1 GPa @ room temperature.•Increasing ductility up to 24% @ 1200 °C is observed.•Compression leads to mixed 〈001〉 and 〈111〉 fiber texture which indicates pencil glide.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The current work compares the deformation behavior of CoCrFeMnNi and CoCrNi in the temperature interval between 295 K and 8 K through a series of quasi-static tensile tests. Temperature-dependent ...yield stress variation was found to be similarly high in these two alloys. Previous investigations only extended down to 77 K and showed that a small amount of ε-martensite was formed in CoCrNi while this phase was not observed in CoCrFeMnNi. The present study extends these investigations down to 8 K where similar low levels of ε-martensite were presently detected. Based on this result, a rough assessment has been made estimating the importance of deformation twinning to the strength. The relative work hardening rates of CoCrFeMnNi and CoCrNi were comparable in value despite the differences in ε-martensite formation during deformation. CoCrFeMnNi deforms by dislocation slip and deformation twinning while deformation in CoCrNi is also accommodated by the formation of ε-martensite at cryogenic temperatures. Additionally, CoNi, a solid solution from the Co–Cr–Fe–Mn–Ni system with low strength, was used for comparison, showing contrasting deformation behavior at cryogenic temperatures.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Fully-dense bulk CoCrFeMnNi high entropy alloy (HEA) has been obtained by high-pressure torsion (HPT) deformation. Elemental, micrometer sized powders were successively mixed in equimolar ...proportions, consolidated and deformed at 5 GPa using imposed shear strain equivalent up to 100 rotations. Transmission electron microscopy, X-ray diffraction and 3D atom probe tomography were performed to study the microstructure evolution resulting from HPT-induced mechanical alloying. Single-phase, nanocrystalline (grain size of 50 nm) alloy with smaller chromium oxide precipitates (of 7–10 nm) exhibited a hardness of 6700 MPa, which is the highest one reported for as-processed bulk CoCrFeMnNi alloys to our best knowledge.
Display omitted
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
5.
On the Hall–Petch relationship in a nanostructured Al–Cu alloy Shanmugasundaram, T.; Heilmaier, M.; Murty, B.S. ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
11/2010, Volume:
527, Issue:
29
Journal Article
Peer reviewed
▶ Yield strength values of Al–4Cu alloy were analysed via Hall–Petch relation. ▶ H–P analysis revealed apparently high ‘
σ
0’ (170
MPa) and a high ‘
k’ (
0.13
MPa
m
) ▶ Precipitates and oxide ...particles are the likely reason for such a high values. ▶ True ‘
σ
0’ and ‘
k’ were evaluated by considering only the grain size contribution. ▶ This agrees with values taken from pure Al.
Mechanical properties of bulk nanocrystalline Al–4Cu alloys with grain sizes from 47 to 105
nm, synthesized by mechanically alloying followed by vacuum hot pressing at different temperatures, were analysed through Hall–Petch relation. Hall–Petch analysis revealed a high frictional stress (170
MPa) and a high positive slope (
0.13
MPa
m
) as compared to pure Al, which has a frictional stress (15–30
MPa) and a slope (
0.06
–
0.09
MPa
m
). From a detailed evaluation of different strengthening mechanisms it is inferred that the Al
2Cu precipitates and oxide particles are the likely reason for such high values of frictional stress and slope.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The effect of Ta on the oxidation resistance of the alloys Tax-(Mo-Cr-Ti-Al)1−x (x=0; 5; 10; 15; 20 at%) in air at 1200 °C was examined. The oxidation behavior improves continuously with the ...increasing Ta concentration. Alloys with Ta concentrations Ta≤10 at% exhibit poor oxidation. Severe evaporation of MoO3 occurred which was quantitatively estimated. The evaporation of MoO3 is drastically reduced in alloys with the higher Ta concentrations. At 15 and 20 at% Ta, protective CrTaO4 scales are found. The CrTaO4 scale formed on Ta20-(Mo-Cr-Ti-Al)80 grows according to the parabolic rate law as a result of oxygen inward diffusion.
•The oxidation behavior improves continuously with the increasing Ta concentration.•The evaporation of MoO3 was quantitatively estimated.•The evaporation of MoO3 is drastically reduced in alloys with the higher Ta concentrations.•The CrTaO4 scale formed on Ta20-(Mo-Cr-Ti-Al)80 grows according to the parabolic rate law due to oxygen inward diffusion.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The yield stress of oxide dispersion-strengthened (ODS) PM2000 steel and nanocluster-strengthened (NCS) 14YWT steel was measured as a function of temperature for grain sizes from 0.2
μm to ∞. In ...combination with data from the literature, it was found that the indirect strengthening (i.e., the Hall–Petch strengthening) for small grain sizes was often greater than the direct strengthening (i.e., the Orowan strengthening due to the dispersoids). The yield stress dropped significantly at a transition temperature near 500
°C even though the grain size remained constant. For small grain sizes, a large fraction of this drop was attributed to a decrease in grain size strengthening. Three published models describing this effect were compared with the experimental data. The observed transition temperatures were consistent with two of them. The strength–temperature curves for PM2000 with different grain sizes tended to intersect, whereas those for 14YWT did not. Because of this feature, the description of the strength–temperature dependence of 14YWT could be narrowed down to a single model. It is concluded that ODS and NCS materials are well suited to examining the temperature dependence of grain size strengthening.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Serrated plastic deformation at temperatures close to 0 K has been previously reported in some metals and alloys, and is associated with two possible origins: (i) thermomechanical instability or (ii) ...mechanical instability. While some recent results indicate that serrations are a mechanical dislocation-based phenomenon, a comprehensive model does not exist. CoCrFeMnNi, an expectedly ideal candidate, exhibits severe serrated plastic deformation with large stress drops in excess of 100 MPa. Furthermore, it also shows cryogenic serrated plastic deformation at a higher temperature (35 K) than previously reported for any other alloy. The exacerbated nature of serrated plastic deformation in CoCrFeMnNi led to the following inferences: (i) temperature and dislocation density are indisputable controlling parameters for cryogenic serrated plastic deformation and they cannot supersede each another; (ii) a phenomenological model is elucidated based on the increasing difficulty for cross-slip with decreasing temperature, leading to sudden massive dislocation proliferation event; (iii) the model establishes a gradual transition from completely non-serrated to completely serrated deformation, mediated by cross-slip, as opposed to the conventional model which proposed a discrete transition; (iv) solute dislocation interaction and associated Stacking Fault Energy (SFE) during deformation plays a key role in controlling dislocation constriction and cross-slip and correspondingly serrated plastic deformation; (v) the need/direct influence of deformation twinning, transformation induced plasticity and especially thermomechanical factors on serrated plastic deformation is invalidated. Some of these points were further clarified through comparisons with CoCrNi and CoNi, also presented in the present article.
Display omitted
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The contribution of dislocation density to strength of Cr–Ni containing austenitic ODS steels is assumed to be limited by dynamic recovery during consolidation of mechanically alloyed powder Seils et ...al. Materials Science and Engineering A 786 (2020) 139452. In order to prevent the reduction in dislocation density by dislocation annihilation subsequent to thermally activated cross slip during recovery, Cr and Ni were replaced by 24 and 34 wt% of Mn in the present study. The comparably lower stacking fault energy leads to larger stacking fault widths and, hence, lower probability of cross slip. The Mn-ODS steels were successfully manufactured by mechanical alloying and subsequent field assisted sintering. X-ray diffraction analysis revealed increased dislocation densities for both Mn-ODS alloys compared to a previously reported Cr–Ni-ODS steel. Formation of additional ε-phase was observed in the lower Mn alloy. Compression and hardness tests confirmed an improved strength and hardness of the Mn-ODS steels. Less thermal stability was found compared to the Cr–Ni-ODS steels due to pronounced ripening of oxide particles as well as grain coarsening in both investigated alloys.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
PDF
