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This paper reports systematic studies on precipitation behavior of a solid-solution treated single-phase Nb-containing CoCrNi-based face-centered-cubic high-entropy alloy, CoCrNi1.5Nb0.2, at a series of aging temperatures of 660–960 °C for different aging times of 1–48 h, and on the effect of precipitation behavior on tensile strength and ductility. Aging at 660 °C for 1–48 h produced disc-shaped coherent γ"-(Ni,Co,Cr)3(Nb,Cr) precipitates of nanometric diameter and thickness with the D022 superlattice, offering precipitation strengthening via dislocation shear mechanism. The diameter monotonically increases with increasing aging time, and the thickness increases with increasing aging time from 1 to 18 h but essentially remains unchanged with aging time of 18–48 h; the volume fraction of γ" precipitates constantly increases with increasing aging time. Consequently, yield strength (YS) and ultimate tensile strength (UTS) of CoCrNi1.5Nb0.2 increase with increasing aging time due to increased volume fraction and dimensions of γ" precipitates. Uniform elongation (UE) of CoCrNi1.5Nb0.2 decreases with increasing aging time due to increased strength of CoCrNi1.5Nb0.2 and dimensions of γ" precipitates. Aging at 760 °C for 1 h yielded dimensions of γ" precipitates similar to, but volume fraction of γ" precipitates lower than, those during aging at 660 °C for 48 h, leading to lower YS and UTS and higher UE than those in CoCrNi1.5Nb0.2 aged at 660 °C for 48 h. Unlike aging at 660 °C for 1–48 h and at 760 °C for 1 h, aging at 860 and 960 °C for 1 h generated semi-coherent lath-shaped ε-(Ni,Co,Cr)3(Nb,Cr) precipitates of submicrometric to micrometric length and submicrometric width with the D019 structure, changing precipitation strengthening mechanism to Orowan dislocation bypass mechanism. Submicrometric to micrometric ε precipitates provide limited precipitation strengthening, only attaining modest YS and UTS. Despite submicrometric to micrometric length of ε precipitates, CoCrNi1.5Nb0.2 still exhibits a relatively high UE due to strength-ductility tradeoff. [ABSTRACT FROM AUTHOR]

It has been well documented that recovery occurring in metals/alloys produced via solid-state quenching involves only annihilation of supersaturated vacancies. Interestingly, in the present study, we observed completely different mechanisms underlying recovery during annealing of an Al-Zn-Mg-Cu (7075 Al) alloy processed via liquid-state quenching, i.e., rapid solidification (specifically melt spinning herein). The as-melt-spun alloy consists of refined grains containing tangled dislocations inside the grains. Following annealing at 393 K (120 °C) for 24 hours, refined grain structure was still retained and grain sizes essentially remained unchanged, but subgrains separated by dense dislocation walls were generated at grain interiors, with a much lower density of dislocations at subgrain interiors than that in the as-melt-spun 7075 Al alloy and dislocation arrays inside some subgrains. The microstructural evolution suggests the absence of recrystallization and the occurrence of recovery primarily via the annihilation and rearrangement of dislocations and the formation of subgrains. Based on the stored energy in dislocations in, and the annealing temperature of, the as-melt-spun 7075 Al alloy, the recovery phenomenon was analyzed and discussed in detail. [ABSTRACT FROM AUTHOR]

This paper reports on a study of the stress-induced grain growth phenomenon in the presence of second-phase particles and solutes segregated at grain boundaries (GBs) during high-temperature deformation of an ultra-fine grained (UFG) Al alloy synthesized via the consolidation of mechanically milled powders. Our results show that grain growth was essentially inhibited during annealing at 673 K (400 °C) in the absence of an externally applied stress, whereas in contrast, grain growth was enhanced by a factor of approximately 2.7 during extrusion at 673 K (400 °C). These results suggest that significant grain growth during hot extrusion was attributable to the externally applied stresses stemming from the state of stress imposed during extrusion and that the externally applied stresses can overcome the resistance forces generated by second-phase particles and solutes segregated at GBs. The mechanisms underlying stress-induced grain growth were identified as GB migration and grain rotation, which were accompanied by dynamic recovery and possible geometric dynamic recrystallization, while discontinuous dynamic recrystallization did not appear to be operative. [ABSTRACT FROM AUTHOR]

This paper reports on a study of the stress-induced grain growth phenomenon in the presence of second-phase particles and solutes segregated at grain boundaries (GBs) during high-temperature deformation of an ultra-fine grained (UFG) Al alloy synthesized via the consolidation of mechanically milled powders. Our results show that grain growth was essentially inhibited during annealing at 673 K (400 °C) in the absence of an externally applied stress, whereas in contrast, grain growth was enhanced by a factor of approximately 2.7 during extrusion at 673 K (400 °C). These results suggest that significant grain growth during hot extrusion was attributable to the externally applied stresses stemming from the state of stress imposed during extrusion and that the externally applied stresses can overcome the resistance forces generated by second-phase particles and solutes segregated at GBs. The mechanisms underlying stress-induced grain growth were identified as GB migration and grain rotation, which were accompanied by dynamic recovery and possible geometric dynamic recrystallization, while discontinuous dynamic recrystallization did not appear to be operative. [ABSTRACT FROM AUTHOR]

This paper reports on a study of the stress-induced grain growth phenomenon in the presence of second-phase particles and solutes segregated at grain boundaries (GBs) during high-temperature deformation of an ultra-fine grained (UFG) Al alloy synthesized via the consolidation of mechanically milled powders. Our results show that grain growth was essentially inhibited during annealing at 673 K (400 °C) in the absence of an externally applied stress, whereas in contrast, grain growth was enhanced by a factor of approximately 2.7 during extrusion at 673 K (400 °C). These results suggest that significant grain growth during hot extrusion was attributable to the externally applied stresses stemming from the state of stress imposed during extrusion and that the externally applied stresses can overcome the resistance forces generated by second-phase particles and solutes segregated at GBs. The mechanisms underlying stress-induced grain growth were identified as GB migration and grain rotation, which were accompanied by dynamic recovery and possible geometric dynamic recrystallization, while discontinuous dynamic recrystallization did not appear to be operative. [ABSTRACT FROM AUTHOR]

In this study, we formulate a mathematical model that can be implemented to calculate the amount of strain energy both introduced to ( U) and stored in ( U) metal powders during mechanical milling. The theoretical analysis presented in this study proposes that the strain energy is primarily induced by normal and shear strains, and moreover, that the contributions from torsion can be neglected. This theoretical framework was implemented to evaluate the influence of various mechanical milling processing parameters on U and U. The calculated results show that the magnitude of U increases with increases in the following processing parameters: attritor diameter, impeller's rotational frequency, and ball-to-powder mass ratio, and the magnitude of U increases with a decrease in diameter of the milling media. The percentage of the shear strains' contribution to the total U is insensitive to the mechanical milling processing parameters, varying within the range of 35 pct to 42 pct. The calculated magnitude of U ranges from a few to a few tens of joules per gram, which is three to four orders of magnitude lower than that of the calculated U. Although with respect to the mechanical milling processing parameters, the calculated U has trends similar to those for U, the changing rates of U are much lower than those for U. [ABSTRACT FROM AUTHOR]

The strain energy stored in mechanically milled 5083 Al powders was investigated using two experimental approaches: thermal and microstructural analysis. The experimental results show that mechanically milled 5083 Al powders store strain energy on the order of a few tens of joules per gram. These experimental results are consistent with the calculated strain energy stored in mechanically milled powders. The experimentally measured strain energy stored in powders increases with an increase in attritor diameter, impeller's rotational frequency, and ball-to-powder mass ratio; however, it decreases with an increase in ball diameter. These trends were in good agreement with the calculated strain energy stored in powders as a function of the corresponding processing parameters. [ABSTRACT FROM AUTHOR]

The evolution of carbides during aging of a spray-formed chromium-containing tool steel was studied. In the as-spray-formed steel, there are two prominent types of carbides: the V-rich proeutectoid MC and the Fe-rich M3C in lower bainite. Evolution of the carbides during aging can be described as follows. While the proeutectoid MC remains unchanged, a portion of the M3C dissolves into the bainitic ferrite matrix, and another portion of it is transformed into Cr-rich M7C3. In addition, fine alloyed carbides, such as M7C3, MC, Cr-rich M23C6, and Mo-rich M6C, precipitate from the matrix consisting of bainitic ferrite, martensite, and retained austenite. [ABSTRACT FROM AUTHOR]