Ultrafine-grained Regions Research Articles

Sustaining strength–ductility synergy of CoCrFeNiMn high entropy alloy by a multilevel heterogeneity associated with nanoparticles

A high tensile yield strength of 1298 MPa and good ductility (uniform elongation: 9.4%) of CoCrFeNiMn high entropy alloy can be sustained by a multilevel structural heterogeneity facilitated by ultrafine grained regions (UFGRs), coarse grained regions (CGRs) and a non-uniform dispersion of TiO(C) nanoparticles in UFGRs. Analysis reveals that the Young's modulus difference and strain gradient associated with the nanoparticles enhance the back stress. Meanwhile, forward stress was estimated quantitatively, which further suggests that nanoparticles also effectively increase resistance to the forward stress, leading to a better strength–ductility synergy than a simple heterogeneous structure with only UFGRs and CGRs.

High strength titanium with a bimodal microstructure fabricated by thermomechanical consolidation of a nanocrystalline TiH2 powder

A titanium rod with a novel bimodal microstructure consisting of α-Ti plates of 1–5µm in width and 10–15µm in length and ultrafine grained (UFG) α-Ti regions with grain sizes in the range of 100–300nm was fabricated by in-situ dehydrogenation and thermomechanical consolidation of a nanocrystalline TiH2 powder. The consolidation process combined spark plasma sintering at 800°C for 5min and hot extrusion at 1100°C, and took less than 15min in total. The titanium rod had a high tensile yield strength of 1052MPa and a limited elongation to fracture of 2.0%. The high yield strength of the titanium rod can be attributed to the high strength of the UFG regions due to grain boundary strengthening and the increased strength of the α-Ti plates due to O solid solution strengthening associated with the high O content of 0.85wt% in the titanium rod. A microstructural evolution model involving nucleation and growth of the α-Ti plates, partition of H between α-Ti and β-Ti phases and Ti-H eutectoid reaction has been proposed to elucidate the mechanism for the formation of the bimodal microstructure.

Significant Corrosion Resistance in an Ultrafine-Grained Al6063 Alloy with a Bimodal Grain-Size Distribution through a Self-Anodic Protection Mechanism

The bimodal microstructures of Al6063 consisting of 15, 30, and 45 vol. % coarse-grained (CG) bands within the ultrafine-grained (UFG) matrix were synthesized via blending of high-energy mechanically milled powders with unmilled powders followed by hot powder extrusion. The corrosion behavior of the bimodal specimens was assessed by means of polarization, steady-state cyclic polarization and impedance tests, whereas their microstructural features and corrosion products were examined using optical microscopy (OM), scanning transmission electron microscopy (STEM), field emission scanning electron microscopy (FE-SEM), electron backscattered diffraction (EBSD), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) techniques. The bimodal Al6063 containing 15 vol. % CG phase exhibits the highest corrosion resistance among the bimodal microstructures and even superior electrochemical behavior compared with the plain UFG and CG materials in the 3.5% NaCl solution. The enhanced corrosion resistance is attributed to the optimum cathode to anode surface area ratio that gives rise to the formation of an effective galvanic couple between CG areas and the UFG matrix. The operational galvanic coupling leads to the domination of a “self-anodic protection system” on bimodal microstructure and consequently forms a uniform thick protective passive layer over it. In contrast, the 45 vol. % CG bimodal specimen shows the least corrosion resistance due to the catastrophic galvanic corrosion in UFG regions. The observed results for UFG Al6063 suggest that metallurgical tailoring of the grain structure in terms of bimodal microstructures leads to simultaneous enhancement in the electrochemical behavior and mechanical properties of passivable alloys that are usually inversely correlated. The mechanism of self-anodic protection for passivable metals with bimodal microstructures is discussed here for the first time.

Microstructure, plastic deformation and strengthening mechanisms of an Al–Mg–Si alloy with a bimodal grain structure

Al6063 alloys with bimodal grain size distributions comprised of ultrafine-grained (UFG) and coarse-grained (CG) regions were produced via mechanical milling followed by hot extrusion. High-energy planetary ball milling for 22.5h with a rotational speed of 350rpm was employed for the synthesis of nanocrystalline Al6063 powders. The as-milled Al6063 powders were mixed with 15, 30, and 45vol.% of the unmilled powders and then the powder mixtures were consolidated via extrusion at 450°C with an extrusion ratio of 9:1. The microstructure of the bimodal extrudates was investigated using optical microscope, transmission electron microscope (TEM) and field emission scanning electron microscope equipped with an electron backscattered diffraction (EBSD) detector. The deformation behavior was investigated by means of uniaxial tensile tests. The bimodal Al6063 exhibited balanced mechanical properties, including high yield stress and ultimate tensile strength resulting from the UFG regions together with reasonable ductility attained from the CG areas. The fracture surfaces demonstrated a ductile fracture mode, in which the dimple size was correlated with the grain structure. The strengthening mechanisms are discussed based on the dislocation models and the functions of the CGs in the deformation behavior and ductility enhancement of bimodal Al6063 are explored.

Harmonic Structure Design of Co-Cr-Mo Alloy with Outstanding Mechanical Properties

Co-Cr-Mo alloy powders were subjected to controlled mechanical milling at room temperature under Ar atmosphere to fabricate bimodal microstructure in the MM powders, having nanosized grains in the surface region and micron-sized coarse grains in the center of the milled powders. Subsequently, the MM powder was compacted by spark-plasma sintering (SPS) process. The sintered compacts indicated two structure areas: (i) ultra-fine grained (UFG) regions, called shell, and (ii) the coarse grained regions called core. The shell and the core correspond to the surface and center of the MM powders, respectively. The shell regions established a continuous three dimensional network of high strength ultra-fine grained regions, which surrounded the discrete coarse grained ductile regions. Such a microstructure is referred as Harmonic Structure. The sintered Co-Cr-Mo alloy compacts exhibited outstanding mechanical properties. The yield strength increased from 605 to 635 MPa, and ultimate tensile strength increased from 1201 to 1283 MPa. Moreover, the elongation was maintained more or less same as that of coarse grained compacts. Therefore, the harmonic structure design leads to the new generation microstructure of Co-Cr-Mo alloy, which demonstrates outstanding mechanical properties, i.e. superior strength and excellent ductility as compared to conventional materials. Keywords: mechanical milling, Co-Cr-Mo alloys, mechanical properties, harmonic structure.

Characterization of bulk bimodal polycrystalline nickel deformed by direct impact loadings

Spark plasma sintering of a blend of powders with nanometer and micrometer sized particles yielded to a composite-like nickel microstructure consisting of ultrafine-grained (UFG) and coarse-grained (CG) volumes with the fractions of 36% and 64%, respectively. Microstructure evolution and nanohardness distributions of specimens submitted to impact loading at various velocities between 12 and 50ms−1 were determined. At a velocity of 12ms−1, cracks were formed in the UFG regions but they were stopped by the CG entities. Higher velocities resulted in crack-free microstructures and considerable grain fragmentation within CG regions. X-ray line profile analysis investigations showed a decrease of mean crystallite size from ~104 (initial state) to ~41nm (highest velocity). The dislocation density first increased up to 20ms−1 then it decreased considerably with increasing impact velocity, indicating recovery in the microstructure due to the conversion of plastic work into heat. Accordingly, the average nanohardness decreased with increasing the velocity from 20 to 31ms−1. No difference between the microstructures impacted at 31 and 50ms−1 was observed.

High temperature thermal stability of ultrafine-grained silver processed by equal-channel angular pressing

The high temperature thermal stability of the ultrafine-grained (UFG) microstructures in low stacking-fault-energy silver was studied by differential scanning calorimetry (DSC). The UFG microstructures in two samples having purity levels of 99.995 and 99.99 at.% were achieved by four passes of equal-channel angular pressing at room temperature. The defect structure was studied by electron microscopy, X-ray line profile analysis, and positron annihilation spectroscopy before and after the exothermic DSC peak related to recovery and recrystallization. The heat released in the DSC peak was correlated to the change of defect structure during annealing. It was found for both compositions that a considerable fraction of stored energy (~15–20 %) was retained in the samples even after the DSC peak due to the remaining UFG regions and a large density of small dislocation loops in the recrystallized volumes. The larger impurity level in Ag yielded a higher temperature of recrystallization and a lower released heat. The latter observation is explained by the much lower vacancy concentration before the DSC peak which is attributed to the segregation of dopants at grain boundaries resulting in a smaller free volume in the interfaces.

Strain Mapping of Al–Mg Alloy with Multi-scale Grain Structure using Digital Image Correlation Method

Al–Mg alloy powder was mechanically milled in liquid N2 (cryomilling) to produce thermally stable powder with nanocrystalline (NC) microstructure for the manufacture of high-strength alloys. A multi-scale microstructure was achieved by blending unmilled coarse-grained (CG) powder with cryomilled powder and subsequently consolidating. The final bulk alloy was comprised of ultra-fine grained (UFG) regions and discrete CG bands. Dynamic observations of tensile deformation of the alloy were recorded using a micro-straining module attached to a light microscope, and the displacements were measured by digital image correlation (DIC). Strain inhomogeneity between UFG regions and ductile CG bands was observed in the micro-strain (strain order of 10−4–10−6) range, and the strain behavior was interpreted in terms of dislocation plasticity. Special emphasis was given to the distinct displacements between adjoining regions during deformation.

The extent and mechanism of nanostructure formation during cold rolling and aging of lath martensite in alloy steel

Transmission electron microscopy was used to study microstructural evolutions during cold rolling and isothermal aging of an Fe–10Ni–7Mn (wt.%) maraging steel at 753 K. The steel shows lath martensite after homogenizing treatment which evolved by cold rolling to a typical deformed structure, consisting of lamellar dislocation boundaries and shear bands. Ultrafine-grained regions were found frequently in the isothermally aged steels. The process of ultrafine grain formation during isothermal aging is attributed to extended recovery to be correlated with precipitate coarsening reactions. At later stages of aging, recrystallization-assisted discontinuous coarsening of grain boundary precipitates was identified at lamellar boundaries, giving rise to a banded structure consisting of ferrite bands and precipitation hardened laths. It was demonstrated that partially-nanostructured multi-phase steel is fabricated by cold rolling and aging of lath martensite.