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Mechanically alloyed 91Fe-8Ni-1Zr (at%) powders were fabricated through high energy ball milling of elemental powder and subsequently consolidated via equal channel angular extrusion (ECAE) at 800 °C and 1000 °C. The resulting microstructure was fine grain with a nano-dispersion of Zr-oxide within the matrix, which was spherical for the 800 °C. ECAE and plate-like (and volumetrically larger) for the 1000 °C ECAE conditions. Atom probe tomography confirmed trace levels of C, N, and Cr impurities within the alloy making it similar to a low-carbon steel. By performing mechanical testing at a quasi-static strain rate (10−3 s−1) and at high strain rate (103 s−1) at room temperature and 473 K, a load drop was noted after yielding. In general, this load drop became more pronounced with increasing strain rate and temperature and has been shown to be a result of dynamic strain aging in the ODS alloy.

The mechanical properties of the weld regions of a 6061-T6 resistance spot welded lap joint are determined. The change in mechanical properties resulting from RSW are linked to the changes observed in the microstructure. Processing currents and strain rates are varied to probe the effects of processing temperature at strain rates from 10−3 to 103 s−1. Results show that material strength decreases within the heat affected zone (HAZ) and fusion zone due to precipitate dispersion. Further, decreased ductility results at quasi-static strain rates from accelerated crack growth arising near voids formed during weld formation, but the short time scale at higher strain rates limits the ability for crack growth from these voids allowing the material to exhibit higher ductility. Overall, significant changes in the mechanical behavior across the weld resulting from a change in microstructure congruent with precipitate dispersion are apparent for all processing conditions.

In this work, various alloy compositions of immiscible copper-tantalum (Cu–Ta) are systematically studied to understand the interplay between cluster stability, precipitation hardening, and the overall stability of the matrix's average grain size. An alloy composition of Cu-3at.%Ta is found to exhibit dramatic improvements relative to other neighboring Ta contents (both higher and lower) only after exposures of more than 300 h at 800 °C. An extremely low steady-state growth rate, i.e., 3.1 nm per 100 h exposure, speaks to the extreme kinetics which allow this composition to retain its high mechanical strength. The key attribute responsible for the NC Cu-3at.%Ta alloy exhibiting such behavior is a high cluster density tailored through the Ta solute content to achieve the best combination of stability without sacrificing strength by limiting Orowan coarsening of the clusters compared to other Ta concentrations. In general, the growth kinetics of this Cu-3at.%Ta alloy are so sluggish that it places it among the most thermally resistant alloys ever produced. The work demonstrates that, if designed properly, bulk nanocrystalline alloys can withstand the prolonged high-temperature exposure required for high-temperature applications, while grain growth is stagnated or halted.

Six-sequential layer deposits of an oxide dispersion strengthened (ODS) Fe91Ni8Zr1 alloy were laser assisted cold sprayed (LACS) at 650 °C and 950 °C. The nanoscale zirconia phase in this alloy precipitated from residual oxygen within the high-energy ball milled constituent elemental powders used for the spray deposition. The laser provided a local heating of the substrate (or the surface of the previous deposited layer), which thermally softens it, increasing the mass deposition efficiency (DE). At a surface temperature of 650 °C during deposition, the ODS alloy was ferrite with a DE of 7.3%. This low DE resulted in a peening/tamping effect of ricocheted powders off the surface resulting in the grains at the substrate/deposit interface being notably refined. At a surface temperature of 950 °C, the ODS alloy was in the austenitic phase field during deposition resulting in a DE increase to 32.4%. With this increase in deposition temperature, the deposited grains grew and the oxide particles coarsened. This resulted in a reduction in Vickers hardness from 598 ± 56 (650 °C) to 293 ± 38 (950 °C). Hall-Petch grain size effects, Orowan strengthening between oxide particles, and Taylor hardening contributions estimated the hierarchy in the strengthening mechanisms with these microstructural changes, with grain size being determined as the dominant mechanism. These deposits demonstrate the advantages of using a laser for relatively difficult-to-spray (high hardness and strength) materials, albeit with associated changes in microstructure with their corresponding changes in hardness.

Small-scale shear punch testing has been applied to a butt joint created by friction stir welding of two adjoining AA2139-T8 plates. Advantages of this technique include the ability to perform a large number of independent tests on a given volume of material and the ability to measure site-specific differences and variations in local material properties. As such, combined with a simultaneous evaluation of the weld morphology, a series of 144 shear punch tests were carried out in a 12×12 grid pattern on the retreating half of the weld. The overlay of the grid pattern onto the etched surface allowed a correlation of the microstructure and mechanical properties measured across the weld at each shear punch site. Two-dimensional color enhanced property maps were generated to provide a powerful site specific visualization of the unique or distinctive microstructural features and how they correlate with the local mechanical response across the weld. One of the more insightful discoveries was the weld nugget region undergoing 2.5 times more strain-hardening than the base plate material, while simultaneously experiencing the Portevin-LeChatelier effect. Aspects of the technique and results of our experiments are described.

Nominally pure nanocrystalline metals do not remain nanostructured under extreme conditions of intense heating and or deformation preventing the study of their physical response under such conditions. Here we present the coupled effect of temperature and strain rate on the mechanical response of a thermally stabilized nanocrystalline Cu alloyed with 10 at% Ta. Compressive mechanical testing was performed from 24 to 1000 °C and strain rates ranging from quasi-static (10−1 s−1) to dynamic (104 s−1) rates. The response of this material exhibits a maximum quasi-static yield stress of 1.05 GPa at room temperature and an approximate yield stress of 0.5 GPa at 600 °C, with an apparently linear temperature response. In contrast to pure coarse-grained Cu, our assessment indicates that this Cu-based composite derives its properties from a combination of very small Cu-rich grains and well-dispersed Ta clusters and nanometer (

Mechanical alloying was employed to produce a nanostructured Mo25Nb25Ta25W25 multi-principal element alloy (MPEA) with enhanced mechanical properties. Overall, a 400% increase in hardness was achieved, as compared to similar cast alloys, via mechanical alloying and optimized long-term annealing treatments. Furthermore, advanced characterization, including aberration-corrected scanning transmission electron microscopy, was conducted to elucidate processing-structure-property relationships in which it was determined that, although the introduction of impurities via mechanical alloying is common and thought to be deleterious, impurities can lead to an impressive enhancement of mechanical properties. More specifically, in this study, Fe and N impurities resulted in the formation of nanoscale, ceramic secondary phases. The observed strengthening was attributed, at least in part, to the ceramic impurity phases. Overall, we suggest that a deliberate doping strategy may be employed in the future to tailor MPEA chemistry and thereby achieve superior mechanical properties.

Al–Mn alloys with a solid-solution Mn content ranging from 0 to 3.1 at% were successfully prepared by high energy mechanical alloying at room temperature of an Al–8 at% Mn sample. The solubility level obtained is up to five times the equilibrium solubility limit of Mn in Al (from 0.62 at% Mn). In general, the observed microstructures are consistent with being a nanocomposite composed of an Al–Mn solid solution matrix with dispersed Mn particles. For alloys with solid solutions up to 3.1 at%, increasing the Mn content correlated with a decrease in the matrix grain size down to a minimum of 12 nm. High hardness values of ~4 GPa were obtained. The main strengthening mechanism of the Al–Mn alloys is attributed to the grain size reduction. Further attempts to increase the dissolved solute content resulted in the precipitation of the Al 6 Mn equilibrium intermetallic phase.

Copper (Cu) was cryogenically milled with tungsten (W) in a high-energy ball mill. The process created W particles dispersed in a nanocrystalline Cu matrix. These “alloys” were then annealed to a maximum temperature of 800 °C. The addition of W stabilized the Cu at∼40 nm during annealing to 400 °C for a 1 at% W composition and to 600 °C for 10 at% W. As evidenced through hardness measurement, the W provided a significant increase in strength over pure Cu, and the 10 at% W material maintained a 2.6 GPa hardness after annealing at 800 °C. The stabilization and strengthening mechanisms are compared against theoretical prediction and found to be in good agreement. Although the strength and stability are significantly improved over pure Cu, the maximum benefit was hindered by an extremely broad W particle size distribution (∼5–5000 nm). For the 10 at% W alloy, only half of the added W was reduced to nanoscale where kinetic pinning and strengthening become most effective.

Using a modified regular solution model for grain boundary solute segregation, the relative thermal stability of a number of Fe-based nanocrystalline binary alloys was predicted with considerable accuracy. It was found that nanocrystalline iron was strongly stabilized by zirconium, moderately stabilized by tantalum, and not significantly stabilized by nickel or chromium. These findings are fully in line with the aforementioned predictions. This success with iron based alloys highlights the utility of this practical approach to selecting stabilizing solutes for nanocrystalline alloys.

It was discovered that mechanical milling of metal powders in an ultra high purity argon atmosphere at cryogenic temperatures can result in argon being incorporated into the metal. This incorporated argon causes expansion by increasing the porosity when the material is annealed. The resulting annealed material can be classified as metal foam due to its highly porous nature. The most porous samples were measured to have nearly 50% porosity. This effect was observed in nominally pure copper and an alloy of 81 at% palladium and 19 at% zirconium.

Fe–Zr nanocrystalline alloys with an as-milled grain size less than 10 nm were synthesized by ball milling. The microstructure changes due to annealing were characterized using X-ray line broadening, microhardness, focused ion beam channeling contrast imaging, and transmission electron microscopy (TEM). Additions of 1/3 to 4 at.% Zr stabilized nanocrystalline grain sizes at elevated annealing temperatures compared to pure Fe. With 4 at.% Zr, a fully nanocrystalline microstructure with a TEM grain size of 52 nm was retained at temperatures in excess of 900 °C. Alloys with lower Zr contents showed less stability, but still significant compared to pure Fe. Bimodal nano–micro grain size microstructures were also observed.

The evaluation of mechanical properties like yield and ultimate tensile strengths from shear–punch tests is important when the availability of material is limited. A shear–punch test setup was built in our laboratory and the mechanical properties for different materials; mild steel, pure Al, Zn, brass (Cu–30% Zn by wt.), Al 6061, Austenitic and Martensitic stainless steels were evaluated. A new method using 1% offset criterion in conjunction with normalized shear–punch curves was used to measure the shear yield strength. A linear correlation between the shear data and tensile data was established for yield and ultimate strengths. The variation of the yield and ultimate shear strength was studied as a function of the sample thickness and die–punch clearance for soft, medium and high strength materials.