Showing papers on "Hardening (metallurgy) published in 2020"

Hierarchically structured diamond composite with exceptional toughness

Abstract: The well known trade-off between hardness and toughness (resistance to fracture) makes simultaneous improvement of both properties challenging, especially in diamond The hardness of diamond can be increased through nanostructuring strategies1,2, among which the formation of high-density nanoscale twins - crystalline regions related by symmetry - also toughens diamond2 In materials other than diamond, there are several other promising approaches to enhancing toughness in addition to nanotwinning3, such as bio-inspired laminated composite toughening4-7, transformation toughening8 and dual-phase toughening9, but there has been little research into such approaches in diamond Here we report the structural characterization of a diamond composite hierarchically assembled with coherently interfaced diamond polytypes (different stacking sequences), interwoven nanotwins and interlocked nanograins The architecture of the composite enhances toughness more than nanotwinning alone, without sacrificing hardness Single-edge notched beam tests yield a toughness up to five times that of synthetic diamond10, even greater than that of magnesium alloys When fracture occurs, a crack propagates through diamond nanotwins of the 3C (cubic) polytype along {111} planes, via a zigzag path As the crack encounters regions of non-3C polytypes, its propagation is diffused into sinuous fractures, with local transformation into 3C diamond near the fracture surfaces Both processes dissipate strain energy, thereby enhancing toughness This work could prove useful in making superhard materials and engineering ceramics By using structural architecture with synergetic effects of hardening and toughening, the trade-off between hardness and toughness may eventually be surmounted

Strain rate dependency of dislocation plasticity

Abstract: Dislocation slip is a general deformation mode and governs the strength of metals. Via discrete dislocation dynamics and molecular dynamics simulations, we investigate the strain rate and dislocation density dependence of the strength of bulk copper single crystals using 192 simulations spanning over 10 orders of magnitude in strain rate and 9 orders of magnitude in dislocation density. Based on these large set of simulations and theoretical analysis, a new analytical relationship between material strength, dislocation density, strain rate and dislocation mobility is proposed, which is in excellent agreement with the current simulations as well as with experimental data. The results show that the material strength is a non-monotonic function of dislocation density and displays two universal regimes (first decreasing, then increasing) as the dislocation density increases. The first regime is a result of strain rate hardening, while the second regime is dominated by the classical Taylor forest hardening. Accordingly, the strength displays universally, as a function of strain rate, a rate-independent regime at low strain rates (governed by forest hardening) followed by a rate hardening regime at high strain rates (governed by strain rate hardening). All the results can be captured by a single scaling function. Finally, the fluctuations of dislocation flow are analyzed in terms of the strain rate dependent distribution of dislocation segment velocities. It is found that the fluctuations are governed by another universal scaling function and diverge in the rate independent limit, indicating a critical behavior. The current analysis provides a comprehensive understanding on how collective dislocation motions are governed by the competition between the internal elastic interactions of dislocations, and the stress required to drive dislocation fluxes at a given externally imposed strain rate.

Strength-ductility synergy of selective laser melted Al-Mg-Sc-Zr alloy with a heterogeneous grain structure

Abstract: In this work, a Sc/Zr modified Al-Mg alloy was processed by both selective laser melting (SLM) and directed energy deposition (DED). Due to different precipitation behavior of primary Al3(Sc,Zr)-L12 nucleation sites, a heterogeneous grain structure was formed in SLMed sample, which consisted of ultrafine equiaxed grains bands and columnar grains domains, while a fully equiaxed grain structure was obtained in DEDed sample. Tensile results showed that the as built SLMed sample had a good combination of strength and ductility. The yield strength of SLMed sample (335 ± 4 MPa) was about 2.8 times that of DEDed sample (118 ± 3 MPa), however, the ductility in uniform elongation (23.6 ± 1.9%) was still comparable to that of DEDed sample (23.8 ± 2.6%). Based on the relationship between the heterogeneous grain structure and strain hardening behavior, the strength-ductility synergy mechanism of the SLMed Al-Mg-Sc-Zr alloy was discussed. Stress partitioning tests showed that the contribution of back stress hardening to flow stress was higher in SLMed sample than DEDed sample, while effective stress hardening showed an opposite trend. Despite the overall strain hardening ability of SLMed sample was limited by the high dynamic recovery rate of ultrafine equiaxed grains, additional back stress hardening, which was caused by strain partitioning between equiaxed grains bands and columnar grains domains, improved its strain hardening ability and resulted in the good combination of strength and ductility.

The Effects of Severe Plastic Deformation and/or Thermal Treatment on the Mechanical Properties of Biodegradable Mg-Alloys

Abstract: In this study, five MgZnCa alloys with low alloy content and high biocorrosion resistance were investigated during thermomechanical processing. As documented by microhardness and tensile tests, high pressure torsion (HPT)-processing and subsequent heat treatments led to strength increases of up to 250%; as much as about 1/3 of this increase was due to the heat treatment. Microstructural analyses by electron microscopy revealed a significant density of precipitates, but estimates of the Orowan strength exhibited values much smaller than the strength increases observed. Calculations using Kirchner’s model of vacancy hardening, however, showed that vacancy concentrations of 10−⁵ could have accounted for the extensive hardening observed, at least when they formed vacancy agglomerates with sizes around 50–100 nm. While such an effect has been suggested for a selected Mg-alloy already in a previous paper of the authors, in this study the effect was substantiated by combined quantitative evaluations from differential scanning calorimetry and X-ray line profile analysis. Those exhibited vacancy concentrations of up to about 10−3 with a marked percentage being part of vacancy agglomerates, which has been confirmed by evaluations of defect specific activation migration enthalpies. The variations of Young’s modulus during HPT-processing and during the subsequent thermal treatments were small. Additionally, the corrosion rate did not markedly change compared to that of the homogenized state.

Additive manufacturing of an Fe–Cr–Ni–Al maraging stainless steel: Microstructure evolution, heat treatment, and strengthening mechanisms

Abstract: Additive manufacturing of a low carbon Fe–Cr–Ni–Al maraging stainless steel (with the brand name CX) through the laser-powder bed fusion (LPBF) process is studied. Since the strength of this material is enhanced through precipitation hardening, the effect of different heat treatment cycles on the hardness and microstructure is assessed. The LPBF-CX is heat treated through a standard heat treatment procedure consisted of austenitization at 900 °C for 1 h followed by air cooling and aging at 530 °C for 3 h. Moreover, the effect of aging treatment (with no austenitization) on the as-built sample is studied. The microstructure of the as-built, austenitized-aged, and aged samples is studied using multiscale electron microscopy techniques. The as-built LPBF-CX consists of the typical lath martensitic structure and minor retained austenite. The martensite laths are featured by high dislocation density, with no evidence of precipitates. Austenitization-aging treatment shows a detrimental effect on the strength of LPBF-CX, due to martensite laths growth and retardation of precipitates evolution. Aging of the as-built LPBF-CX results in strength enhancement due to the evolution of nanometric and coherent β-NiAl precipitates, and martensite laths refinement. Moreover, the pre-existing dislocation networks play a key role in the strength of the aged material. The strength enhancement of the aged LPBF-CX is investigated through the fundamentals of alloy hardening.

A Review of Friction Stir Processing of Structural Metallic Materials: Process, Properties, and Methods

Abstract: Friction stir processing (FSP) has attracted much attention in the last decade and contributed significantly to the creation of functionally graded materials with both gradient structure and gradient mechanical properties. Subsurface gradient structures are formed in FSPed metallic materials due to ultrafine grained structure formation, surface modification and hardening with various reinforcing particles, fabrication of hybrid and in situ surfaces. This paper is a review of the latest achievements in FSP of non-ferrous metal alloys (aluminum, copper, titanium, and magnesium alloys). It describes the general formation mechanisms of subsurface gradient structures in metal alloys processed by FSP under various conditions. A summary of experimental data is given for the microstructure, mechanical, and tribological properties of non-ferrous metal alloys.

Achieving highly strengthened Al–Cu–Mg alloy by grain refinement and grain boundary segregation

Abstract: An age-hardenable Al–Cu–Mg alloy (A2024) was processed by high-pressure torsion (HPT) for producing an ultrafine-grained structure. The alloy was further aged for extra strengthening. The tensile strength then reached a value as high as ~1 GPa. The microstructures were analyzed by transmission electron microscopy and atom probe tomography. The mechanism for the high strength was clarified in terms of solid-solution hardening, cluster hardening, work hardening, dispersion hardening and grain boundary hardening. It is shown that the segregation of solute atoms at grain boundaries including subgrain boundaries plays a significant role for the enhancement of the tensile strength.