Hardness Evolution Research Articles

Profiling hardness and microstructural evolution in the soft zone of 1.25Cr-0.5Mo heat-resistant steel subjected to varied creep durations

Simulated welding has been employed to precisely locate and track the soft zone of 1.25Cr-0.5Mo heat resistant steel. Hardness and microstructural variations in the soft zone subjected to designed creep durations have been systematically explored. It has been demonstrated that, as the creep duration increases, the hardness of the soft zone gradually increases, which is primarily driven by the grain size reduction of equiaxed ferrites. In addition, population of carbide precipitates and accumulation of geometrically necessary dislocations also contribute synergistically to hardness dip.

Advanced use of waste rubber and fly ash to ensure an efficient circular economy

The manufacture of high-performance products that have in their structure both rubber waste and fly ash represents the objective of the research which is a common one with that of the international community, in order to be able to ensure an efficient circular economy for the waste. Research has focused on the creation of new composite materials that contain as large amounts of fly ash as possible, respectively materials obtained from the processing of rubber waste. Thus, during the research, the use of a fairly large proportion of fly ash (30 phr), but also reclaimed rubber (25 phr) and rubber powder with size 80–100 µm (15 phr) was considered. The new composite materials were tested from several points of view, with the aim of determining their properties, but also the possibilities of use in different applications. Thus, the microscopic structure (SEM) was analyzed to observe how the connections between fly ash and rubber are made, but also how they behave in thermal shocks, the evolution of hardness, respectively the adhesion of dirt particles on the surface of the samples. The tests performed have shown that by using a KWQ coupling agent, a fairly homogeneous structure of the materials can be obtained, with a very good incorporation of the fly ash into the rubber matrix. It has also been demonstrated that by using fly ash in combination with reclaimed rubber or rubber powder, a very good topography of the surface layer of the samples can be obtained, which prevents dust from adhering to the surfaces of products containing a large amount of fly ash or regenerated rubber. It is noteworthy that composite materials containing a large amount of fly ash (FA) and regenerated rubber retain their properties very well after applying an aging process. Thus, the use of FA and regenerated rubber allows to obtain composite materials that have an aging elasticity loss of only 0.4% compared to rubber without FA, when this loss of elasticity was 25.5%. At the same time, the use of a suitable coupling agent allows the creation of new materials that have a fairly high hardness, without the addition of fly ash and regenerated rubber causing a large reduction in hardness. It is noteworthy that the reduction in hardness was very low, from 790ShA for the rubber without fly ash to 740ShA for the rubber containing FA. The rubber mixtures used in the research were also subjected to a TGA-DTGA analysis demonstrating that the addition of an optimal amount of FA ensures a weight loss comparable to that of rubber mixtures that do not contain FA. Thus, it was demonstrated that an efficient circular economy can be ensured for waste from rubber products as well as for FA.

Effect of high temperature exposure on microstructure, mechanical and tribological properties of cold sprayed NiCoCrAlTaY coatings

NiCoCrAlTaY coating is widely used in high temperature oxidation resistance, gas erosion protection and gas path sealing of hot-end parts of aeroengines and gas turbines. In this study, NiCoCrAlTaY coatings were prepared on cobalt-based superalloy substrates by high pressure cold spray technology. The microstructure, mechanical property and tribological behaviors of the coatings before and after thermal exposure at 1050 °C were systemically studied. The results showed that NiCoCrAlTaY coatings with very dense structure could be obtained by high-pressure cold spraying with nitrogen as carrier gas. The coating remained completely intimate with the substrate without any cracking and spalling after thermal exposure at 1050 °C for 500 h. Due to elemental diffusion at high temperatures, the coating formed a four-layer structure, including alumina film on the surface, β-NiAl phase depletion layer near the surface, intermediate layer containing β-NiAl phase and β-NiAl phase depletion layer near the substrate. With the prolongation of thermal exposure at 1050 °C, the thicknesses of β-NiAl phase depletion layers both near the surface and the substrate increased. The thickness and content of the intermediate β-NiAl phase layer decreased gradually during thermal exposure. Besides, the average hardness values of the coating rapidly decreased by about 30 % after thermal exposure at 1050 °C for 5 h. In particular, the hardness of the β-NiAl phase depletion layer near the surface substrate was lower than that of the intermediate area containing β-NiAl phase. Due to the microstructure and hardness evolution, the wear resistance of the coating decreased significantly after thermal exposure. The friction coefficient of the coating increased from 0.2 to 0.5, and the volume wear rate of the thermally exposed coating was about 4.5 times higher than that of the as-sprayed coating. The results of this study provide guidance for design and application of high-performance MCrAlY coatings.

Influence of processing temperature on microhardness evolution, microstructure and superplastic behaviour in an Al–Mg alloy processed by high-pressure torsion

Experiments were conducted to assess the effect of processing temperature on the hardness evolution, microstructure and the flow properties of an annealed Al–3Mg alloy processed by high-pressure torsion (HPT) at either 300 or 450 K. The results demonstrate that HPT processing at room temperature (RT) leads to higher microhardness values with a more uniform hardness distribution in the Al alloy compared with processing at 450 K. After 10 HPT turns at RT, the microstructure displayed a more intense grain refinement, a higher dislocation density and smaller Al3Mg2 precipitates than after HPT at 450 K. Accordingly, the metal processed at RT showed enhanced strength at RT and consistently exhibited superplastic flow during deformation at 523 K, with tensile elongations of ∼530–650% for strain rates from 10−3 to 10−2s−1. Conversely, a maximum elongation of ∼110% was recorded at 523 K in the alloy processed by HPT at 450 K. This behaviour is attributed to an enhanced thermal stability in the metal processed at RT compared with HPT at 450 K where deformation at 523 K led to the onset of abnormal grain coarsening due to a more disperse distribution of grain sizes and the partial dissolution and coalescence of Al3Mg2 precipitates after HPT processing.

Evolution of morphology, microstructure and hardness of bodies and debris during sliding wear of carbon steels in a closed tribosystem

Debris generated during sliding wear of steels have an important role on tribosystem's behavior. However, the evolution of morphology, microstructure and hardness of debris and solid bodies, and their role on friction and wear is not quite evident, especially for closed tribosystems. Interrupted pin-on-disk wear tests were carried out on low, medium, and high carbon steel tribopairs, while an adaptation of the test apparatus effectively prevented debris ejection from wear track, simulating a closed tribosystem. The experiment was designed according to a repeated measures ANOVA model for multiple dependent variables that includes running-in duration, wear rates, surface hardness and their ratios, and debris size and shape. Plasticity-dominated severe wear took place in all cases, generating two types of metallic debris: flake-type via adhesion and delamination processes and, after sufficient sliding distance, cutting-type due severe shear deformation caused by the abrasive action of hard asperities. Debris were, at least, 20% softer than worn pins and disks, and provided some lubricating effect when adhered, sheared and rolled between sliding surfaces. Low carbon steel wear generated more soft and adherent debris, resulting in prolonged running-in periods. Finally, steel carbon content did not affect wear mechanisms, debris morphology (size and shape), worn surface hardness and wear rates in closed tribosytems, when a debris-flow controlled process prevails.

Relationship between precipitation behavior and loading orientations of the creep-aged Al–Cu–Li single crystal

In present work, typical loading orientations were applied on an Al–Cu–Li single crystal during creep aging, and the relationship between precipitation behavior and loading orientations of the creep-aged Al–Cu–Li single crystal was studied by high angle annular dark field - scanning transmission electron microscope (HAADF-STEM) observation along multiple zone axes. The results show that the precipitation behavior of the precipitates (i.e., T1, θ′) in creep-aged Al–Cu–Li alloy was directly attributed to the loading directions. The number density of T1-phase in the creep-aged single crystal loaded close to <001>α direction is highest, and that of T1-phase is the lowest when the loading direction is < 111>α. Moreover, the oriented-precipitation of T1-phase was found in the creep-aged sample loaded close to <111>α direction, while that of θ′-phase was observed in the creep-aged sample loaded close to <011>α direction. The hardness evolution of the creep-aged samples along different loading orientations is also in agreement with the corresponding precipitation behavior. The present work proposes a novel method to regulate the precipitation behavior of the creep-aged Al–Cu–Li alloy by controlling the loading direction, and the ideal loading direction is < 001>α for the Al–Cu–Li alloy to obtain better mechanical property.

Mark McCleod-Harrison, Saving the Neanderthals: Sin, Salvation, and Hard Evolution

Mark McCleod-Harrison, Saving the Neanderthals: Sin, Salvation, and Hard Evolution(Lanham, MD: Lexington Books, 2020), xiv + 155 pp. $95 (hbk), ISBN: 978 1978706545.

Effect of carburizing process on bending fatigue performance of notched parts of 18CrNiMo7-6 alloy steel

Since stress concentration is easily formed at the notch, improving the fatigue performance of notched parts is critical. The carburizing process is widely used to manufacture notched parts. In this research, the effects of carburizing process and notch stress concentration on fatigue performance were analyzed by bending fatigue tests. In addition, the microstructure and hardness evolution processes of the carburized notched 18CrNiMo7-6 specimens were observed quasi-in situ. The results showed that macro-plasticity has little effect on the bending fatigue performance of notched samples and that the improvement in fatigue performance is primarily attributable to the high hardness and residual compressive stress introduced by carburizing. The fatigue notch sensitivity is therefore reduced after carburizing. Compared to the uncarburized specimens, the fatigue limit of the carburized specimens is increased by up to 128%. With the increase of the effective hardening layer depth, the fatigue limit increases first and then decreases. Moreover, the brittle phase transformation product (twinning martensite) of metastable retained austenite is easy to produce stress concentration. Conversely, the 'film-like' retained austenite with an extremely high stability can effectively avoid the phase transformation products.

Effect of Ni content and crystallographic orientation on mechanical properties of single-crystal (CoCr)100-x Ni x medium-entropy alloy

Nanoindentation analyses of (CoCr)100-x Ni x medium-entropy alloys with different Ni contents and crystal orientations were carried out by molecular dynamics simulations. Analyses show that the force-displacement curves during elastic deformation are in good accordance with the Hertz contact theory and the elastic modulus is closely related to the Ni contents and crystal orientations. The elastic-plastic transition point appears later in (CoCr)67Ni33 than in other alloys. The plastic deformation was studied by exploring the instantaneous microstructure, which was found to be dominated by homogeneous nucleation of Shockley partial dislocations and the accumulation of stacking faults, and different levels of dislocation density were produced in the alloys with different Ni contents and crystal orientations. By analyzing the evolution of dislocation density and hardness, a linear relationship between the square root of dislocation density and hardness can be revealed, which agrees well with the classical Taylor hardening model, and the empirical constant is found closely related to crystal orientations.

The role of hardening and roughening during the incubation period in water droplet impingement erosion of Ti–6Al–4V

The present work investigates the role of surface hardening and roughening on the damage accumulation during the incubation period in water droplet erosion (WDE). Water droplet erosion tests were carried out on Ti–6Al–4V alloy at an impact velocity of 250 m/s. The evolution of hardness and surface roughness during the incubation period were evaluated using microhardness tester and Confocal Laser Scanning Microscope (CLSM). Scanning Electron Microscopy (SEM) was conducted on the impact area at several intervals during the incubation period. Finite Element (FE) simulations were performed to obtain the impact stresses on both smooth and rough surfaces. It was found that the solid surface plastically deforms by the action of the initial impingements, and as a result, hardness and surface roughness increase progressively until the end of the incubation period. The cyclic increase in hardness results in gradual improvement in the erosion strength, making the surface more resistant to the subsequent droplet impacts. However, the finite element results revealed that the dynamic surface roughening process results in higher impact stresses for the same impact pressure due to geometrical stress concentration. It is concluded that the increase in impact stresses resulting from surface roughening negates the improvement in the erosion resistance due to hardening. Hence, cyclic hardening and roughening are crucial parameters in the damage accumulation process during the incubation period.

The effects of strain rate and strain on the deformation-induced grain refinement process and hardness evolution of a polycrystalline cobalt

In this study, the effects of strain rate and strain on the evolution of microstructure and hardness of a polycrystalline cobalt subjected to high-speed impact and quasi-static compression, with the strain rate of ∼ 103 s−1 and 3 × 10−3 s−1, respectively, were investigated. During high-speed impact, the high strain rate promoted the activation of various types of twinning: 101̅2, 101̅1, 112̅1 and 112̅2 twinning, as well as the 101̅1−101̅2 double twinning. Meanwhile, the temperature rise during high-speed impact activated the incomplete dynamic recrystallization. Deformation twinning, deformation-induced martensitic phase transformation and dynamic recrystallization contributed to the grain refinement. During quasi-static compression, dislocation slip and twinning dominated the deformation. Only 101̅2 twins with different orientations were triggered. The number of twins increased with the increase of strain, but was less than that in the high speed compressed sample. The high speed compressed samples exhibited higher hardness values compared with the quasi-statically compressed samples. The more pronounced hardening effect mainly accounted for the higher density of twins produced during high-speed compression. Although the recrystallization during high-speed compression may lead to strain softening, such softening effect was overwhelmed by the strengthening effect of grain refinement.

Defect evolution of neutron irradiated ITER grade tungsten after annealing

The microstructural evolution of neutron irradiated tungsten (W) after annealing and its correlation with the corresponding mechanical properties provides valuable insight on the defect interactions and their annihilation processes. This would result in the identification of recovery mechanisms, leading to the design of healing processes and thus, enabling the lifetime extension of fusion reactor components. Within this framework, samples from ITER specification forged W bar were neutron irradiated to a dose of 0.2 displacements per atom (dpa) at 600 °C in the Belgian Material Test Reactor (BR2) and subsequently annealed at 800 and 1000 °C. The evolution of the irradiation induced defects after annealing has been assessed by transmission electron microscopy (TEM), positron annihilation lifetime spectroscopy and electrical resistivity and its effect on the mechanical properties are discussed in terms of Vickers hardness. Neutron irradiation results in the formation of dislocation loops and voids. TEM observations show an increase in the size of both defect types after post irradiation (PI) annealing at both temperatures, accompanied by an initial increase in their density after PI annealing at 800 °C, followed by a subsequent decrease after PI annealing at 1000 °C. The presence of TEM-irresolvable defects of both types is revealed in the as-irradiated state, which is evidenced by the evolution of Vickers hardness, resistivity and positron lifetimes and their correlation after the PI annealing. In the as-irradiated state, as well as after PI annealing at both temperatures, the hardening is dominated by voids.

Precipitation of carbides in a nickel-based cast heat-resistant alloy during thermal exposure: evolution of microstructure, hardness and corrosion properties

Abstract Ni-based G-NiCr28W alloy, which is widely preferred in high-temperature applications, has an austenite matrix as well as carbides dispersed in the structure. The morphology of the carbides in the structure varies depending on the chemical composition and heat treatment. Carbide phases commonly seen in these alloys are MC, M7C3, and M23C6. These carbides can transform into each other and the mechanical properties of the alloy can be affected by the carbide transformation. In this study, the effects of the carbides on the morphology, distribution, and strength of the alloy were investigated following the solution treatment applied at different temperatures and durations. Samples were characterized by light metal microscopy, SEM, EDS, XRD analysis, hardness, and potentiodynamic polarization tests. After the process, due to the excess of more stable carbides and their homogeneous distribution within the structure, the highest hardness and corrosion values were revealed in the sample that was processed at 1000 °C for 100 h.

Effect of Al Content on Microstructure and Properties of Cu-14Ni-xAl Alloys

This paper focuses on the influence of Al content on the microstructure and properties of Cu-14Ni-xAl alloys, with the mass fraction of aluminum element ranging from 0 to 5% as the object. First, the evolution of Vickers hardness and electrical conductivity of Cu-14Ni-xAl alloys during aging were tested. Then a comparative study of the friction and wear behavior of Cu-14Ni-xAl alloys with different Al contents were studied. The results show that the alloy with Al content of 3% possesses the best mechanical and friction performance, with the hardness of HV 275.0 and friction coefficient of 0.7 in the peak-aged state. Microstructure analysis show that the precipitates are denser and the volume fraction of aging induced precipitates are more in the Cu-14Ni-3Al alloy, compared with that in the other studied alloys. Finally, the correlations between the Al content, microstructure, the mechanical and tribological properties of the Cu-Ni-Al alloys were further discussed.

Development of a Periodic Laminate Structure in H13 Steel Using Laser Powder Bed Fusion: Effects of Tempering on Hardness Evolution

H13 steel with a sub‐millimeter‐order periodic laminate structure is fabricated using the laser powder bed fusion process by changing the input energy periodically, and the effect of heat treatment on the hardness distribution of the alloy is examined. When the as‐built specimen is directly tempered at 550 °C to induce secondary hardening, the hardness values at the matrix area of the specimen, which average 638 HV under the as‐built condition, increase to an average of 685 HV, whereas those of intermediate areas, which initially average 658 HV, reach an average hardness of 722 HV, which is higher than that of the matrix area even under the tempered condition. Consequently, the hardness distribution maintains the initial trend, wherein the hardness varies periodically depending on the area, exhibiting a shift toward higher hardness values under the tempered condition. The factors affecting the hardness and tempering behavior are investigated by analyzing the phase constituents and microstructural evolution. The tempered specimen comprises α′‐martensite, indicating that retained austenite present in the as‐built microstructure is almost completely decomposed. At the nanolevel, only V segregates at the lath boundaries under the as‐built condition, while various metallic carbides are formed at the lath boundaries after tempering.

Microstructure and thermal stability of nanocrystalline AZ31 Mg alloys reinforced by ultrafine vanadium particles

In this paper, the microstructure evolution and thermal stability of nanocrystalline (NC) AZ31-2.5 wt%VP magnesium matrix composites were studied, while the hardness was analyzed. The results showed that the NC AZ31-2.5 wt%VP still kept NC scale after annealing at 300 °C and 350 °C. Even at 400 °C and 450 °C, the grain size of the annealed composite only grew to 113 nm and 132 nm, which was still in ultrafine grain scale, illustrating excellent thermal stability of NC AZ31-2.5 wt%VP. Furthermore, the V particles, which was uniformly distributed in the Mg matrix, had no obvious change in dimension after annealing, while it also had no new phase formed. The stabilization mechanism was expounded and found that the pinning effect of V particles effectively inhibited the grain growth of Mg matrix at high temperature. In addition, the grain growth kinetics equation (D6−D06=kt) was utilized to explain the grain growth behavior of NC AZ31-2.5 wt%VP, and the activation energy (Ea) of grain growth was calculated to be 130.9 kJ/mol, which was much higher than that of pure Mg (92 kJ/mol). Eventually, the hardness of NC AZ31-2.5 wt%VP was contrastive studied and the hardness evolution curves at different temperatures were obtained. After annealing treatment was performed at 450 °C for 180min, the hardness remained at 80 HV, which was higher than that of as-cast AZ31 (53 HV).

The microstructure and texture evolution of Mg-5Sn-1Si-0.8Y alloy during the reciprocating extrusion process (REP)

In this paper, the microstructure and texture evolution of Mg-5Sn-1Si-0.8Y alloy fabricated by reciprocating extrusion process (REP) at 340 ℃ for 2–8 passes is characterized by optical microscopy (OM), Scanning Electron Microscope (SEM) and Electron Backscatter Diffraction (EBSD).The results show that the microstructure was refined significantly, dynamic recrystallization (DRX), fragmentation of the second Mg2Si phase and precipitation of the Mg2Sn phase occurred during REP. With the increase of the REP passes, the fraction of low angle grain boundaries (LAGBs) showed a decreasing trend and the secondary phases distribution became more and more homogeneous and the degree of DRX was increased. At the same time, the initial extruded fiber basal texture was gradually weakened and became random as the REP passes increases. The hardness of the alloy also shown an increasing trend with the increase of REP passes. The grain refinement mechanism, texture evolution and increase of hardness were mainly related to the dynamic recrystallization and the particle stimulation nucleation (PSN) effect of the second phases.

The irradiation effects on the nanoindentation hardness and helium bubbles evolution mechanism of Ni-based alloy

High-dose neutron irradiation could produce various kinds of microstructural defects, and the radiation resistance of nickel alloy is put forward for higher requirements under the high temperature. To simulate the (n, α) reaction, helium ions with an energy of 540 keV were implanted into nickel-based alloy 617 at 700 °C. The microstructure and mechanical properties were established by comparing the nanoindentation hardness before and after irradiation. For the low dose irradiation, the microstrain induced by vacancy-type defects like He-vacancy clusters and helium bubbles is slight, which would not result in a large increase of hardness. The hardening effect of high-temperature irradiation was much lower than that of room-temperature irradiation due to the recovery of matrix defects. The actual distribution of helium bubbles observed by Transmission Electron Microscope (TEM) was comparable to the SRIM simulation result. The helium bubbles were spherical near the surface of the alloy and polyhedral deeper in the matrix. Moreover, chromium carbide had a strong trapping effect on helium bubbles. The shapes and distribution of the helium bubbles were analyzed to evaluate irradiation effects on nickel-based alloys.

Study of microwave cooking of legumes with excess water: The case of chickpeas

In the context of the agro-ecological transition towards sustainable systems, legumes are a real opportunity in terms of agronomy and nutrition. The development of vegetable seed consumption is currently hampered by the problem of the long hydration and cooking stages of current processes. Low CO2 emission processes, such as microwave treatment, are preferred. The efficiency of these processes relies on the transformation of the energy contained in an electromagnetic wave into heat within materials with dielectric losses. The objective of the presented work is to study the evolution of the characteristics of leguminous seeds during the operation of cooking in excess of water by microwave treatment. Kinetics of texture modification and mass gain of the seeds were performed for three different levels of microwave power (between 400 and 1200 W) and compared to hot plate cooking. Empirical models by multi-parameter equations were proposed and validated by a 1000 W treatment. The results show that the evolution of hardness and mass gain is strongly related to the evolution of mass transfer coupled with volume heat transfer phenomena, specific to electromagnetic wave heating. Two different profiles of hardness and mass gain were observed depending on the type and intensity of the treatment. This work is a preliminary experimental study allowing to express hypotheses, which will be validated later by a more thorough study.

Synergetic effect of natural ageing and pre-stretching on the ageing behavior in Tʹ/ηʹ phase-strengthened Al-Zn-Mg-Cu alloys

Al-Zn-Mg-Cu alloys with different major strengthening precipitates are subjected to a novel combinatorial pre-treatment, including natural ageing and pre-stretching. The evolution of hardness and microstructure during the combinatorial pre-treatment and subsequent artificial ageing has been investigated. The results reveal that the growth rate of hardness in alloy B (Zn/Mg = 10.0) is much higher than that of alloy A (Zn/Mg = 1.5) due to the fast precipitation kinetics of η′ phase compared with T′ phase. Both GP I zones and dislocations introduced by the combinatorial pre-treatment can act as heterogeneous nucleation sites for precipitation, resulting in more precipitates and higher hardness than pre-stretched alloys A and B. Dislocations distribute uniformly in combinatorial pre-treated alloys owing to the existence of GP I zones and dislocations, which promote the precipitation and refine the precipitate size. Moreover, these alloys with distinct pre-stretching (2%–10%) show similar precipitation behavior and peak hardness, and it indicates that the dislocation-induced precipitation will not be affected by the density of dislocations when plenty of GP I zones pre-exist.