Microhardness Evolution Research Articles

Enhanced ultra-cryogenic impact toughness in 9 wt% Ni steel through lamellar microstructure refinement

The research aims to effectively enhance the low-temperature toughness of 9 wt% Ni steel, which is used in storage containers for alternative energy sources such as liquefied natural gas. To achieve this, a lamellarizing heat treatment was applied to 9 wt% Ni steel, initially manufactured using the conventional quenching-tempering (QT) process, thereby introducing a quenching-lamellarizing-tempering (QLT) process. The Charpy impact toughness of the 9 wt% Ni steel was then evaluated under two extreme cryogenic conditions: 196 °C and −253 °C. The 9 wt% Ni steels subjected to the QLT treatment demonstrated exceptional Charpy impact toughness values of 238 J at −196 °C and 217 J at −253 °C. This study presents an examination of the microstructural and micro-hardness evolution throughout the various stages of the QLT heat treatment process. A comparative investigation was undertaken by contrasting these results with samples treated using quenching (Q), quenching-tempering (QT), and quenching-lamellarizing (QL) treatments. The QLT treatment induces a substantial volume fraction of retained austenite, combined with a strategically distributed micro-hardness profile. In this profile, the soft phase interspersed among hard phases can alter the crack propagation path and absorb energy, thereby enhancing toughness. This synergy contributes to the exceptional Charpy impact toughness observed at −253 °C. These results suggest that the introduction of the QLT heat treatment is an effective method for enhancing the cryogenic impact toughness of 9 wt% Ni steel at temperatures below −196 °C, compared to the conventional QT process.

Microstructure and Microhardness Evolution of Mg-8Al-1Zn Magnesium Alloy Processed by Differential Speed Rolling at Elevated Temperatures.

Mg-8Al-1Zn magnesium alloy was successfully processed using deferential speed rolling (DSR) at temperatures of 400 and 450 °C for thickness reduction of 30, 50, and 70% with no significant grain growth and dynamic recrystallization. Using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), the rolled microstructures were examined. Although the results indicate a slight reduction in grain size from the initial condition, the DSR processing of alloy at an elevated temperature was associated with a significant number of twins and a distribution of the fine particles of the second phase. The strength in terms of microhardness measurements and strain hardening in terms of shear punch testing was significantly improved in the rolled microstructure at room temperature. The existence of twins and widely distributed second-phase fine particles at twin boundaries reflected positively on the extent of the elongations in terms of shear displacements when microstructures were tested at elevated temperatures in the shear punch testing.

Additive manufacturing of functionally graded stellite6/17-4 PH fabricated via direct laser deposition

Multi-material structures with variable functionalities can lead to unique solutions for engineering problems compared to single-material structures. Additive manufacturing of Functionally Graded Materials (FGMs) using Direct Laser Deposition for the multi-material deposition of Stellite6 and 17–4 PH stainless steel has been investigated in this study. Process parameter design was successfully applied to achieve FGM structures with expected composition ratio and defect free. Results revealed that clads with insufficient laser power had unmelted 17-4 PH powders, while those with excessive power resulted in micro-cracks and disrupting the expected composition for the previous layer. Energy Dispersive Spectroscopy analysis results showed that the composition was almost as expected, with less than 10% deviation. The Stellite6 composition in FGM exhibits a minimum ratio near the substrate due to the penetration of 17-4 PH elements. Meanwhile, in multi-track clads, the Stellite6% decreases along the transverse direction due to increased heat input, multiple cladding tracks, higher dilution, and slower solidification rate. The results of the micro-hardness tests illustrated the gradual evolution of micro-hardness along the build direction starting from the base metal, which had a hardness of approximately 300 HV for 17-4 PH and for the compositions of 25%, 50%, 75%, and 100% Stellite6 showed increases of 10%, 39%, 43%, and 63% respectively, reaching up to 490 HV for fully Stellite6 at the top of the specimen.

High-temperature oxidation behavior of AlCoCrFeNi2.1 eutectic high-entropy alloy: Microstructure evolution and microhardness

In this work, the high-temperature oxidation behavior at 820 °C of an AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) prepared by vacuum induction melting was studied, while the evolution of microstructure and microhardness at different oxidation times (1 h, 3 h, 5 h, 10 h, 20 h, 30 h, and 50 h) were also investigated. The prepared AlCoCrFeNi2.1 EHEA shows a typical FCC/BCC bi-phase microstructure with Cr-rich precipitation phase. Compared to 304 stainless steels, the AlCrCoFeNi2.1 EHEA exhibits a better oxidation resistance, which is due to the continuous and dense Al2O3 generated during high-temperature oxidation process. The AlCoCrFeNi2.1 EHEA also has an excellent high-temperature structure stability, proved by that the phase composition and elemental composition of each phase remain almost unchanged with the increase in oxidation time. After high-temperature oxidation, the microhardness of AlCoCrFeNi2.1 EHEA decreases as expected, however, with the increase in oxidation time, the microhardness shows a noticeable increasing trend due to the solid solution strengthening of the σ phase and formation of the α-Cr phase.

Experimental and Computational Study of Microhardness Evolution in the HAZ for Al–Cu–Li Alloys

The Laser Beam Welding (LBW) of aluminum alloys has attracted significant interest from industrial sectors, including the shipbuilding, automotive and aeronautics industries, as it expects to contribute to significant cost reduction associated with the production of high-quality welds. To comprehend the behavior of welded structures in regard to their damage tolerance, the application of fracture mechanics serves as the instrumental tool. However, the methods employed overlook the changes in the microstructure within the Heat-Affected Zone (HAZ), which leads to the degradation of the mechanical properties of the material. The purpose of this study is to simulate microhardness evolution in the HAZ of AA2198-T351 LBW. The material represents the latest generation of Al-Cu-Li alloys, which exhibit improved mechanical properties, enhanced damage tolerance behavior, lower density and better corrosion and fatigue crack growth resistance than conventional Al-Cu alloys. In this work, the microhardness profile of LBW AA2198 was measured, and subsequently, through isothermal heat treatments on samples, the microhardness values of the HAZ were replicated. The conditions of the heat treatments (T, t) were selected in line with the thermal cycles that each area of the HAZ experienced during welding. ThermoCalc and DICTRA were employed in order to identify the strengthening precipitates and their evolution (dissolution and coarsening) during the weld thermal cycle. The microstructure of the heat-treated samples was studied employing LOM and TEM, and the strengthening precipitates and their characteristics (volume fraction and size) were defined and correlated to the calculations and the experimental conditions employed during welding. The main conclusion of this study is that it is feasible to imitate the microstructure evolution within the HAZ through the implementation of isothermal heat treatments. This implies that it is possible to fabricate samples for fatigue crack growth tests, enabling the experimental examination of the damage tolerance behavior in welded structures.

Microstructure and Mechanical Properties of TiC/WC-Reinforced AlCoCrFeNi High-Entropy Alloys Prepared by Laser Cladding

By employing the technology of laser cladding, AlCoCrFeNi–TiC20−x/WCx high-entropy alloy coatings (where x = 0, 5, 10, 15, and 20 is the mass fraction) were fabricated on 316L stainless steel (316Lss). The effects of changes in different mass fractions on the morphology, phase composition, microstructure, microhardness, and corrosion resistance of the composite coatings were studied. This demonstrates that the addition of TiC and WC powder produces an FCC phase in the original BCC phase, the morphology and size of the coatings from top to bottom undergo some changes with x, and the grain size evolution follows a cooling rate law. The evolution of microhardness and corrosion resistance of the coatings exhibit a trend of increasing first and then decreasing with an increase in x. The coatings exhibited their best microhardness and corrosion resistance when x = 15, and their corrosion resistance and microhardness were much better than those of the substrate.

Investigation on microstructural and microhardness evolution of GH4698 superalloy under transiently varying strain rates

The high-temperature deformation behavior of GH4698 superalloy was explored via isothermal compressive tests under transiently varying strain rates. The impacts of loading types and deformation temperature on flow stress, microstructural evolution, and microhardness were explored, and the underlying nucleation mechanisms for dynamic recrystallization (DRX) were elucidated. Experimental results indicated that the flow stress decreased when the strain rate decreased transiently. Specifically, under loading type I (interrupted strain = 0.11), the flow stress was higher than that under loading type II (interrupted strain = 0.36). Moreover, the flow stress decreased with increase in deformation temperature. When the initial interrupted strain was high, a high degree of DRX was observed in the deformed microstructure. The DRX fraction of types I and II are 48 % and 61 %, respectively. Higher deformation temperatures accelerated the growth of DRX grains, and at a deformation temperature of 1100 °C, DRX fraction reaches 87 %. The microhardness of the DRX grains was higher than that of the deformed grains. At higher deformation temperatures, the γ′ phase dissolved, and the DRX grains grew, resulting in smaller microhardness of the DRX grains than that at lower deformation temperatures. However, microhardness was not sensitive to the loading type (i.e., the value of interrupted strain). Furthermore, discontinuous dynamic recrystallization (DDRX), activated by the initial bulging of grain boundaries, served as the fundamental mechanism for DRX nucleation. Similarly, continuous dynamic recrystallization (CDRX), characterized by the formation of subgrains through dislocation cell structures within individual grains, as well as the γ′ phase and twin boundaries, also played crucial roles in the nucleation of DRX during transiently variation states conditions.

Unraveling microforging principle during in situ shot-peening-assisted cold spray additive manufacturing aluminum alloy through a multi-physics framework

Cold spray (CS) is a highly potential solid-state additive manufacturing (AM) technique. In situ shot-peening-assisted CSAM was proposed to additively manufacture fully dense deposits using cost-effective and renewable nitrogen gas. The role of in situ shot-peening particles is critical but remains unclear. Here, the process was quantitatively modeled to visualize the dynamic deformation, energy conversion, as well as cell/sub-grain size and microhardness evolutions, compared to those during the conventional CSAM process, identifying the key role of in situ shot-peening particles in the AA6061 extreme deformation and microstructure characteristics during in situ shot-peening-assisted CSAM. High-fidelity modeling was verified fully by comparing the experimental and model-reproduced deformation profiles, cell/sub-grain size distributions, and increases in microhardness. The results show that the kinetic energy of in situ shot-peening particles was 470 times higher and dissipated mainly through AA6061 plastic deformation (86.36% of total energy), leading to significant enhancement of microhardness and tensile strength. Moreover, the mixing ratio of large-size SS410 particles required to create a fully dense deposit was evaluated from an energy perspective, in good agreement with the experiment. This study elucidates the microforging principle during in situ shot-peening-assisted CSAM, providing scientific guidelines for high-quality and low-cost CSAM of high-strength aluminum alloys.

Evolution of microstructure, microhardness, crystallographic texture, and grain size variation in dissimilar joints used in concentrated solar power plants application

High carbon stainless steel 347H (SS347H) and Inconel 625 (IN625) are high-strength alloys, commonly used in high-temperature applications. These metals are commonly used in the fabrication of concentrated solar power energy storage tanks and pipelines. Construction of such tanks involves dissimilar welding as the primary manufacturing process. The critical factor to consider when joining two different materials is the development of microstructure, as it directly impacts the mechanical characteristics of the joints. According to the available literature, an increase in the heat input (HI) promotes alloying element segregation and susceptibility to weld cracking. Traditional arc welding processes, such as metal inert gas (MIG) welding, produce a higher HI. Therefore, there is a need to develop welding techniques that result in reduced HI. In the present study, a pulse-current MIG (PC-MIG) welding method was employed, utilizing current pulsing to minimize HI during the welding process. The welding was followed by metallographic and mechanical characterization. The microstructural examination found variations in microstructure at different regions. Electron backscatter diffraction analysis shows the crystallographic orientation at different regions. Analysis shows that both SS347H and IN625 have a face-centred cubic austenite structure. The inverse pole figure discloses strong texture formation. The average tensile strength of dissimilar weldments was found to be 479 MPa, while the average hardness value of the fusion zone was measured to be 235 HV. The research indicates that the PC-MIG technique is suitable for joining dissimilar materials with reduced HI, effectively eliminating the risk of cracking and enhancing the efficiency of the joint.

Evolution of structural-phase state and properties of hypereutectoid steel rails at long-term operation

The methods of modern physical materials science were used to analyze the evolution of microhardness, tribological properties, dislocation substructure and phase composition of the rails with increased wear resistance and contact endurance of DT 400 IR category after missed tonnage of 187 million gross tons on the experimental ring of Russian Railways. It is shown that extremely long-term operation of the rails is accompanied by a decrease (3.1 times) in wear parameter of the rolling surface and an increase (1.4 times) in microhardness, scalar dislocation density (1.5 times) and Fe3C carbide content (1.24 times). Operation of the rails led to a decrease in the crystal lattice parameter, which correlates with an increase in the content of iron carbide. We made the assumptions about physical causes of the change in parameters.

Mechanical behavior of bilayer Ti6Al4V composite under quasi-static and dynamic compression

Bilayer structure and homogeneous structure of Ti6Al4V (TC4) alloy were fabricated by High energy ball milling (HEBM) and Fast Hot-Press Sintering (FHPS). The flake TC4 (F-TC4) powders were obtained from spherical TC4 (S-TC4) powders by HEBM, and the bilayer F/S-TC4 composite was prepared from F-TC4 and S-TC4 powders. The microstructure and micro-hardness evolution along the interface of the bilayer composite were investigated. It was found that the hardness decreased gradually from the F-TC4 side to the S-TC4 side. Moreover, an obvious morphology difference in the interface was observed, typical Widmanstatten structure on the S-TC4 side while an equiaxed structure on the F-TC4 side. In addition, the mechanical properties of the alloy and composite with different structures were studied under quasi-static and dynamic compression. The results revealed that the bilayer F/S-TC4 composite showed great strength improvement with little plasticity sacrifice as compared to traditional S-TC4 alloy.

Evolution of microstructure, microhardness and surface roughness of Al alloy thin-walled tube during flaring process

Flaring forming has been widely utilized in connecting tubes, fluid pipe circuit components and flight fairings. Surface quality of the flared inner wall significantly affects the connection properties and service reliability. However, the surface microstructure, hardness and roughness of flared thin-walled tubes have long lacked corresponding characterization and correlation analysis. In the present study, wear mechanism of a 5052 Al alloy flared tube is analyzed based on multi-scale characterization. Furthermore, the impact of flaring process parameters including spindle speed, feed rate and holding time on the surface roughness is investigated by considering the evolution of microstructure and microhardness. The results reveal that adhesive wear is the main wear mechanism in the flared tube, accompanied by a small amount of abrasive wear. The effect of process parameters on dynamic recrystallization (DRX) and grain orientation are also analyzed in detail. Based on ANOVA analysis, the spindle speed is found as the most crucial controllable parameter affecting the surface roughness. The feed rate is the second important parameter, while the holding time is found to be insignificant. Simulation and experimental results indicate that with the increase of strain, surface roughness first increases and then decreases. This non-monotonic trend is associated with the evolution of strain (strain rate, strain path, etc.) and microstructure (DRX, texture, hardening, etc.).

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.

Local Microstructure and Texture Development during Friction Stir Spot of 5182 Aluminum Alloy

The local microstructure, texture gradient and mechanical properties through the shoulder dimension (10 mm) of upper and lower AA5182 aluminum sheets were investigated using electron backscatter diffraction (EBSD) and Vickers microhardness after friction stir spot welding (FSSW). Based on the microstructural features (mean grain size, grain boundary type and dynamic recrystallization (DRX)), the upper sheet was found to be mainly composed of the stir zone (SZ) and thermomechanically affected zone (TMAZ) due to the high deformation induced simultaneously by the tool rotation and the shoulder download force, while the SZ, TMAZ, heat-affected zone (HAZ) and base metal (BM) were detected in the lower sheet due to the limited effect of the shoulder on the lower sheet. The texture changes, due to the nature of the deformation, demonstrated a shear-type texture at the SZ to a plane strain compression deformation type texture at the TMAZ and then a recrystallization texture at the HAZ and BM. The microhardness gradually decreased with the increasing distance from the keyhole along the SZ, TMAZ and HAZ regions. Eventually, the microstructure and microhardness evolutions were correlated based on the Hall–Petch relationship.

Microstructural response and mechanical properties of α-precipitated Ti–5Al–5Mo–5V–3Cr alloy processed by high-pressure torsion

The aged Ti–5Al–5Mo–5V–3Cr alloy which is composed of acicular α precipitates and β matrix was processed by high-pressure torsion (HPT). The different flow behaviors of these two phases have a great influence on the grain refinement process. The homogenization of microstructure and microhardness along the radial direction is very slow, which is reached after HPT processing of 100 revolutions. And the α precipitates might enhance the refinement. It is shown that HPT processing produces an ultrafine microstructure with a grain size of less than 50 nm and a high dislocation density of approximately 8 × 1016 m−2. The microhardness evolution as a function of equivalent strain follows a bell-shaped curve, for which the hardness initially increases with strain, reaches a peak value (∼450 Hv) and then decreases to a saturated condition (∼413 Hv). The refined β gains, the high density of dislocations and the α grains with a certain size contributes to the peak hardness. But the strengthening caused by α phase will weaken as increasing the strain.

Effect of ultrasonic vibration on the microstructure and microhardness of laser cladding Fe-based crystalline/amorphous composite coatings

The laser cladding technique with and without ultrasonic vibration was employed to prepare Fe-based crystalline/amorphous composite coatings and the effect of ultrasonic vibration on the evolution of microstructures and microhardness was investigated via SEM observation and microhardness testing. The results show that, during the laser cladding process assisted with ultrasonic vibration, the content of amorphous phase is increased by 1.4 % and the growth of columnar crystals at interface is inhibited. The average length of columnar crystal at interface is measured to be around 17.2 ± 5.1 μm which is diminished by 35.8 %, and the microhardness of coating is about 866.2 ± 57.5 HV0.2 which is improved by 27.9 % compared to that without ultrasonic vibration.

Microstructure and Microhardness Evolution of Additively Manufactured Cellular Inconel 718 after Heat Treatment with Different Aging Times

The manufacture of cellular structures using high-performance materials is possible thanks to the additive manufacturing of metals. However, it is well known that the mechanical and microstructural properties of metals manufactured by this technique do not correspond to those of the same metals manufactured by conventional methods. It is well known that the mechanical properties depend on the direction of manufacture, the size of the pieces, and the type of cell structure used. In addition, the effect of heat treatments on parts manufactured by additive manufacturing differs from parts manufactured by conventional methods. In this work, the microstructure and microhardness of cellular structures of Inconel 718, manufactured by additive manufacturing under heat treatments with different aging times, were evaluated. It was found that the time of the first aging impacts the microhardness and its homogeneity, affecting the microstructure. The highest hardness was obtained for an aging time of 8 h, while the lowest standard deviation was obtained at 10 h. Finally, it is shown that the aging time influences a more homogeneous distribution of the elements and phases.

Microstructure and mechanical properties of cast Al-Ce-Sc-Zr-(Er) alloys strengthened by Al11Ce3 micro-platelets and L12 Al3(Sc,Zr,Er) nano-precipitates

The microstructure and mechanical properties of two die-cast hypoeutectic Al-Ce-based alloys modified with Sc, Zr and Er micro-additions – Al-1.5Ce-0.14Sc-0.03Zr with high Sc content and Al-1.4Ce-0.02Sc-0.06Zr-0.003Er (at%) with low Sc content – are studied, with focus on the two strengthening phases: micron-scale Al11Ce3 platelets formed on solidification and L12 Al3(Sc,Zr,Er) nanoprecipitates formed during aging. The evolution of microstructure and microhardness upon isothermal aging at 350 and 400 °C, and the creep resistance at 300 °C, are studied and compared to alloys containing only one of these strengthening phases. The Al11Ce3 and L12 phases form mostly independently of each other, except in the low-Sc alloy where scavenging of Er and Si by Al11Ce3 on solidification reduces the kinetics of precipitation and number density of the L12 precipitates formed on subsequent aging. The two strengthening phases, while mostly independent of each other chemically, interact synergistically in terms of strengthening. At ambient temperature, the hardness of the Al11Ce3- and L12-strengthened alloys is higher than that of similar alloys containing only one strengthening phase. At 300 °C, the creep strain rate is reduced by as much as five orders of magnitude in the Al11Ce3- and L12-strengthened alloys compared to alloys containing only L12 precipitates. This improvement in both room and elevated-temperature mechanical properties is modeled through a combination of load transfer from the Al11Ce3 micro-platelets, and precipitation strengthening from the Al3(Sc,Zr,Er) nanoprecipitates.

Thermal Stability of the Precipitates in Dilute Al-Er-Zr/Hf Alloys at Elevated Temperature

The temporal evolution of microhardness and Al3(Er,Zr/Hf) precipitates are investigated in Al-Er-Zr/Hf alloys during annealing at 450 °C and 500 °C. The microhardness of the alloys decreases continuously with the prolonged annealing time due to the coarsening of the precipitates. Different weakening amplitudes are observed because of the disparity of the precipitate coarsening rate that is related to the disparity in their intrinsic diffusivities of Er, Zr, and Hf solute atoms in an Al matrix. The addition of Hf element is beneficial to enhancing the coarsening resistance, thus improving the thermal stability of the alloys. Introducing such elements to improve the thermal stability of precipitates can provide a new idea or choice for the development of heat-resistant aluminum alloys.

Grain boundary status dependent mechanical property of annealed nanocrystalline Ni(Fe) alloy

The strength and hardness of nanocrystalline (NC) metals and alloys have been often correlated with their grain size. We have synthesized NC Ni(Fe) alloys by electrodeposition and studied the evolution of microstructure and microhardness with annealing. By minimizing the interval of annealing temperature from previously utilized 50–150 °C to 10 °C, we have found unique step-type changes, instead of previously observed continuous changes, in the microhardness of annealed NC Ni alloys. The microhardness decreases slightly within two wide temperature ranges and largely drop within two narrow temperature ranges (20–40 °C). Our experimental results and analysis suggest that the microhardness of annealed NC Ni(Fe) alloy is dependent not only on the grain size but also largely on the grain boundary status, i.e., the segregation and desegregation of solute and impurity atoms at grain boundaries (GBs), the grain boundary (GB) relaxation, and the GB nanoprecipitation. Our studies also suggest that the change of mechanical property with annealing in NC metals and alloys should be examined within a narrow range of annealing temperature, e.g., 10 °C, to observe and study their unique, or even abnormal, structure–property relations, which may have been neglected in previous studies.