Heat Treatment Strategy Research Articles

Multi-alloying of nanomet: conception and implementation of homogeneous nanocrystallization in high-flux density soft magnetic alloys

This study demonstrates how multi-alloying the Fe-Si–B–P–Cu (Nanomet®) can avoid the strict requirements on the annealing scheme in terms of high heating rate and narrow annealing temperature range in order to grow a homogeneous ultrafine nanocrystalline structure. The rather restricted amorphization capability sets a low limit of the maximum thickness of the amorphous precursor. These shortcomings have their origin in the existence of detrimental pre-existing nuclei in the amorphous precursors, which in turn potentially lead to a heterogeneous crystallization. Here, we have multialloyed Nanomet with CoCNi- and CoCMo- to avoid the creation of these pre-existing nuclei. This leads to improved amorphization capability and changes a potentially heterogeneous crystallization to a homogeneous nanocrystallization over a much broader temperature range than for unalloyed Nanomet. Thus, the requirements for the annealing are much relaxed. This work encompasses quenching the amorphous precursors using melt-spinning, investigating the crystallization temperatures by calorimetry, showing the depletion of pre-existing nuclei by magneto-thermo-gravimetry, conceptualizing the crystallization dynamics using isothermal calorimetry, and finally revealing the excellent soft magnetic properties over a broad annealing temperature interval (390–490 °C for the substituted alloys compared to 410–470 °C for unalloyed Nanomet). The multi-elemental substitution of Fe with CoCMo and CoCNi in Nanomet alloy nearly maintains the saturation magnetization and the coercivity. We believe the substituted alloys provide a better alternative to Nanomet with improved amorphization capability and homogeneous nanocrystallization without any special heat treatment scheme.

Competing nucleation of single- and double-layer Guinier\u2013Preston zones in Al\u2013Cu alloys

Solid-state precipitation is a key heat-treatment strategy for strengthening engineering alloys. Therefore, predicting the precipitation process of localized solute-rich clusters, such as Guinier–Preston (GP) zones, is necessary. We quantitatively evaluated the critical nucleus size and nucleation barrier of GP zones in Al–Cu alloys, illustrating the precipitation preferences of single-layer (GP1) and double-layer (GP2) GP zones. Based on classical nucleation theory using an effective multi-body potential for dilute Al–Cu systems, our model predicted GP1 and GP2 precipitation sequences at various temperatures and Cu concentrations in a manner consistent with experimental observations. The crossover between formation enthalpy curves of GP1 and GP2 with increasing cluster size determines the critical conditions under which GP2 zones can nucleate without prior formation of GP1 zones. This relationship reflects competing interactions within and between clusters. The results illustrate the underlying mechanisms of competing nucleation between zones, and provide guidance for tailoring aging conditions to achieve desired mechanical properties for specific applications.

Effects of Heat Treatments on Microstructures and Mechanical Properties of Ti6Al4V Alloy Produced by Laser Solid Forming

The steep thermal gradient and complicated thermal cycle occur in the fabrication of Ti6Al4V alloy during laser solid forming (LSF). That leads to obvious anisotropic mechanical properties and requires essential heat treatments to improve its performance. In this work, different heat treatment strategies under vacuum condition were used to study the evolution of microstructures and mechanical properties of Ti6Al4V alloy produced by LSF. The results show that transformation from α phase into lamellar α + β dual-phases structure is introduced at low temperature (550 °C), and the α phase is broken and refined at higher temperature (800 °C). Tensile tests present the increase of elongations in the horizontal and vertical directions by 12.4% and 13.2% for specimens treated by two-step heat treatment (750 °C × 4 h + 500 °C × 1 h). Fatigue crack growth (FCG) lives of LSFed Ti6Al4V alloy after different heat treatments were improved due to the elimination of residual tensile stress and the transformation of α phase into α + β dual-phase structure. Specimens treated at 800 °C for 4 h exhibit a higher fatigue life among those heat-treated alloys. The low sensitivities of the FCG behavior in the Paris-zone to different heat treatments under vacuum condition are explored in the FCG testing of Ti6Al4V alloy.

Dual‐Active Sites Engineering of N‐Doped Hollow Carbon Nanocubes Confining Bimetal Alloys as Bifunctional Oxygen Electrocatalysts for Flexible Metal–Air Batteries

Since the sluggish kinetic process of oxygen reduction (ORR)/evolution (OER) reactions, the design of highly-efficient, robust, and cost-effective catalysts for flexible metal-air batteries is desired but challenging. Herein, bimetallic nanoparticles encapsulated in the N-doped hollow carbon nanocubes (e.g., FeCo-NPs/NC, FeNi-NPs/NC, and CoNi-NPs/NC) are rationally designed via a general heat-treatment strategy of introducing NH3 pyrolysis of dopamine-coated metal-organic frameworks. Impressively, the resultant FeCo-NPs/NC hybrid exhibits superior bifunctional electrocatalytic performance for ORR/OER, manifesting exceptional discharging performance, outstanding lifespan, and prime flexibility for both Zn/Al-air batteries, superior to those of state-of-the-art Pt/C and RuO2 catalysts. X-ray absorption near edge structure and density functional theory indicate that the strong synergy between FeCo alloy and N-doped carbon frameworks has a distinctive activation effect on bimetallic Fe/Co atoms to synchronously modify the electronic structure and afford abundant dual-active Fe/Co-Nx sites, large surface area, high nitrogen doping level, and conductive carbon frameworks to boost the reversible oxygen electrocatalysis. Such N-doped carbon with bimetallic alloy bonds provides new pathways for the rational creation of high-efficiency energy conversion and storage equipment.

Investigation of microstructural attributes of steel surfaces through magneto-optical Kerr effect

Magneto-optical Kerr effect (MOKE) microscopy is a versatile technique to image magnetic domains and evaluate the behavior of magnetic materials. However, MOKE has been rarely applied to the research of low magnetic steels with complex microstructure including micron-sized features. This study reveals the potential of implementing MOKE microscopy for analyzing the microstructure and stress state of martensitic steels with high carbide density (example conducted on AISI M2 steel grade). For the first time, the resulting MOKE domain images are used to analyze the size and morphology of martensite lath structures without chemical etching of the material's surface. Furthermore, MOKE microscopy allowed the confirmation and volumetric fraction estimation of retained austenite, despite the small size and volumetric fraction of under 1%. The validity of the MOKE microscopy analysis is cross-checked with scanning electron microscopy of etched samples and X-ray diffraction. Finally, MOKE microscopy in combination with Vickers indentation is applied to determine the residual stress state of the selected steel, which was varied by processing the steel material with conventional heat treatment scheme and heat treatment including deep cryogenic treatment (DCT). The method is proven effective in indicating the residual stress qualitatively. The measurements proved, that the residual stresses are significantly reduced in DCT steel samples, as theoretically anticipated. The conducted pioneering work provides important research towards the application of MOKE microscopy for the research of steel microstructure, properties and residual stresses as well as delivers a new perspective for determining the effectiveness of DCT on the steel properties.

Synergistic alloy design concept for new high-strength Al–Mg–Si thick plate alloys

With the aim of fully exploiting the advantageous strength-to-weight ratio evident in Al–Mg–Si alloys, this study presents measures for increasing the yield strength of an EN AW-6082 type plate alloy. In addition to describing the thermodynamic simulation-based adjustment of age-hardenable elements (Si, Mg and Cu) and a modified artificial ageing treatment, it investigates the effects of adding a small amount of Zr. The significant strengthening induced by adding Zr is correlated with sub-grain boundary hardening in a recovered microstructure after solution annealing at 570 °C, compared with the almost entirely recrystallized microstructure in an unmodified EN AW-6082 alloy. In combination with a maximum dissolvable number of age-hardenable elements and interrupted quenching, which comprises an improved heat treatment strategy for thick plates, it is seen that the yield strength can be increased by more than 40% to 411 MPa compared to conventional EN AW-6082 base material as verified by tensile testing. In the study scanning electron microscopy and scanning transmission electron microscopy were performed for microstructural characterization with a focus on particle and deformation analysis. All individual contributions which generated the superior strength are calculated and discussed in order to reveal the microstructure-property relationship.

Designing efficient spin Seebeck-based thermoelectric devices via simultaneous optimization of bulk and interface properties

We describe two strategies to enhance the thermoelectric conversion efficiency of spin Seebeck devices and demonstrate a simultaneous realization of them via a simple heat treatment scheme.

Efficient antibacterial activity of hydroxyapatite through ROS generation motivated by trace Mn(iii) coupled H vacancies.

Hydroxyapatite (HA) has attracted wide attention for medical application due to its biocompatibility and bioactivity. However, the infection problems of HA remain among the leading reasons for implantation failure. Thus, it is urgent to endow HA biomaterials with antibacterial activity. Herein, the high antibacterial activity was achieved by introducing trace Mn3+ and H vacancy couples in HA through a facile heat-treatment strategy in air. The theoretical results indicated that Mn3+ was preferentially substituted for the Ca(2) site in the HA structure with a charge-compensating H vacancy appearing at the adjacent OH- site. The antibacterial tests showed that Mn-HA possessed antibacterial activity towards both E. coli and S. aureus with trace Mn content at the ppm level, and implied that Mn3+ and centers may play an important role in the antibacterial process. The Mn3+ and couples in Mn-HA, serving as oxidative and reductive centers respectively, could then collectively participate in the CoQ/CoQH2 redox cycling and synergistically facilitate the accumulation of CoQ˙- and ROS radicals. This enhanced ROS production was the main factor to endow Mn-HA with efficient antibacterial activity. Moreover, the in vitro bioactivity assay showed that Mn-HA materials exhibited enhanced osteogenic activity and good biocompatibility. Therefore, this work not only provides a feasible method to control the oxidation state of Mn elements in HA, but also proposes a novel trace Mn3+-doped HA for potential applications in tissue engineering.

3D N-doped carbon continuous network supported P-doped ZnCo2O4 nanosheets with rich oxygen vacancies for high-performance asymmetric pseudocapacitor

N-doped carbon supported MOFs derived P-doped ZnCo2O4 nanosheets (ZCP/NC) with rich oxygen vacancy are rationally designed and synthesized via a chemical heat treatment strategy. In the unique structure, N-doped carbon as a skeleton can provide an electrically conductive 3D continuous network with large surface area for the growth of active materials. P-doped ZnCo2O4 nanosheets are rich in oxygen vacancies and have a large exposure area, which can enhance the charge storage capabilities. Electrochemical measurement displayed that the novel electrodes can deliver a highest specific capacitance of 1581.5 F g−1 at 1 A g−1. An asymmetric supercapacitor (ASC) fabricated by ZCP/NC and activated carbon can reach a high energy density of 47.8 Wh kg−1 at power density of 800 W kg−1. Besides, The ASC also exhibits a cyclic retention of 90.6% for 5000 cycles. The reasons for excellent properties of ZCP/NC are discussed in detail.

Research on influence of heat treatment scheme of Ti10V2Fe3Al alloy on technological surface integrity after electrodischarge machining

The paper focus on the importance of heat treatment procedure of Ti10V2Fe3A alloy for the removal process during EDM. The goal of the research was to analyse the influence of Ti10V2Fe3Al alloy microstructure on the surface integrity after electrodischarge machining (EDM). By varying the time and temperature of the heat treatment operation the samples with various morphologies and a volume fraction of the α phase have been obtained. Then, all the samples were electrodischarge machined in sinking kinematics. The results analysis focuses on the microstructural changes, microhardness and geometrical properties of the machined surface. In the surface layer cross-section typical zones differing in chemical composition, microstructure and microhardness between samples with different initial morphologies and a volume fraction of the α phase were distinguished. Also significant differences in 3D surface roughness parameters such as Sa, Sz, Sq, Sk, obtained for each investigated sample after EDM were detected. Observed changes in surface layer corresponds to the temperature of heat treatment and annealing time and the resulting volume fraction of the α phase.

Synergistic Alloy Design Concept for New High-Strength Al-Mg-Si Thick Plate Alloys

With the aim of fully exploiting the advantageous strength-to-weight ratio evident in Al-Mg-Si alloys, this study presents measures for increasing the yield strength of an EN AW-6082 type plate alloy. In addition to describing the thermodynamic simulation-based adjustment of age-hardenable elements (Si, Mg and Cu) and a modified artificial ageing treatment, it investigates the effects of adding a small amount of Zr. The significant strengthening induced by adding Zr is correlated with sub-grain boundary hardening in a recovered microstructure after solution annealing at 570°C, compared with the almost entirely recrystallized microstructure in an unmodified EN AW-6082 alloy. In combination with a maximum dissolvable number of age-hardenable elements and interrupted quenching, which comprises an improved heat treatment strategy for thick plates, it is seen that the yield strength can be increased by more than 40% to 411 MPa compared to conventional EN AW-6082 base material as verified by tensile testing. In the study scanning electron microscopy and scanning transmission electron microscopy were performed for microstructural characterization with a focus on particle and deformation analysis. All individual contributions which generated the superior strength are calculated and discussed in order to reveal the microstructure-property relationship.

Mechanical properties and formability of EN AW-7075 in cold forming processes

Due to a low density and high tensile strength, the aluminum alloy EN AW 7075 T6 offers a high lightweight potential for structural components. Since its formability is limited at room temperature in the T6 temper state, the potential of this alloy for automotive bodies is only utilizable by adapted deep drawing processes. In recent years, process chains suited for warm and hot forming have been researched and developed. However, warm and hot forming solutions require additional process steps and a complex tooling system in comparison to cold forming processes. Alternatively, the forming of such blanks at room temperature in the W temper state is favorable since conventional tools can be used. The W temper state is a heat treatment condition achieved after solution heat treatment and subsequent quenching, which is characterized by an increased ductility. However, this condition is unstable, due to the onset of natural ageing. With increasing time after the quenching step, the strength of the material increases, which leads to a reduction of formability. Another phenomenon that occurs after quenching is the Portevin Le-Chatelier effect. This effect causes the formation of flow lines during cold forming and results in a decrease of ductility. Hence, the objective of the investigations was to determine the formability of EN AW 7075 as a function of the natural ageing time after solution heat treatment and quenching. Therefore, tensile tests of various aged samples were carried out. The results show a relation of the formability to the natural ageing time and a dependency on the quenching rate. Furthermore, a heat treatment strategy for EN AW-7075 was developed, that considers manufacturing processes like the cathodic dip coating. The influence of the quenching rate, ageing time and temperature as well as the influence of temperature of the paint baking process after the cathodic dip coating were considered. Therefore, a design of experiments and tensile tests were carried out. Thus, the deep drawing of EN AW-7075 at room temperature is particularly promoted.

The effect of homogenization temperature on the microstructure and high temperature mechanical performance of SLM-fabricated IN718 alloy

Microstructure, mechanical performances at elevated temperature (650 °C) and their correlation of Inconel 718 manufactured by Selective Laser Melting (SLM) were investigated in this paper. Four kinds of heat treatment schemes including homogenization, solution treatment and conventional aging process were carried out to regulate the microstructure in order to obtain the optimum comprehensive mechanical performances at high temperature. The dimension and morphology of grains, subgrains, different precipitates including δ phase, strengthening phase, carbides and nitrides were investigated. In addition, their evolution mechanism was also analyzed in detail. Furthermore, for figuring out the effects of microstructures on the mechanical performance, creep rupture test and tensile test were carried out to compare the performance differences of heat-treated samples. The results demonstrated that the heat-treated samples show the better creep rupture performance and higher tensile strength at elevated temperature than that of the wrought. In addition, the sample under homogenization heat treatment in 1080 °C, solution treatment in 980 °C and conventional aging treatment shows the optimum mechanical properties at 650 °C.

Wire-arc additive manufacturing of a novel high-performance Al-Zn-Mg-Cu alloy: Processing, characterization and feasibility demonstration

Wire-arc additive manufacturing (WAAM) is a feasible technology for manufacturing of metallic components of medium complexity with a high deposition rate. Manufacturing of aluminum alloys for e.g. structural components of the aero plane fuselage by this technology has been impeded by the unavailability of high-performance alloys with good processability and low susceptibility to hot cracking. Therefore, a novel alloy system has been developed based on the Al-Zn-Mg-Cu system and successfully processed by WAAM without the occurrence of any hot cracks. Heat treatment strategies were developed allowing for optimum mechanical properties. A homogeneous grain structure was observed with few elongated grains. Upon heat treatment, the formation of T phases was verified with a precipitate size in the range of ~10 nm. These are responsible for the observed pronounced age-hardening response of this alloy. An isotropic proof stress of up to ~340 MPa and a fracture strain of up to ~11% are evidence of the high quality of the WAAM deposited material allowing for an extended use of these alloys for advanced applications. Finally, the applicability and processability of the alloy was proven by fabrication of a motorcycle piston.

Effects of double heat treatment of NiO hole transport layer on the performance of QLEDs

Parameters affecting the performance of QLED devices using NiO hole transport layer (HTL) thin film via the sol–gel process route were investigated. In the preparation of the NiO HTL thin film, a double heat treatment scheme, crystallization heat treatment at elevated temperatures followed by vacuum annealing treatment at lower temperatures, was introduced to form a crystalline NiO single phase as well as surface phases with reduced defect concentration. The decoupling of crystallization and vacuum annealing treatment of NiO HTL thin film was effective in enhancing the performance of the QLEDs. Also, the insertion of an interlayer between QD EML and NiO HTL was very effective in enhancing the performance of the device. With the modification of processing conditions, the luminance of QLED with the NiO HTL was improved from 30 to 11500 cd/m2 and the current efficiency from 0.25 to 23 cd/A, respectively.

Investigation of Mechanical Properties of Large Shape Memory Alloy Bars under Different Heat Treatments.

Shape memory alloys (SMAs) are a class of functional materials that possess unique thermomechanical properties, such as shape memory effect (SME), superelasticity (SE), damping, and good fatigue and corrosion resistance, which enable them to become ideal materials for applications in earthquake engineering. Numerous studies have shown that the mechanical properties of superelastic SMAs mainly depend on the wire form, or the relationship between the microstructure and thermally induced phase transitions. However, extremely few studies have elucidated the effects of the heat-treatment strategy, size effect of large diameters, and cyclic loading. Herein, the mechanical properties of SMA bars, such as residual strain, energy dissipation, and equivalent damping ratio, were studied with different heat-treatment strategies, cyclic loadings, and strain amplitudes; this was achieved by conducting cyclic tensile tests on SMA bars with four different diameters. The results indicate that the maximum phase transformation stress was obtained with a 14 mm SMA bar subjected to heat treatment at 400 ℃ for 15 min. The mechanical properties were relatively stable after five loading–unloading cycles, which should be considered in engineering applications. The test results provide a mechanical basis for using large SMA bars in self-centering structures in seismic regions.

Antibacterial Effect of the Controlled Nanoscale Precipitates Obtained by Different Heat Treatment Schemes with a Ti-Based Nanomaterial, Ti-7.5Mo-5Cu Alloy.

It is well known that copper is an excellent option for a Ti-based alloy component as a β-stabilizer that provides improved biocompatibility and antibacterial ability. The development of a Ti-based nanomaterial containing Cu is a promising strategy for addressing implant-associated infections (OII). However, the antibacterial mechanism of copper-related alloys is still unknown. There are two popular hypotheses: copper ion release sterilization and alloy contact sterilization. The main mechanism of contact sterilization may be Cu-related phase (Ti2Cu) precipitation. Because excess copper can lead to cytotoxicity and reduce the β-Ti phase content, molybdenum needs to be added to the alloy given its well-known and widely researched β-stabilizer characteristics, which can provide satisfactory mechanical properties, wear resistance, and biocompatibility. Our study created a Ti-based nanomaterial, Ti-7.5Mo-5Cu, and performed two kinds of heat treatment schemes at different solution temperatures: 750 and 900 °C. The above schemes resulted in homogeneous and heterogeneous nucleation on the precipitation behavior of the Ti2Cu crystal phase, which controlled its amount, distribution, and size. Finally, our results showed that Ti-7.5Mo-5Cu, especially at 900 °C, possessed excellent antibacterial ability, corrosion resistance, cytocompatibility, and induced osteogenic differentiation, indicating its potential for use as a biomedical antibacterial alloy in the future.

Preparation of soft magnetic composite from Fe-6.9wt%Si by different heat treatment strategies.

Present study investigated the effect of isothermal heat treatment strategies between 800 °C and 1150 °C on the magnetic properties of toroidal samples made from Fe-6.9wt%Si powder. The samples were prepared by classical powder metallurgy method since the classical sheet forming methods no longer work with the high silicon content. Our results presented here are part of a series of comparative experiments where we study the effectiveness of the insulating layers created during and before the compacting of soft magnetic composites (SMCs). Our goal was to create a soft magnetic composite made of ferromagnetic and inorganic insulating material with a frequency limit already in the megahertz range and a Snoek limit of few gigahertz. In the case of samples made from Fe-6.9wt%Si powder, the computed tomography results showed that significant porosity is to be expected after pressing. Its positive effect occurred during the heat treatment in the atmospheric agent, where silicon is precipitated and deposited on the surface of the particle. This coating is an electrically insulating layer at the grain boundaries. Depending on the heat treatment strategy, 1 or 2 ferromagnetic phases were observed. The frequency limit approached the target values, but due to the low value of static permeability, the Snoek limit did not reach the gigahertz range. However, there is a significant improvement in magnetic properties compared to the heat-treated samples in a protective gas.

The effect of subsequent heat treatment on the evolution behavior of second phase particles and mechanical properties of the Inconel 718 superalloy manufactured by selective laser melting

In this work, the influence of subsequent heat treatment on the evolution behavior of Laves phase, δ phase and carbides, and mechanical properties were investigated systematically for the Inconel 718 superalloy manufactured by selective laser melting (SLM). Two different subsequent heat treatment schemes were employed for SLM manufactured Inconel 718 superalloy in our work: 980 °C * 1 h/AC + 720 °C * 8 h/FC to 620 °C * 8 h/AC (the standard heat treatment scheme, SA980) and 1080 °C * 1 h/AC + 720 °C * 8 h/FC to 620 °C * 8 h/AC (the optimized heat treatment scheme, SA1080). The results show that the δ phase can nucleate around the residual Laves phase, and then the composite phase particles are formed at the subgrain boundaries during SA980 heat treatment of SLM manufactured Inconel 718. Moreover, the entangled dislocations around these composite phase particles can promote the initiation of microcrack during the tensile process at elevated temperature, leading to premature failure. On the other hand, two types of carbides were found to exist in the SA1080 heat-treated samples: the nanoscale TiC particles along subgrain boundaries originally formed during the SLM process, and the large-size NbC particles along the grain boundaries formed during the solution heat treatment process. The heat treatment schemes of SA1080 can achieve excellent combination of strength and elongation at room temperature and elevated temperature, which is more suitable for the subsequent heat treatment of SLM manufactured IN718.

Tailored bainitic-martensitic microstructures by means of inductive surface hardening for AISI4140

Inductive surface hardening processes are widely used in the manufacturing of automotive parts. They combine short process times with high economic and energy efficiency. Because of their high surface hardness, induction hardened steel parts feature beneficial wear resistance and also good fatigue properties due to the induced compressive residual stresses in the surface. Considering the positive effects on the mechanical properties by creating a bainitic-martensitic microstructure, outlined by severel authors, this study includes the implementation of a temperature controlled inductive surface hardening process on a conventional hardening machine. Therefore an innovative inductor design, which allows a controlled sample cooling is presented. Supported by dilatometric studies as well as different heat treatment strategies, limitations are identified to gain a profund process understanding. Hereby an accelerated bainite formation after short time austenitization is detected. By using different heat treatment strategies, the fraction of the bainitic phase can be adjusted in the surface of the part made of AISI 4140. The results of this innovative heat treatment method are compared to short time inductive hardening and tempering processes regarding microstructure, hardness and residual stresses. The generated surface layer states, including mixed microstructures, show promising properties resulting in a possible enhancement of the fatigue strength of induction hardened parts.