Desired Microstructure Research Articles

Influence of niobium and silver on mechanical properties and shape memory behavior of Cu-12Al-4Mn alloys

Abstract The advantages of Cu-based shape memory alloys (SMAs) over the more popular Ni-Ti alloys are their low cost and easy processability coupled with good electrical and thermal conductivity. However, these alloys suffer from severe brittleness due to highly ordered structure causing difficulty in cold working; consequently, their applicability in the form of wires and sheets is impeded. In an attempt to improve the ductility of these alloys, varying amounts of Nb and Ag were added to Cu-12Al-4Mn, a known SMA, and the effect of these additives on the shape memory properties as well as the microstructure through grain refinement were analyzed. Detailed microstructural observations indicated that the addition of more than 2 wt% of Ag or Nb was detrimental to the martensitic structure that was formed on quenching. Furthermore, the addition of Nb led to the formation of a more desirable martensitic microstructure, namely the fine β1’ phase, whereas the inclusion of Ag caused the formation of the coarse γ′1 phase. Based on the observed strength and shape memory effect, it was concluded that Nb significantly improved the mechanical as well as the shape memory properties of the base alloy.

Designing of a thin Mo diffusion barrier towards strong and reliable Ti/Cu dissimilar brazing

The dissimilar brazing of Ti and Cu has been recognized to be challenging owing to the affinity of forming brittle intermetallic compounds at joint interface. In the current investigation, it was demonstrated that such detrimental interfacial reaction products can be readily circumvented by employing a Mo diffusion barrier deposited on Ti substrate. Combining Mo interlayer and a Ag–Cu–Ti active filler alloy, intermetallic compounds free Ti/Cu brazed joints consisting of Ti–Mo solid solution, Mo interlayer and remnant braze can be obtained. As a result of the eradication of brittle phases, the resultant joints exhibit excellent structural integrity with bonding strength comparable to the parent material properties. Additionally, as a consequence of sluggish solid state diffusion at the Ti/Mo interface and limited dissolution of the Mo interlayer by filler alloy during brazing, the desirable joint microstructure can be reproduced within a wide range of bonding duration. Such results indicate that strong and reliable Ti/Cu dissimilar brazing can be achieved by the combination of a Mo diffusion barrier and Ag–Cu–Ti active filler alloy.

Effect of the Microsegregation on Martensitic and Bainitic Reactions in a High Carbon-High Silicon Cast Steel

Casting processes show some weaknesses. A particular problem is presented when the workpiece needs to be subjected to heat treatments to achieve a desired microstructure. This problem arises from the microsegregation phenomena typically present in cast parts. The effect of the microsegregation on the martensitic and bainitic transformations has been investigated in a high carbon-high silicon cast steel, with the approximate composition Fe-0.8C-2Si-1Mn-1Cr (in wt. %), which was poured into 25 mm keel block-shaped sand molds. The microsegregation maps of Cr, Si, and Mn characterized by electron probe microanalysis (EPMA) show that interdendritic regions are enriched while dendrites are impoverished in these elements, implying that their partition coefficients are lower that the unity (k < 1). As-quenched martensitic and austempered bainitic microstructures (at 230 °C) were obtained and analyzed after applying an austenitization heat treatment at 920 °C (holding for 60 min). The thermal etching method used to reveal the prior austenite grain size showed a bimodal grain size distribution, with larger grains in the dendritic regions (≈22.4 µm) than in the interdendritic ones (≈6.4 µm). This is likely due to both the microsegregation and the presence of small undissolved cementite precipitates. Electron Backscatter Diffraction (EBSD) analysis carried out on the martensitic microstructure do not unveil any differences in misorientation distribution frequency and block size between the dendritic and interdendritic zones related to the microsegregation and bimodality of the austenite grain size. On the contrary, the bainitic transformation starts earlier (incubation time of 80 min), proceeds faster and bainitic ferrite plates are longer in the dendritic zones, were prior austenite grains are larger and impoverish in solute. The presence of these microsegregation pattern leads to the non-uniform development of the bainitic reaction in cast parts, modifying its kinetics and the resulting microstructures, which would probably have a major impact on the mechanical properties.

The role of microstructure and strain rate on the mechanical behavior of Ti-6Al-4V produced by powder metallurgy

Hydrogen sintering and phase transformation (HSPT), a powder metallurgy process, has been previously shown to produce desirable microstructures in Ti-6Al-4V components with corresponding excellent static and fatigue tensile properties. In the current study, three microstructures produced by HSPT and one produced by traditional wrought processing were tested under compression at strain rates of 0.001, 0.1, and > 1000 s−1. The wrought condition exhibited the best toughness at 0.001 s−1, but had the worst performance at both 0.1 and > 1000 s−1 strain rates. The HSPT condition with a globularized microstructure performed the best at 1000 s−1. From microstructural analyses, it is shown that the wrought condition had strong macroscopic texture consistent with hot rolling above the β-transus temperature. It is proposed that this macrotexture made the wrought material susceptible to relatively early-onset shear banding at all but the lowest strain rate. Being a powder metallurgical process with no mechanical working, this macrotexture does not form during the HSPT process.

Post heat treatment of additive manufactured AlSi10Mg: On silicon morphology, texture and small-scale properties

Post-fabrication heat treatment, including solution treatment (at 520 °C for 1 h) followed by water quenching (WQ), air cooling (AC) and furnace cooling (FC), was performed on a selective laser melted AlSi10Mg alloy. The objective is to assess the effect of various cooling rates on the microstructure (specifically eutectic-Si morphology) and small-scale mechanical properties, measured by employing a depth-sensing nanoindentation platform, of the selective laser melted AlSi10Mg alloy. Results show extensive evolutions in the microstructure and the mechanical properties of the heat-treated materials relative to the as-fabricated sample. Upon solutionizing treatment, the eutectic Si is first fragmented, then spheroidized, and finally coarsened when cooled with slow rates. The microstructural evolution directly affects the mechanical properties, where the as-fabricated and the FC are the hardest and the softest structures, respectively. This is directly attributed to the size and morphology of the eutectic-Si within the microstructure. The findings of this study could help to adjust the optimized heat-treatment process to fabricate SLM AlSi10Mg parts with desirable microstructure and mechanical properties.

Predicting structure zone diagrams for thin film synthesis by generative machine learning

Thin films are ubiquitous in modern technology and highly useful in materials discovery and design. For achieving optimal extrinsic properties, their microstructure needs to be controlled in a multi-parameter space, which usually requires too high a number of experiments to map. Here, we propose to master thin film processing microstructure complexity, and to reduce the cost of microstructure design by joining combinatorial experimentation with generative deep learning models to extract synthesis-composition-microstructure relations. A generative machine learning approach using a conditional generative adversarial network predicts structure zone diagrams. We demonstrate that generative models provide a so far unseen level of quality of generated structure zone diagrams that can be applied for the optimization of chemical composition and processing parameters to achieve a desired microstructure.

Numerical modelling and experimental casting of 17%Mn–4%Al–3%Si–0.45%C wt-% TWIP steel via the horizontal single belt casting (HSBC) process

ABSTRACT This paper presents optimum operating parameters for the production of thin strips of 40 mm in width and ∼4 mm in thickness of Fe–17%Mn–4%Al–3%Si–0.45%C wt-% TWIP (TWinning Induced Plasticity) steel using the present version of the Horizontal Single Belt Casting (HSBC) process. A two-dimensional model was developed to examine the flow of molten metal in the HSBC process, primarily focusing on investigating the instabilities/turbulence that arises when molten metal encounters the moving belt. For computational fluid dynamics modelling, three belt speeds were tested, i.e. 0.4, 0.8 and 1.2 (m s−1), against a constant molten metal velocity of 0.8 m s−1 at the nozzle slot outlet. It was observed that the molten metal/air interface fluctuations were appreciably suppressed/reduced when the belt and molten metal velocities approached each other. The fluctuations formed are damped further downstream, and any remaining surface perturbations can be eliminated via hot plastic deformation. An appropriate heat treatment was also designed for the TWIP steel strips, in order to achieve the desired microstructure and mechanical properties.

Interactive optimization of process parameters and coating analysis of laser cladding JG-3 powder

As an advanced additive manufacturing technology, laser cladding has become a research hotspot in the fields of rapid manufacturing and surface modification in recent years. The quality of the cladding layer directly depends on the choice of process parameters. In order to obtain high quality cladding layer, the effects of laser power, scanning speed, and powder feeding rate on the quality of JG-3 iron-based powder cladding layer were studied by interaction orthogonal experiments. Three-dimensional measurement laser microscopy (3D MLM) and variance analysis were used to analyze the results. It was found that the interaction between laser power and powder feeding rate directly affects the optimum process parameters. Laser power 750 W, scanning speed 420 mm/min, and powder feeding rate 6.96 g/min were selected as the optimum process parameters. Scanning electron microscope (SEM), X-ray diffraction (XRD), and Thermo-Calc software were used to analyze the microstructure, phase composition, and solidification process of the coating, and compared with the experimental results. The results show that under the optimum process parameters, a dense crack-free and non-porous coating was obtained. The coating can be divided into three areas: coating zone (CZ), bonding zone (BZ), and heat affected zone (HAZ). The CZ is composed of α-Fe, Cr2B, Fe2B, and Cr23C6 phases. The calculated results obtained from the Thermo-Calc software are in good agreement with the experimental data. It is beneficial to the coating design for a desirable microstructure and mechanical properties.

Multivariable control of ball-milled reactive material composition and structure

In reactive bimetallic compounds such as Ni–Al multilayers, desirable thermo-kinetic properties upon ignition require simultaneously controlled geometric microstructure and material composition. This article establishes fundamental dynamical models of plastic deformation and material diffusion in ball milling processing of particulates from Ni and Al powders, for the purpose of designing and implementing feedback control strategies for process control. The role of heat dissipation from plastic yield and friction slip in affecting compressibility and diffusivity of the material is elucidated. The different sensitivity of compressibility and diffusivity to thermal power is exploited by introducing multivariable control of both bilayer thickness and penetration depth simultaneously, using a real-time computational model as an observer with adaptation informed by infrared measurements of external vial temperature. The proposed control scheme is tested on a laboratory low-energy ball milling system and demonstrated to effectively modulate power intensity and process duration to obtain the desired microstructure and material composition.

Femtosecond laser sintering of silver nanoparticles for conductive thin-film fabrication

Femtosecond laser sintering of metal nanoparticles for fabricating thin metal films/patterns has drawn substantial attention owing to its potential in fabrication of thin-film electrodes in various applications. In such applications, the electrical conductivity and the adhesion strength of the sintered film need to be optimized by forming desired microstructures. However, the physical mechanisms of sintering by ultrashort laser pulses without the effect of inter-pulse heat accumulation, especially microstructure formation, have not yet been clarified. Also, the relation between the microstructure of various sintered films and their properties are largely unknown. In this work, the microstructures and the properties (electrical conductivity and adhesion strength) of sintered film are analyzed by varying the process conditions for Ti:sapphire femtosecond laser sintering of silver nanoparticles. Sintering occurred mainly by surface necking at low laser fluences while by melting of the particles at high laser fluences. When laser fluence was further increased, the balling phenomenon occurred by capillary instability of the molten pool. These sintering phenomena were largely similar to those observed in typical thermal sintering. Both the electrical conductivity and the adhesion strength increased with the laser fluence to reach maximum when melting of the particles began before triggering the balling phenomenon. The maximum electrical resistivity ~ 8.7 μΩ cm obtained in this work was similar to that obtained by other thermal-sintering processes.

Lightweight aggregate obtained from municipal solid waste incineration bottom ash sludge (MSWI-BAS) and its characteristics affected by single factor of sintering mechanism

ABSTRACT To solve the disposal problem of municipal solid waste incineration bottom ash sludge (MSWI-BAS), using it as the main raw material to prepare lightweight aggregates (LWA) for resource utilization. Sintering is an important process to achieve the desired microstructure and material properties. This paper investigates the characteristics of LWA affected by single factor of sintering mechanism (sintering temperature, heating rate and soaking time). Results show that sintering temperature increased from 1130°C to 1160°C caused high-density microstructure materials gradually formed in LWA, leading to particle strength increased from 0.1 MPa to 3.64 MPa, particle density showed an overall upward trend, reaching a maximum of 916 Kg/m3 at 1160°C, and 1 h water absorption reduced from 68% to 25%. The heating rate of 15 K/min was beneficial to the formation of dense phase structure which could increase the particle strength, and the water absorption rate reached the lowest at this time, while the particle density was less affected by heating rate. When soaking time extended from 5 min to 20 min, particle strength and compressive density were gradually increased, and 1 h water absorption showed an overall downward trend, indicating that a longer soaking time was not conducive to the retention of pores. This study demonstrates that the utilization of MSWI-BAS to make high-performance LWA is feasible, along with the preferable environmental and economic benefits. Implications: MSWI-BAS were selected to produce lightweight aggregate (LWA), so that the sludge disposal problem is reduced. The effects of sintering temperature, heating rate and soaking time on the characteristics of LWA were investigated. Compact glass structures are formed at 1150°C and 1160°C which greatly improve the strength. The heating rate has little influence on the physical properties of LWA products. The particle density of LWA increases after the sintering soaking time reaches 15 minutes.

Highly conducting Sc and Y co-doped ZrO2 thin film solid electrolyte on a porous Ni/YSZ electrode prepared via simple drop-coating method

Dense and crack-free thin film solid electrolyte of scandium and yttrium co-doped zirconia (Sc0.08Y0.08Zr0.84O1.92) or 4S4YSZ has been successfully deposited on a NiO-YSZ porous electrode substrate via simple drop-coating deposition method. The 4S4YSZ precursor powder was synthesized using sol-gel method and the NiO-YSZ substrate, with 1:1 wt% composition, was prepared via modified glycine-nitrate combustion method. The NiO-YSZ substrates were pre-sintered at different temperatures, 1000 °C and 1300 °C, to investigate the effect on the thin film deposition and its microstructure. XRD results showed that both 4S4YSZ and NiO-YSZ exhibited a cubic crystal structure with no observable impurity peaks. SEM-EDS revealed a dense morphology of 4S4YSZ thin film with thickness of about 7μm after sintering while maintaining a desirable porous NiO-YSZ microstructure of the electrode substrate for the 1000 °C pre-sintered substrate as compared to a still porous solid electrolyte for the higher substrate pre-sintering temperature at 1300 °C. And from the conductivity measurements using EIS, the 4S4YSZ thin film revealed a notable high total conductivity of about 1.2x10-1 Scm-1 at 700 °C with 0.73 eV activation energy. Reduction of NiO/YSZ to Ni/YSZ further increases the porosity of the electrode substrate while maintaining a dense solid electrolyte thin film.

Tribological studies of aqueous poly quenched medium carbon steel

Abstract In the recent time, aqueous polymer quenchants are gaining importance in the steel heat treatment processes over conventional quenchants. It has been observed that desirable microstructure and mechanical properties can be achieved by proper adjustment of the quench parameters in the polymeric media. Reduction of distortion and cracking of heat treated steel has also been reported. However, the effect of this comparatively new technique of heat treatment on the tribologicalbehavior of steel has not been reported to a greater extent. In the present experimental study the effect of aqueous PEG-400 (poly ethylene glycol) solution as a quenching medium on medium carbon steel has been studied followed by the study of friction and wear. Multi tribotester ‘TR-25’ (DUCOM) has been utilized for this purpose. This paper also highlights the different applications of the new generation polymer quenchants and various critical quenching system parameters that need to be considered during the design stage of the quenching system for the new generation polymer quenchants. The results have been discussed along with necessary figures

An Investigation into Friction Stir Welding of Aluminium Alloy 5083-H116 Similar Joints

Abstract In friction stir welding (FSW), process parameters play an essential role for obtaining desirable mechanical properties and microstructures in the welded zone. Therefore, a study of the effect of the welding parameters on mechanical properties was undertaken in FSW of 5083-H116 aluminium. The microstructure of the joint was also investigated. The maximum tensile strength alongside with the impact energies is presented in this paper. The state of weld nugget and the structure of the grains have been investigated. A numerical study of the thermal cycle in the FSW was carried out, and the results were validated experimentally by using K-type thermocouples at specific locations. A good agreement was obtained between the numerical and the experimental results

The Synergistic Role of Mn and Zr/Ti in Producing Θ'/L12 Co-Precipitates in Al-Cu Alloys

Microstructural stability is a critical factor to consider when designing new alloys for high-temperature applications. An Al-Cu alloy with Mn and Zr additions has recently been developed to withstand extended exposures of up to 350 °C. The addition of Mn in combination with Zr and their segregation to precipitate interfaces play a significant role in stabilizing the metastable θ' precipitates responsible for the alloy's hardness; however, adding Zr and Mn separately only improves the stability to 200 °C and 300 °C, respectively. To this end, the effect of the synergistic additions on interfacial structure and chemistry was studied in detail using atom probe tomography and scanning transmission electron microscopy for Al-Cu-Mn-Zr/Ti-containing alloys subjected to long-term annealing (up to 2,100 h) in the critical temperature range, 300 °C and 350 °C, to investigate the role of Zr/Ti in increasing the θ'-precipitate stability. The results reveal how the addition of Mn allows Zr to segregate to θ' interfaces and eventually create a θ'/Al3(Zrx,Ti1-x) L12 co-precipitate structure along the interface. The co-precipitate is highly stable, as shown by density functional theory calculations, and is a key factor that governs microstructural stability beyond 300 °C. This study reveals how solute additions with different stabilization mechanisms can work in concert to stabilize a desired microstructure, the results provide insights that can be applied to other high-temperature alloy systems.

Design and development of a dual-phase TRIP-TWIP alloy for enhanced mechanical properties

Triggering transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) mechanisms in metastable β titanium alloys (bcc, body centered cubic) have helped reaching unprecedented mechanical properties for Ti-alloys, including high ductility and work-hardening. Yet the yield strength of such alloys generally remains rather low. So far, mostly single-phase metastable bcc alloys have been developed. In this study, a dual phase TRIP/TWIP alloy is designed and investigated. While the β-matrix is expected to display TRIP/TWIP deformation mechanisms, the addition of a second phase, α in the present study, aims at increasing the yield strength. The composition was designed in the Ti-Cr-Sn system, based on Calphad prediction and on the semi-empirical d-electron alloy design approach. Results were compared to the published full β Ti – 8.5Cr – 1.5Sn (wt%) TRIP/TWIP alloy. The dual-phase alloy was prepared and processed to reach the desired microstructure containing about 20% α. It displays remarkable mechanical properties such as a ductility of 29%, an ultimate tensile strength of 1200 MPa and a yield strength of 760 MPa, 200MPa higher than the Reference single-phase β alloy. Analysis of the mechanical properties and deformation microstructures confirm the TRIP and TWIP effects, validating the proposed approach.

DETERMINAÇÃO DAS TEMPERATURAS DE TRANSFORMAÇÃO AR1 E AR3 EM BARRAS DE AÇO CA-50 LAMINADAS A QUENTE / DETERMINATION OF THE TRANSFORMATION TEMPERATURES AR1 AND AR3 FOR HOT ROLLED CA-50 STEEL REBARS

This paper has as objective to determinate the critical temperatures of transformation A r1 and A r3 for steel rebars from ABNT/NBR CA-50 class [1], which are produced by hot rolling process. To determinate these temperatures, quenching heat treatments are going to be made at various temperatures, determining the start of the Austenite formation (A r1 temperature) and complete Austenite formation (A r3 temperature). To have these temperatures known is so important to reach the desired microstructure in the product after the hot rolling process and consequently, control its mechanical properties. Furthermore, rolling the steel at inter critic field (located between the A r1 and A r3 temperatures) requires greater rolling efforts, reducing the rolling chain service life. That way, the determination of the A r1 and A r3 temperatures are of great importance for the steel industry. The used methods, metallographic and hardness method showed efficient for the critical temperatures calculation.

Structure and Properties of Layered Ti-6Al-4V-Based Materials Fabricated Using Blended Elemental Powder Metallurgy

Due to the high specific strength of Ti, materials on its base are indispensable when high-strength and low-weight requests are a chief demand from the industry. Reinforcement of Ti-alloys with hard and light particles of TiC and TiB is a credible pathway to make metal matrix composites (MMC) with enhanced elastic moduli without compromising the material’s low-weight. However, reinforcement of the alloy with hard particles inevitably lowers the value of toughness and plasticity of material. Yet, in many applications simultaneous high hardness and high plasticity are not required through the entire structure. For instance, parts that need enhanced wear resistance or resistance upon ballistic impact demand high hardness and strength at the surface, whereas their core necessitates rather high toughness and ductility. Such combination of mechanical properties can be achieved on layered structures joining two and more layers of different materials with different chemical composition and/or microstructure within each individual layer.Multi-layered structures of Ti-6Al-4V alloy and its metal-matrix composites (MMC) with 5 and10% (vol.) of TiC and TiB were fabricated in this study using blended elemental powder metallurgy (BEPM) of hydrogenated Ti. Post-sintering hot deformation and annealing were sometimes also employed to improve the microstructure and properties. Structure of materials were characterized using light optical microscopy, scanning electron microscopy, electron backscattered diffraction, x-ray microscopy, tensile and 3-point flexural tests. The effect of various fabrication parameters was investigated to achieve desirable microstructure and properties of layered materials. Using optimized processing parameters, relatively large multilayered plates were made via BEPM and demonstrate superior anti-ballistic performance compared to the equally sized uniform Ti-6Al-4V plates fabricated by traditional ingot and wrought technology.

Influence of Deep Cryogenic Treatment and Secondary Tempering on Microstructure and Mechanical Properties of Medium-Carbon Low-Alloy Steels

Deep cryogenic treatment (DCT) and secondary tempering for 40CrNiMoA steel were carried out to obtain the desirable microstructures corresponding to excellent mechanical properties suiting for the preparation of flex spline of harmonic drive reducer. The effects of DCT and secondary tempering on microstructural characteristics and mechanical properties were investigated by x-ray diffraction, scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy, in association with property measurements. The results show that the DCT promotes the transformation of retained austenite to martensite, the precipitation and homogeneous distribution of carbides as well as the refinement of martensitic substructures. The structural and morphological changes significantly improve the hardness, yield and tensile strength of steels, but slightly lower the elongation of them. Further secondary tempering at special temperatures can successively increase the elongation and fracture toughness of the DCT-treated 40CrNiMoA steels at the cost of slight decrease in hardness and strength due to the reduction in dislocation density. Therefore, the DCT in combination with secondary tempering can improve the comprehensive mechanical properties of 40CrNiMoA steels to produce the flex spline with a higher lifetime.

Development of fresh and fully recrystallized microstructures through friction stir processing of a rare earth bearing magnesium alloy

In the present study the friction stir processing (FSP), as an effective thermomechanical processing routine, has been conducted under the different rotational speeds on a solution treated rare earth bearing magnesium alloy. The various processing conditions result in a wide range of Zener-Holloman parameters to elaborate the desired microstructure in respect of the grain size. The obtained results indicate that applying the process on the primary solution treated material leads to the development of fully recrystallized microstructures, where the capability of refinement is directly affected through controlling the strain rate and temperature. In this regard, the resulting microstructures have been classified into “fresh and recrystallized” and “recrystallized and deformed” regions. The grain orientation speared (GOS) maps have also been employed to separate the recrystallized grains and deformed ones. Interestingly, the “recrystallized and deformed” microstructure holds a specified recrystallization texture, and exhibits unexpected hardenability during subsequent room temperature deformation. This is supported by Schmid factor analysis and fractography examinations revealing activation of basal slip and twinning systems in “recrystallized and deformed” microstructure.