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Sn-doped TiFe0.5Ni0.5Sb1−xSnx (x = 0, 0.05, 0.1, 0.2) were synthesized by vacuum arc melting (VAM). In addition to the half-Heusler phase, secondary phases of Fe–Sb-rich compound and Ti-rich compounds were obtained after VAM. The alloys were then subjected to ball milling for 1 h and 5 h. Ball milling for 1h led to microcrystalline grains, while that for 5 h led to nanocrystalline grains. Ball milling followed by spark plasma sintering (SPS) at 1173 K led to significant reduction in size of secondary phases in the microstructure. The undoped sample exhibited a ZT of 0.008 at 873 K for both 1h and 5h BM-SPS samples.

Kinetics of face-centered cubic (FCC) phase evolution in equiatomic AlCoCrFeNi alloy has been studied in this work, measuring the phase fraction from X-ray diffraction (XRD). As-cast alloy showed a body-centered cubic (BCC)+B2 structure. Heat treatments performed at different temperatures showed the formation up-to 30 to 35 pct FCC phase between 1073 K and 1373 K. A systematic decrease in hardness from 540 to 390 HV10 with increase in temperature suggested an increase in the FCC volume fraction. Kinetics of FCC evolution were analyzed using the Johnson–Mehl–Avrami–Kolmogorov equation and Arrhenius law to calculate the activation energy for the phase transformation. Furthermore, a time-temperature-transformation diagram was constructed from the isothermal transformation studies. Detailed microstructural investigation suggests faster kinetics of FCC phase formation near dendritic boundaries compared to interdendritic regions. The Kurdjumov–Sachs orientation relationship between FCC and BCC phases suggested a coherent interface between these phases. Results of the present study pave the way to decide on heat treatment practices in AlCoCrFeNi alloy.

The present work reports the effect of Ta on densification and microstructure evolution during non-isothermal and spark plasma sintering of nanocrystalline W. Nanocrystalline W-5wt.%Ta alloy powder was synthesized using mechanical alloying. The nanocrystalline powder was characterized thoroughly using X-ray diffraction line profile analysis. Furthermore, the shrinkage behavior of nanocrystalline powder was investigated during non-isothermal sintering using dilatometry. Subsequently, the alloy powder was consolidated using spark plasma sintering up to 1600 {\deg}C. The role of Ta on stabilizing the microstructure during spark plasma sintering of nanocrystalline W was investigated in detail using electron backscatter diffraction. The average grain size of spark plasma sintered W-5wt.%Ta alloy was observed as 1.73 micron., Comment: 14 pages, 3 figures

Two different processing routes of mechanical alloying followed by the spark plasma sintering (powder metallurgy) and vacuum arc melting (casting route) were employed to understand the role of processing routes on the phase and microstructural evolution in an equiatomic CrMoNbTiW refractory high-entropy alloy. Besides a major BCC solid solution, a small fraction of carbide, σ phase, nitride, and oxide phases were observed in the alloys prepared by the powder metallurgy route in contrast to a single-phase BCC solid solution in the casting route. The milling atmosphere (dry milling in air and Ar) has significantly influenced the phase and microstructural evolution, illustrating the substantial role of contaminants. Good thermal stability of microstructure at high homologous temperatures was shown based on the long-term heat treatment at 1300 °C for 240 h. The phase evolution predictions via Calphad studies were found to be in reasonable agreement with the experimental observations, albeit with some limitations.

High-entropy alloys (HEAs) are a new generation of materials that exhibit unique characteristics and properties, and are demonstrating potential in the form of thermal spray coatings for demanding environments. The use of HEAs as feedstock for coating processes has advanced due to reports of their exceptional properties in both bulk and coating forms. Emerging reports of thermal sprayed HEA coatings outperforming conventional materials have accelerated further exploration of this field. This early-stage review discusses the outcomes of combining thermal spray and HEAs. Various synthesis routes adopted for HEA feedstock preparation and their properties are discussed, with reference to the requirements of thermal spray processing. The HEA feedstock is then compared and correlated with coating microstructure and phase composition as a function of the thermal spray processing route. Subsequently, the mechanical behavior of thermal spray HEA coatings is summarized in terms of porosity, hardness, and tribological properties, along with their oxidation and electrochemical properties, followed by their potential applications. The thermal spray methods are contrasted against laser cladding and surface alloying techniques for synthesizing thick HEA coatings. Furthermore, HEAs that have displayed excellent properties via alternative processing routes, but have not been explored within the framework of thermal spray, are recommended.

In the present investigation, nanocrystalline W and W-xRe (x = 3, 5 wt.%) alloy powders were produced by mechanical milling/alloying using high energy ball milling. The nanocrystalline nature (∼50 nm) of these powders was validated by the Rietveld refinement of their respective X-Ray diffraction patterns. Subsequently, spark plasma sintering of the ball milled powders was carried out. It was observed that pure W was not able to densify completely (relative density of 93%) at a temperature of 1500 °C. However, the addition of 5 wt.% Re resulted in near complete densification (relative density of 97%) at the same sintering temperature. The enhanced densification of W-Re powders is mainly attributed to the ductilising effect of Re assisted by the nanocrystallinity of powders, and the application of pressure during sintering.

Nanocrystalline CoCrFeNi and CoCrFeMnNi high entropy alloys have been processed by mechanical alloying followed by spark plasma sintering. Grain growth kinetics has been estimated for both the alloys by subjecting them to heat treatment in the temperature range 1073–1373 K. These alloys possess a thermally stable single phase FCC structure along with Cr7C3 contamination. Electron back scattered diffraction (EBSD) has been used to determine grain size of all the heat treated samples. Both CoCrFeNi and CoCrFeMnNi alloys exhibit a grain growth exponent, n = 3, suggesting long-range diffusion-controlled grain growth in these alloys. Activation energies for grain growth are 134 and 197 kJ/mol for CoCrFeNi and CoCrFeMnNi, respectively, which are significantly lower than the activation energy of lattice diffusion in these alloys. Hardness is measured for CoCrFeMnNi alloy as function of grain size and is found to follow the Hall-Petch type relation. The strength coefficient (slope of Hall-Petch relation) is calculated as 1.92 GPa, which is nearly three times that of the value reported in literature for coarse grained CoCrFeMnNi. Presence of carbides enhances the hardness of these HEAs. The maximum contribution to strengthening comes from the FCC-carbide phase boundaries.

Mechanical alloying (MA) followed by sintering has been one of the most widely adopted routes to produce nanocrystalline high-entropy alloys (HEAs). Enhanced solid solubility, room temperature processing, and homogenous alloy formation are the key benefits provided by MA. Spark plasma sintering has largely been used to obtain high-density HEA pellets from milled powders. However, there are many challenges associated with the production of HEAs using MA, which include contamination during milling and high propensity of oxidation. The present review provides a comprehensive understanding of various HEAs produced by MA so far, with the aim to bring out the governing aspects of phase evolution, thermal stability, and properties achieved. The limitations and challenges of the process are also critically assessed with a possible way forward. The paper also compares the results obtained from high-pressure torsion, another severe plastic deformation technique.

In the present study, the phase evolution and microstructure of CrMoNbTiW, a new equi-atomic refractory high-entropy alloy, are studied. The alloy was synthesized through mechanical alloying (MA) followed by consolidation using spark plasma sintering. After MA, a major BCC solid solution along with residual Cr and Nb were observed. However, secondary phases such as Laves and carbides were also observed in addition to the major BCC solid solution after sintering. Unsolicited contamination from the milling media is found to be one of the reasons for the formation of secondary phases. The high hardness of 8.9 GPa after sintering was attributed to the presence of secondary phases along with the nanocrystalline nature of the alloy. To understand the phase evolution, calculation of phase diagram was carried out using CALPHAD. Further, binary phase diagram inspection and simple empirical parameters were also used to assess their effectiveness in predicting phases.

A novel oriented attachment growth mechanism of nanograins has been suggested for spark plasma sintering and annealing of NiO nanopowder. A hierarchical microstructure built by cube-shaped nanograins was observed during pressure-less spark plasma sintering at temperatures above 1000 °C and when the powders were annealed in ambient atmosphere at and above 700 °C. Irrespective of the processing method used, a rotation assisted grain attachment of irregularly shaped nanocrystals results in such a microstructure with parallel epitaxy, twinning or line defects at the grain interface. The unique microstructural features and the governing factors for the attachment process have been discussed.

Mechanically activated annealing was employed successfully for the synthesis of nanocrystalline half-Heusler (TiNiSn) alloy for the first time. TiNiSn could be synthesized by annealing, after 5 h of mechanical alloying of equiatomic elemental blend, at a temperature as low as 600 °C in 2 h. The nanocrystalline nature and resultant increased surface area coupled with large number of defects induced by mechanical alloying has resulted in a significant reduction in temperature and time of annealing in comparison to earlier reports (800 °C–400 h). This could lead to energy efficient low temperature synthesis of nanocrystalline half-Heusler alloys for thermoelectric and magnetic applications.

The present investigation deals with the processing of [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 bulk metallic glass (BMG) using different techniques. The maximum plate thickness for the amorphous phase formation has been found to be 1.25 mm using a conventional BMG casting technique (forced infiltration casting). For enhancing the dimension, an alternate processing route was employed to produce the BMG in the form of compacts. In this case, the dimensions are not restricted by the glass forming ability of the alloy. The process involved the preparation of amorphous ribbons using a melt spinner followed by high energy ball milling to crush the ribbons into very fine particles. The powder was then compacted in the form of pellets using spark plasma sintering technique. Density achieved was almost near to theoretical density (99%). The density was found to increase with a decrease in the particle size of glassy powder, which could be achieved by increasing the duration of high energy ball milling. The maximum densification occurred when the holding temperature was within undercooled liquid regime (∆Tx). Further increase of temperature beyond crystallization temperature (Tx) resulted in decreasing the compact density.

In this study, dense fine-grained ZrB2 and TiB2 were fabricated using reactive spark plasma sintering (RSPS) of ball-milled Zr/B and Ti/B mixtures. Systematic investigations were carried out to understand the mechanisms of reactive sintering. Two densification mechanisms were found to be operating during RSPS. The first stage of densification was due to self-propagating high temperature synthesis reaction leading to formation of ZrB2 and TiB2 compacts having relative density of similar to 48 and similar to 65%, respectively. The second stage of densification occurred at temperatures more than 1100 degrees C and resulted in final relative density of more than 98%. Electron backscatter diffraction and electron microscopy studies on interrupted RSPS samples as well as dense samples showed deformed grains and presence of slip steps while grain orientation spread map and pole figure analysis confirmed plastic flow. Plastic flow-aided pore closure is shown as major mechanism during reactive sintering.

Effect of Sc addition and T6 aging treatment on the secondary dendritic arm spacing (SDAS), modification of eutectic Si morphology, β-Al 5 FeSiand π-Al 8 Mg 3 Si 6 Fe 1 phases and its effect on mechanical properties in A356 alloy has been investigated. Addition of 0.4 wt%Sc in A356 alloy resulted in a 50%reduction in the secondary dendritic arm spacing (SDAS). Sc addition changed the morphology of eutectic Si from plate like to fibrous and globular. The needle like morphology of β-Al 5 FeSi phase in A356 alloy changed to Al 5 Fe(Si,Sc) phase having smaller size and irregular morphology. Transmission electron microscopy (TEM) diffraction pattern and Energy dispersive spectroscopy (EDS) analysis revealed the presence of β-Al 5 FeSiand π-Al 8 Mg 3 Si 6 Fe 1 phases in A356 alloy which changed to β-Al 5 Fe(Si,Sc), π-Al 8 Mg 3 (Si,Sc) 6 Fe 1 and additional V-AlSi 2 Sc 2 phase was observed in Sc containing alloys. Addition of 0.4 wt%Sc to A356 alloy improved its Vickers hardness, Ultimate tensile strength (UTS), Yield strength (YS) and ductility by 20%, 25%, 20% and 30% respectively. Artificial aging treatment resulted in significant improvement in the tensile properties for both A356 and Sc added A356 alloys.

Ba0.8Pb0.2TiO3 nanocrystalline ferroelectric ceramics were prepared by high-energy ball mill followed by spark plasma sintering and conventional sintering techniques. Sintering behavior, microstructure, dielectric properties, and ferroelectric properties were investigated by XRD, SEM, dielectric spectrometer, and ferroelectric P-E loop tracer. It was found that the densification process was greatly enhanced during SPS. The sintering temperature was 673 K (400 °C) lower and the microstructure was much finer than that of conventionally sintered ceramics, and dense compacts with a higher density of over 99 pct were obtained. The average grain size in the conventionally sintered samples is up to about 200 nm, while that in the spark plasma sintered prepared sample is only about 100 nm. The room temperature and transition temperature dielectric constant of spark plasma-sintered samples is higher than that of the conventionally sintered sample. There is a noticeable difference in processing time between conventional sintering and spark plasma sintering, and the latter offers potential advantage in time saving also.

Dense (>98%) submicron sized monolithic zirconium diboride (ZrB 2 ) was fabricated by reactive spark plasma sintering (RSPS) of 8 h ball milled Zr and B elemental mixtures at 1200 °C. During RSPS, ZrB 2 formed in-situ and the densification takes place by a two-step process. Plastic flow of ZrB 2 resulting from application of higher pressure enhanced densification significantly. Improvement in nanohardness, elastic modulus and indentation fracture toughness were observed with increased sintering pressure.

The extent of oxidation and mechanism involving glassy and nanocrystalline phases are found to be different than that in case of normal crystalline metals and alloys. A short review on oxidation behavior of metallic glass and nanocrystalline alloys is presented in the light of work by the present authors and other researchers. The main factors, which are important in controlling the oxidation of metallic glass, are: (i) structure of glass and its devitrified states, (ii) composition of the alloy, and (iii) structure of the oxide layer. Hence, these factors are discussed with the help of isothermal and non-isothermal oxidation both in air and in the presence of oxygen of a number of Zr-based glassy alloys. The oxidation resistance of the glassy alloys decreases in the following order: microcrystalline alloy�>�fully nanocrystalline alloy�>�partially nanocrystalline alloy�>�relaxed amorphous alloy�>�quenched amorphous Zr-based alloy. � 2015, The Indian Institute of Metals - IIM.

In this study, the low temperature synthesis of dense titanium diboride (TiB2) compacts by reaction spark plasma sintering (RSPS) of ball milled mixtures of titanium and boron powders is presented. The influence of milling time and sintering temperature on the in-situ reactive synthesis, densification, grain size and mechanical properties of TiB2 was investigated. TiB2 compacts with relative density above 99% have been obtained at a significantly low sintering temperature of 800 �C. X-ray diffraction analysis confirmed the formation of TiB2 along with a small amount of secondary TiB phase. The effect of TiB on the grain size and the hardness and fracture toughness was investigated. TiB2 compacts processed were found to have a nanohardness &gt, 26 GPa and an elastic modulus &gt, 570 GPa and indentation fracture toughness more than 3.30 MP a.m1/2. It is shown that RSPS can be an attractive route for synthesis of ultrahigh temperature ceramics at low temperatures. � 2014 Elsevier Ltd.

Partially vitrified Zr60Cu10Al15Ni15 bulk metallic glass has been synthesized using water cooled copper mold drop casting technique. Kinetically favorable microstructures having different morphologies are observed throughout the volume of the bulk metallic glass sample. X-ray diffraction studies indicate formation of hard intermetallic compounds such as Zr3Al2 and Zr2Ni in certain regions along with amorphous structures. Microindentation studies carried out in different regions of the sample reveal microstructure dependent deformation behavior. Highest hardness is observed in the fully crystallized regions compared to pure glassy regions in the same sample. Further nanoindentation in the same sample is used to understand dynamic mechanical properties of microstructures in different regions. The pile-up morphologies around the indent and differences in load-displacement curves provide vital information on deformation behavior of sample in different microstructure sensitive regions. � 2014 Elsevier Ltd.

Aluminum exhibits excellent corrosion resistance in the pH range of 4–8 due to the presence of a thin “passive film” upon the metal surface [1–9]. Breakdown of the passive film caused by several metallurgical (defects, impurities, secondary phases etc.) and environmental (halide ions, high or low pH, temperature) factors leads to the significant localized corrosion [9–17]. The corrosion of Al is escalated by the presence of intermetallics, which are formed due to the alloying additions and impurities as most of the elements possess limited solid solubility in Al and therefore intermetallics form during production, processing, and service conditions [18–22].

High-energy ball milling (HEBM) of Al and Al based alloys results in the nanocrystalline alloys in the form of powder, which subsequently needs to be consolidated for engineering applications, as well as for the investigation of associated properties. Consolidation of HEBM nanocrystalline alloy powders is not a trivial task. High mechanical strength (Chap. 4) and poor thermal stability (Chap. 5) of nanocrystalline materials make consolidation into fully dense bulk products challenging. Various conventional and non-conventional consolidation techniques used for consolidation of ball milled Al powders are summarised in Table 3.1 and described briefly in this chapter.

High-energy ball milled Al alloys have demonstrated extraordinary properties due to grain refinement to the nanoscale, and increased solid solubility of alloying elements. For example, the compressive yield strength of 1200 and 1105 MPa was observed in Al-Fe [1] Al-Cr alloys [2], both prepared via HEBM followed by consolidation. The specific compressive yield strength of an Al-20Cr alloy was compared with that of the commercial alloys and high-energy ball milled alloy surpassed the specific yield strength of the commercial alloys indicating a possibility of a development of a new class of light metals with high strength (Fig. 7.1). Moreover, corrosion performance of the nanocrystalline alloys was shown to improve significantly. For example, Gupta et al. [2, 3] reported a significant improvement in corrosion performance of both nanocrystalline alloys. In contrary to commercial alloys, where corrosion performance diminished with strength, high-energy ball milled alloys exhibited simultaneous improvement in corrosion resistance and strength due to the extended solid solubility of suitable solute and grain refinement to nanoscale. In summary, the high-energy ball milled Al alloys exhibited revolutionary properties and showed promise to develop future lightweight materials exhibiting ultra-high strength and improved durability. In spite of the attractive properties, the research on these alloys is in an early stage and further research is required to gain fundamental insight and ensure the engineering applications.

Alloying of elemental blends achieved through high-energy ball milling (HEBM) is referred to as mechanical alloying (MA), which is a solid-state powder processing technique involving the repeated deformation, fracture and welding of powder particles [1–4]. This technique was originally developed to produce oxide-dispersion strengthened (ODS) nickel and iron-base superalloys for aerospace applications [5]. Later, MA has been substantiated to be capable of synthesizing a variety of equilibrium and non-equilibrium phases, including nanocrystalline and amorphous materials. Recently MA has been demonstrated to be a most versatile and economical process for synthesis of nanocrystalline materials, due to its simplicity, low cost, and ability to produce large amount of material [1–4, 6]. Historically, from the point of Al based alloys, MA was used to produce dispersion hardened Al alloys [7–9]. Commercial production of Al alloys by ball milling was first reported by INCO alloys in year 1989 [10].

High entropy alloys (HEAs) represent a new class of materials that present novel phase structures and properties. Apart from bulk material consolidation methods such as casting and sintering, HEAs can also be deposited as a surface coating. In this work, thermal sprayed HEA coatings are investigated that may be used as an alternative bond coat material for a thermal barrier coating system. Nanostructured HEAs that were based on AlCoCrFeNi and MnCoCrFeNi were prepared by ball milling and then plasma sprayed. Splat studies were assessed to optimise the appropriate thermal spray parameters and spray deposits were prepared. After mechanical alloying, aluminum-based and manganese-based HEA powders revealed contrary prominences of BCC and FCC phases in their X-ray diffraction patterns. However, FCC phase was observed as the major phase present in both of the plasma-sprayed AlCoCrFeNi and MnCoCrFeNi coatings. There were also minor oxide peaks detected, which can be attributed to the high temperature processing. The measured porosity levels for AlCoCrFeNi and MnCoCrFeNi coatings were 9.5 ± 2.3 and 7.4 ± 1.3 pct, respectively. Three distinct phase contrasts, dark gray, light gray and white, were observed in the SEM images, with the white regions corresponding to retained multicomponent HEAs. The Vickers hardness (HV0.3kgf) was 4.13 ± 0.43 and 4.42 ± 0.60 GPa for AlCoCrFeNi and MnCoCrFeNi, respectively. Both type of HEAs coatings exhibited anisotropic mechanical behavior due to their lamellar, composite-type microstructure.

In the present study, influence of molybdenum and niobium additions on phase formation during mechanical alloying and spark plasma sintering of CoCrFeNi high entropy alloy was studied. Major FCC and minor BCC phase were observed after mechanical alloying of CoCrFeNi. However, major FCC and sigma phase were observed after spark plasma sintering. A maximum relative density of 95% was obtained with the hardness of 570 HV in CoCrFeNi HEA. The phase formation behavior was not significantly affected by the addition of molybdenum or niobium. However, addition of Mo to CoCrFeNi increased the hardness from 570 HV to 620 HV, and the hardness increased to 710 HV with combined addition of molybdenum and niobium. After sintering, major FCC phase with crystallite size of 60-70 nm was observed in all the compositions. Further, the microstructure and hardness retention was observed in CoCrFeNiMo0.2 with annealing temperature up to 800 degrees C.

Influence of laser welding (LW) and shielded metal arc welding (SMAW) processes on Type IV cracking behavior of modified 9Cr-1Mo (P91) steel has been investigated in this paper. The study involved comparison of stress rupture lives of modified 9Cr-1Mo steel weldments prepared by SMAW and continuous wave CO2 laser welding processes. Width of the heat affected zone (HAZ) in laser weldment was found to be 1.0mm, whereas it was ~2.5mm in SMAW weldment. The rupture lives of laser weldment were found to be higher than SMAW weldment at higher stress level and comparable at lower stress level. Under similar stress levels, the creep rupture lives of 875�C heat treated specimens were found to be lower than that of the base metal and cross weld specimen. These results clearly suggest that the instability of microstructure in the intercritical heat affected zone (ICHAZ) is responsible for lower creep rupture lives of P91 steel weldment than the base metal. The experimentally observed variations in creep cavitation have been corroborated with the results of finite element (FE) analysis. � 2014 Elsevier B.V.

Pure W and W-2Y(2)O(3) materials were fabricated using powder metallurgy method. The microstructures of the materials were investigated by electron back-scattered diffraction and transmission electron microscopy techniques. Both materials contain 1-2 mu m size W grains. In the case of W-2Y(2)O(3), the material contains yttria particles having sizes between 300 and 900 nm. The W matrix in W-2Y(2)O(3) shows stronger texture than that of pure W. Berkovich hardness values are 4.5 GPa in pure W and 4.9 GPa in W-2Y(2)O(3) for a 10 N load. Ion irradiation experiments were performed on both materials at the JANNuS facility (Saclay, France) using Fe and He ions with energies of 24 MeV and 2 MeV, respectively. Radiation loops are present on the W grains whereas on yttria particles, radiation-induced damages appear as voids. Berkovich hardness values of irradiated materials are slightly higher than the non-irradiated materials. Results of the microstructure and microhardness of irradiated as well as non-irradiated materials are presented in detail. (C) 2014 Elsevier Ltd. All rights reserved.

Carbide-free bainite, a fine mixture of bainitic ferrite and austenite, is a relatively recent development in steel microstructures. Apart from being very strong and tough, the microstructure is hydrogen-tolerant. These characteristics make it well-suited for weld metals. In the current work, an armor-grade quenched and tempered steel was welded such that the fusion zone developed a carbide-free bainitic microstructure. These welds showed very high joint efficiency and ballistic performance compared to those produced, as per the current industrial practice, using austenitic stainless steel fillers. Importantly, these welds showed no vulnerability to cold cracking, as verified using oblique Y-groove tests. The concept of carbide-free bainitic weld metal thus promises many useful new developments in welding of high-strength steels. � 2014, The Minerals, Metals & Materials Society and ASM International.

Analysis of isothermal grain growth kinetics of nanocrystalline Fe-9Cr-1Mo and Fe-9Cr-1W-based ferritic oxide dispersion strengthened alloys is reported. Fe-9Cr-1Mo-0.25Ti-0.5Y2O3 alloy exhibited ~900 and ~250 pct enhancement in grain-coarsening resistance at 1073 K (800 °C) in comparison with Fe-9Cr-1Mo-0.5Y2O3 alloy and Fe-9Cr-1W-0.5Y2O3 alloy, respectively. Comparison of grain growth time exponents also revealed that addition of Ti and Y2O3 to nanocrystalline Fe-9Cr alloy has significantly enhanced the grain growth resistance. This is attributed to the possible presence of Y-Ti-O-based nanoclusters (

AlxCoCrCuFeNi (x = 0.45, 1, 2.5 and 5 mol) multi-component high entropy alloys synthesized by mechanical alloying were spark plasma sintered to produce high dense compacts. X-ray diffraction and scanning electron microscopy studies reveal that these sintered alloys exhibit varying microstructures from single phase to three phases depending on Al content. The thermal stability studies carried out in the temperature range of 400-600 C for duration of 2-10 h in Ar atmosphere suggest that these alloys exhibit excellent thermal stability in terms of phases and crystallite size. Highest specific hardness of 160 (HV/g cm-3) is achieved in the sintered Al5CoCrCuFeNi alloy and there is no significant change in the hardness after heat treatment of Al 0.45CoCrCuFeNi and AlCoCrCuFeNi alloys. Hall-Petch analysis based on hardness measurements carried out on sintered samples reveals that solid solution strengthening seems to increase with increase in Al content. � 2013 Elsevier B.V. All rights reserved.

In this study, corrosion behavior of Cu60Zr20Ti 20 (at. %) metallic glass is investigated in acidic and neutral chloride solutions, at varying molar concentrations of chloride and also in alkaline solutions. Further, to investigate Cu60Zr 20Ti20 glassy ribbon in biological conditions, it is evaluated in phosphate buffered saline (PBS) solution. Results indicate that corrosion resistance decreases with increase in chloride concentration in case of acidic and neutral solutions. While in case of alkaline solution, corrosion resistance is found to increase with increase in pH of solution. The glassy alloy has exhibited excellent corrosion resistance in PBS medium. In order to understand the corrosion behavior, the surfaces of the samples were studied using scanning electron microscopy (SEM). SEM examination reveals pitting type of corrosion. The possible reasons for pitting corrosion in metallic glasses have been addressed in the present study. � 2013 Elsevier B.V. All rights reserved.

In the current study, the phase evolution of multicomponent equiatomic CoCrCuFeNi, CoCuFeNi, CoCrCuNi, and CoCrFeNi alloys synthesized by mechanical alloying (MA) followed by annealing was studied. From the phase evolution studies, CoCrFeNi, CoFeMnNi, CoCuFeNi, and CoFeNi were chosen to correlate the densification together with phase evolution during spark plasma sintering (SPS). MA resulted in a major face centered cubic (fcc) phase and a minor body centered cubic (bcc) phase in Cr-containing alloys, and a single fcc phase in all other alloys. After SPS, CoFeMnNi and CoFeNi remained as single fcc phase. However, CoCuFeNi transformed to two fcc phases, and CoCrFeNi had a major fcc phase with minor sigma phase. From densification studies, it was evident that CoCrFeNi showed delayed densification, albeit maximum final densification in comparison to other alloys. This behavior was attributed to distinctly different phase evolution in CoCrFeNi during SPS as compared to other alloys. Detailed phase evolution studies were carried out on CoCrFeNi by annealing the powders at different temperatures followed by conventional x-ray diffraction (XRD) and in situ high-temperature XRD of mechanically alloyed powders. The results obtained from the annealing and in situ high-temperature XRD studies were correlated with the densification and alloying behavior of CoCrFeNi alloy. � 2013 The Minerals, Metals & Materials Society.

Topological factors such as mismatch entropy and configurational entropy, along with thermodynamic entity such as enthalpy of chemical mixing, are found to control glass formation in metallic systems. Taking both these factors into consideration, a parameter called P HS was proposed to correlate glass forming ability successfully in the Cu-Zr-Ti system. The parameter P HS (=∆H chem × ∆S σ /k B ) is a product of enthalpy of chemical mixing and mismatch entropy. Our study indicates that the more negative is the PHS value within the configurational entropy (∆S config/R) range of 0.9 to 1.0, the higher is the stability of glassy phase resulting in a larger diameter of bulk metallic glass rods. Observed theoretical predictions are supported by experimental results in which the compositions with high negative P HS resulted in easy amorphous phase formation in comparison with less negative P HS compositions by mechanical alloying. This criterion was extended to Cu-Zr-Al and Cu-Zr-Ag systems as well, thus establishing a strong correlation between P HS and the glass forming ability of alloys. The role of size effect, probability of atomic arrangements, and heat of formation among constituent elements in obtaining a larger dimension bulk metallic glasses was addressed in this study.

In the present work, nanocrystalline FeAl and FeAl–Al2O3 composite were synthesized by high energy ball milling and subsequent compaction by spark plasma sintering. Microstructural changes during all stages of processing are studied using X-ray analysis. After 20 h of milling, the disordered FeAl and some amount of Fe rich solid solution was obtained in both of these compositions. Subsequent heat treatment results in formation of ordered FeAl. However, disordering of FeAl was observed in both compositions after spark plasma sintering. Nanocrystallinity is retained in both the compositions even after sintering at high temperature of 1,000 °C. Very high hardness of ~575 HV1 and ~600 HV1 was exhibited by FeAl and FeAl–Al2O3 composite.

The present study deals with phase evolution of oxide dispersed AlCoCrFe high entropy alloy during mechanical alloying and spark plasma sintering. Mechanical alloying of AlCoCrFe resulted in a single BCC phase. However, ordering of BCC phase with evolution of chromium carbide and sigma phase were observed after spark plasma sintering. High hardness of 1,050 � 20 HV1 and 1,070 � 20 HV1 was observed for AlCoCrFe high entropy alloy without and with oxide dispersion, respectively. Significant contribution from solid solution strengthening effect in high entropy alloys appears to have overwhelmed the effect of oxide dispersion on hardness. � 2013 Indian Institute of Metals.

A new thermodynamic parameter P HSS ( P HSS = Δ H C (Δ S σ / k B )(Δ S C / R )) is proposed, to describe and predict the glass forming ability (GFA) of Fe based alloys. Here Δ H C is the chemical enthalpy of mixing, Δ S σ is the mismatch entropy and Δ S C is the configurational entropy. The P HSS parameter incorporates enthalpy of mixing, size mismatch and the number of elements in the systems which influence GFA significantly. It was observed from rapid solidification processing (RSP) and mechanical alloying (MA) studies that, all alloys with P HSS in between −0.55 kJ/mol to −6.00 kJ/mol would form glass in the Fe–Zr–B system. MA and RSP experiments on multi-component Fe–Cr–Ni–Zr–B alloys indicated that P HSS is a better parameter to predict GFA of the system than P HS , a parameter used in earlier studies. It was also observed that bulk metallic glass (BMG) forming alloys can be observed in between a P HSS range of −3.00 kJ/mol to −6.00 kJ/mol, with the maximum thickness of the BMG increasing with decreasing P HSS within the above range.

The effect of initial heat treatment on microstructure and mechanical properties of boron-free and boron-containing modified 9Cr-1Mo steel (P91 and P91B, respectively) has been studied under different heat-treatment conditions. The prior austenite grains evolved in P91 steel, having different prior austenite grain sizes, were found to be similar in size after heat treatment in the range of 1073 K to 1448 K (800 °C to 1175 °C) for 5 minutes. The microstructural evolution in P91B steel having different prior austenite grain sizes appeared to be uniform when subjected to different heat-treatment temperatures with the prior austenite grain size being similar to that of initial grain size. Lath martensite was observed in P91B steel after all heat treatments. On the other hand, lath martensite was observed in P91 steel only when subjected to high-temperature heat treatment, whereas subgrain/substructure as well as coarse precipitates were observed after a lower temperature heat treatment. Large differences in the hardness/strength values between different microstructures corresponding to coarse-grained heat-affected zone (CGHAZ) and intercritical HAZ (ICHAZ) of P91 steel weldment were due to the distinct difference in these microstructures. The difference in hardness/strength values between the CGHAZ and ICHAZ was found to be insignificant in P91B steel under similar heat-treatment conditions.

Two different heats of modified 9Cr-1Mo steel, one without boron (P91) and the other with controlled addition of boron (P91B) and very low nitrogen were used for the present study. Microstructure of P91 steel, annealed at 875 °C (inter-critical region) consisted of very fine prior austenite grains without lath martensite. Grain size increased with increase in heat treatment temperature and showed coarse prior austenite grains with clearly defined lath after 900 °C heat treatment. In contrast, the microstructures of P91B steel subjected to identical heat treatment did not vary much with heat treatment temperature, prior austenite grain size remaining similar to that observed in the as received material. Significant improvement in the creep properties of P91B steel compared to that of P91 steel subjected to inter-critical temperature is explained based on the difference in microstructures observed in these two steels after the heat treatment.

The microstructure, hot hardness, and indentation creep of ultrafine grained (ufg) 9Cr-1Mo and 9Cr-1W based ferritic oxide dispersion strengthened (ODS) alloys prepared by mechanical alloying and spark plasma sintering are investigated. Studies on hot hardness revealed that the hardness of Mo-containing alloys is higher than that of W-containing alloys. This is attributed to the complete dissolution of Mo while W did not dissolve completely during mechanical alloying. Hardness of Ti containing alloys is higher compared to the base and Y 2O 3 containing alloys and this is attributed to the presence of Ti-Y-O based nanoprecipitates. Study of hot hardness behaviour of ufg ferritic ODS alloys revealed a sharp transition at around 700-800K. This is possibly due to the enhanced grain boundary mediated plasticity and pronounced reduction in Hall-Petch coefficient with increase in temperature. � 2012 Elsevier B.V.

Aluminium alloys reinforced with transition metal aluminide (Al3Ti, Al3Fe, Al3Ni, etc.) particles possess high specific strength both at ambient and elevated temperature. The improved strength of these alloys are the results of slower coarsening rate of the intermetallic particles due to low diffusivity of the transition metals in aluminium. However, the strength can be enhanced further by refining the microstructure of the alloys to nanometer range. The authors have successfully attempted two important non-equilibrium processing techniques i.e. rapid solidification processing (RSP) and mechanical alloying for the refinement of the microstructure in various aluminium alloys. In this report, authors present a short review of their work on RSP of Al–Ti and Al–Fe alloys to produce nanocomposites.

The paper attempts to study the shrinkage behavior of nanocrystalline tungsten mechanically alloyed with molybdenum (5, 10, 15 and 20 wt.%). The dilatometric analysis was performed by Setsys Evolution TMA (ambient to 1600 °C) using constant heating rate (CHR) method. The significant improvement in shrinkage with alloying of molybdenum is attributed to reduced grain size, lowered tungsten carbide contamination and enhanced diffusion kinetics. The initial stage sintering kinetics of W–20Mo alloy has been investigated. The densification starts with Mo diffusion (calculated activation energy = 128 kJ/mol) and proceeds with the diffusion of both along the grain boundaries (calculated activation energy = 307 ± 1 kJ/mol).

Hall-Petch (HP) relation and the superposition of different strengthening mechanisms in different aluminum alloys with grain sizes in the range of 50 nm-70 ?m were analyzed. HP parameters as high as ? 0 ? 200 MPa and k HP ? 0.14 MPa m 1/2 were observed when the HP relation was fitted directly using the experimentally measured yield strength values. In contrast, considering only the grain size strengthening (by subtracting other strengthening contributions), HP constants of ? 0 ? 10 MPa and k HP ? 0.06 MPa m 1/2 were obtained over wide range of grain sizes for the case of a linear superposition of the various strengthening mechanisms present. This is in excellent accord with the values obtained for pure Al justifying the chosen superposition exponent of n equal unity for the different alloys investigated. Copyright � 2012 WILEY-VCH Verlag GmbH &amp, Co. KGaA, Weinheim.

Deformation behavior of boron free (P91) and boron added (P91B) modified 9Cr-1Mo steel weldments was studied at room temperature using an indentation technique. A sub-structure consisting of sub-grains was observed in the inter-critical heat affected zone (ICHAZ), whereas lath martensitic structure was observed in the coarse grain heat affected zone (CGHAZ) of the P91 weldment. Consequently, weldment showed significant variation in hardness as well as strain rate sensitivity in different regions of the HAZs. On the other hand, P91B steel weldment showed small variation in hardness and strain rate sensitivity, which was commensurate with uniform microstructure consisting of lath martensite. These differences in microstructures as well as hardness values also reflected in large variation in creep depth in P91 compared to the small variation in P91B weldment. � 2012 Elsevier B.V.

The microstructural evolution in the heat-affected zones (HAZs) of boron-added modifi ed 9Cr-1Mo steel (P91B) weldment is distinctly different from that of steel without boron addition (P91). This is accompanied by less reduction in hardness in the inter-critical heat-affected zone (ICHAZ) of the weldment of the boroncontaining steel than what is observed in boron-free steel. Though, tensile properties of both the boron-containing steel weldments and boron free-steel weldments do not differ much at room and at high temperatures, there is a signifi cant improvement in the creep strength of the weldment of the boron-containing steel over that of the weldments of boron-free steel. In fact, for creep tests so far completed for weldments of P91B, fracture occurred in the base metal, not in typical Type IV mode (fracture at the ICHAZ or fi ne grained heataffected zone) as occurred for the P91 weldment for identical conditions of testing. This improvement is explained based on the role of boron in modifying the HAZ microstructure of the steel.

CALPHAD approach has been used to predict the stable phases, their relative amounts and compositions in multicomponent equiatomic high entropy alloys. The results show a good match between the predictions and experimental results on the phase formation for two equiatomic high entropy alloys (CrCoCuNi and CrCuMnNi alloys) prepared by mechanical alloying, considering the kinetic constraints of the non-equilibrium processing route. � Indian Institute of Metals 2012.

The roles of boron and heat-treatment temperature in improving the type IV cracking resistance of modified 9Cr-1Mo steel weldment were studied. Two different heats of P91 steel, one without boron, designated as P91 and the other with controlled addition of boron with very low nitrogen, designated as P91B, were melted for the current study. The addition of Boron to modified 9Cr-1Mo steel has increased the resistance against softening in fine-grained heat-affected zones (FGHAZ) and intercritical heat-affected zones (ICHAZ) of the weldment. Creep rupture life of boron containing modified 9Cr-1Mo steel weldment, prepared from 1423 K (1150 �C) normalized base metal, was found to be much higher than that prepared from 1323 K (1050 �C) normalized base metal because of the stabilization of lath martensite by fine M 23C 6 precipitates. This finding is in contrast to the reduction in creep rupture life of P91 weldment prepared from 1423 K (1150 �C) normalized base metal compared with that of the weldment prepared from 1323 K (1050 �C) normalized base metal. The trace of failure path from the weld metal to ICHAZ in P91B weldment was indicative of type II failure in contrast to type IV failure outside the HAZ and base metal junction in P91 weldment, which suggested that boron strengthened the microstructure of the HAZ, whereby the utilization of boron at a higher normalizing temperature seemed to be significantly greater than that at the lower normalizing temperature. � 2012 The Minerals, Metals &amp, Materials Society and ASM International.

In the present work, synthesis of bulk Fe amorphous alloy by mechanical alloying of 70Fe-15Cr-4Mo-5P-4B-1C-1Si powders followed by consolidation of powders using Spark Plasma Sintering process is reported. The evolution of phase, microstructure, relative density and hardness of amorphous alloy as a function of spark plasma sintering temperature is discussed. Transmission electron microscopy investigation has shown partial divitrification of amorphous matrix into nanocrystalline phase during spark plasma sintering of amorphous Fe alloy. The hardness of amorphous alloy sintered at 525 – 575°C are in the range of 675 - 725 MPa.

IntroductionCurrently, nanocrystalline materials are being widelyinvestigated due to their unique properties. This interest hasspawned the development of various techniques to synthe-size materials exhibiting grain sizes (\20 nm) wherephysical properties are the most exotic. Such processesinclude: high-energy ball milling [1, 2], pulsed electrode-position [3], chemical vapour condensation [4], inert gascondensation [5] etc. However, nanocrystalline samplesproduced from these techniques are either in the formof powders or thin films. Successful consolidation to arte-fact free, fully dense material is required if structuralapplications are ever to be adopted. Many methods toconsolidate these materials (i.e., hot compaction [6],explosive consolidation [7], hot isostatic pressing [8], pre-annealing–compaction–sintering [9] etc.) have been repor-ted in the literature with partial success. In general, thesuccess of these techniques for consolidation hinges on theapplication of high temperatures and pressures, which leadto explosive grain growth in nanocrystalline materials.In situ consolidation technique as developed by Youssefet al. [10] seems promising for the synthesis of consolidatednanocrystalline materials as the step of sintering to bulkdensity is no longer a need and can be eliminated. However,this technique has not been reported widely in the literatureand has been limited to nanocrystalline Zn, Al, Al alloys,copper and copper-based alloys [10, 11]. This study showsthat in situ consolidation of a FeCrNi alloy is possible usingcombination of room temperature (RT) milling and liquidnitrogen temperature milling.Nanocrystalline materials produced by high-energy ballmilling have not been well characterized and the micro-structure of these materials is not well understood. Theprimary challenge in characterization is TEM samplepreparation from ball-milled powders. Nanocrystallinepowders as produced by ball milling are too coarse to beexamined under TEM and the sample preparation for TEManalysis on such powders is not a trivial task. In general,two methods are used for the preparation of conventionalmetallic samples for TEM analysis: electro jet polishingand dimpling followed by ion milling. Nanocrystallinematerials produced by consolidation of ball-milled pow-ders can be problematic for these preparation techniques asthe consolidated samples must first be mechanically thin-ned to \100 lm before being chemically thinned to thefinal thickness. If the material exhibits poor inter-particlebonding, the sample may not withstand the final thinning

Bulk nanostructured Al-4Cu alloy was synthesized by high energy ball milling followed by vacuum hot pressing. Its thermal stability was investigated up to 450°C using differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. The results showed that grain growth in this Al-4Cu alloy was very limited and grain sizes in the range of 100 nm were still present in the alloys after exposure to 450 °C corresponding to a temperature as high as 0.77 T/Tm. The TEM investigations reveal that such a high thermal stability against grain growth observed in this alloy is attributed to the presence of ultrafine dispersoids.

The objective of this study was to examine the potential of Al-5Ti-1B and Al-1Ti-3B master alloys in reducing the hot tearing susceptibility of AZ91E magnesium alloy. The low penetration of magnesium alloys for structural automotive applications can be attributed to their poor castability during permanent mould casting. An improvement in the castability of magnesium alloys will result in the production of larger castings for automotive applications and a reduction in vehicle weight. The addition levels examined for both master alloys were 0.1, 0.2, 0.5 and 1.0 wt.%. The master alloys were added to the AZ91E alloy and stirred for 30 seconds. For the graphite mould castings used to observe grain refinement, the pouring and mould temperatures were 720 and 750 °C respectively. The hot tear castings were produced using a “dog bone” shaped H-13 tool steel mould. The pouring and mould temperatures were 720 and 180 °C respectively. Without master alloy addition, the base AZ91E casting had severe hot tears. The addition of Al-5Ti-1B slightly reduced hot tears while Al-1Ti-3B addition significantly reduced hot tears. The addition of Al-1Ti-3B also significantly reduced the grain size of the castings from 113 µm in the base alloy to 72 µm with 1.0 wt.% addition. The addition of Al-5Ti-1B did not lead to a reduction of hot tears because of large TiAl3 particles acting as stress risers during solidification.