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To improve the ablation resistance of graphite, AlSi alloy was used as a dissipative agent to fill the pores of graphite matrix by pressure infiltration method to prepare AlSi/graphite composites. The microstructure evolution and ablation behavior of the obtained AlSi/graphite composites were investigated under oxy-acetylene flame after ablating for varying lengths of time. Subsequently, the effect of heat dissipation through the AlSi alloy was quantitatively analyzed by data processing technique. Experimental results showed that the AlSi alloy reduced the surface temperature by about 300 °C in the first 20 s, and the linear ablation rate of the AlSi/graphite composite is found to be 119.3% lower than that of the graphite itself ablated for 10 s. The strengthening of ablation resistance is mainly achieved by heat dissipation and oxygen dissipation. However, it gradually decreases with continuous consumption of the AlSi alloy during ablation. Among these, the contribution of heat dissipation to the ablation resistance gradually increased from 10.1% at 10 s to 62.1% at 60 s.

The aim of the current study is to design multiaxial forging (MAF) schedules in order to achieve submicron-grained (KQ, Kee and J-integral values of ultrafine grained (UFG) samples and correlating them with the microstructure, besides evaluating the other mechanical properties. Fatigue strength in the high cycle fatigue (HCF) regime were also investigated and fracture mechanisms analyzed and comparison established between differently processed samples. The microstructural analysis was performed using transmission electron microscopy (TEM) and Electron backscatter diffraction (EBSD) and results corroborated with the mechanical properties. Superior combinations of yield strength (YS), ductility (% El.), fracture toughness (Kee) and high cycle fatigue strength (σf) were obtained under certain conditions, i.e., i) MA steel: intercritical (α+γ) phase regime (~Ar1) controlled and 15-cycle multiaxially forged (MAFed) (YS=1027MPa, %El.=8.3%, σf=355MPa and Kee=90MPa√m), and ii) IF steel: ferritic region (r1) controlled 18-cycle MAFed (YS=881MPa, %El.=11.2%, σf=255MPa and Kee=97MPa√m). In the case of MA steel, an enhancement of the fatigue and fracture toughness properties can be ascertained following the formation of uniformly distributed nanosized fragmented cementite (Fe3C) particles (~35nm size) present in the submicron sized (average ~280nm size) ferritic microstructure. In contrast, in the case of IF steel, this is ascribed to the development of submicron sized ferrite grains (average ~320nm) along with a high density of dislocation substructures. These fine dislocation cells/substructures along with the nanosized Fe3C particles could effectively block the initiation and propagation of cracks and thereby enhance the fatigue endurance and fracture toughness of the steel. Superior fracture toughness along with high mechanical properties in submicron-grained condition render the two steels highly useful for high-strength structural applications.

In the present study, Fe-42% Ni, Fe-2% Y2O3, and Fe-42% Ni-2% Y2O3 (30-60 nm) were prepared by mechanical alloying using high energy SPEX8000M ball mill. Consolidation of the milled sample was done by spark plasma sintering (SPS) at 800, 900, and 1000 °C at a pressure of 60 MPa with a holding period of 5 min in an argon atmosphere. The sintered density was found to be higher at a higher temperature, e.g., 98% at 1000 °C as compared to 78% at 800 °C. The addition of nanosize Y2O3 in Fe or Fe-Ni system showed a strong influence on the formation of intermetallic compounds and final grain size of the resultant material. x-ray diffraction analysis of the sintered Fe-42% Ni-2% Y2O3 alloy indicates the presence of Fe-Ni phase with additional peaks of intermetallic phases (Fe17Y2, Ni5Y) and oxides (Fe3O4, NiO). The nanoindentation hardness of the sintered (at 1000 °C) alloys was found to be the maximum for the Fe-42% Ni-2% Y2O3 (7.9 GPa), followed by Fe-2% Y2O3 (7.2 GPa) and Fe-42% Ni alloy (5.8 GPa). Dry sliding wear resistance of the spark plasma sintered (SPSed) (sintered at 1000 °C) Fe-42% Ni-2% Y2O3 was found to improve due to higher hardness and formation of oxide rich layer at the contact surface. Moreover, the SPSed specimen of the Fe-42% Ni-2% Y2O3 alloy showed a better corrosion resistance (Icorr = 0.78 μA/cm2) as compared to the corrosion resistance (Icorr = 1.34 μA/cm2) of the Fe-42% Ni alloy, both sintered at 1000 °C. The corrosion tests were conducted in a freely aerated 3.5% solution of NaCl electrolyte. A significant amount of grain refinement (due to the addition of nanosize yttria), formation of intermetallic phases (Fe17Y2, Ni5Y) as well as uniform distribution of fine oxide particles (Fe3O4, NiO) played the pivotal role in the improvement of wear, corrosion and mechanical properties of the SPSed Fe-42% Ni-2% Y2O3 alloy.

Ferritic steel with oxide dispersion strengthening is a promising material for fusion and fission reactor components. In the present study, the influence of Mo, V, and Zr on microstructural evolution, thermal stability, and mechanical properties of yttria-dispersed ferritic Fe-14Cr-1Ti-0.25Y2O3-0.3 wt pct X (X = Mo/V/Zr) steels was investigated. This work is inspired by the concept of MA957 alloy, where Mo was replaced by V/Zr to develop new alloy compositions with possible improvement of thermal stability and mechanical properties through grain refinement and oxide dispersion strengthening. These steels were developed by mechanical alloying (MA) and subsequently consolidated by spark plasma sintering (SPS) at different temperatures (900 °C, 1000 °C, and 1050 °C) in high-purity argon atmosphere. The relative sintered density was found to be ~ 97 to 98 pct for specimens spark plasma sintered (SPSed) at 1050 °C. Microstructural analysis of the SPSed specimens (using scanning electron microscopy/transmission electron microscopy-selected area diffraction (SEM/TEM-SAED)) confirmed the formation of uniformly dispersed Y-Ti-O, TiO, and Ti-Cr-O nanosize complex oxide particles within the ultrafine ferritic matrix grains (~ 200 nm). The nanoindentation hardness value is found to correlate well with the compressive strength and wear resistance of the corresponding batches. The influence of V addition in Fe-14Cr-1Ti-0.25Y2O3 alloy is established to yield better thermal stability and superior mechanical properties (nanoindentation hardness of 16.7 GPa, compressive strength of 3068 MPa) as compared to Mo/Zr-stabilized alloys. This was analyzed and discussed in terms of microstructural evolution and strengthening mechanisms involved in comparison to the Mo/Zr-added steels.

Abstract An effort has been made to establish a relation between Zener–Hollomon parameter, flow stress and dynamic recrystallization (DRX). In this context, the plastic flow behavior of Ti + Nb stabilized interstitial free (IF) steel was investigated in a temperature range of 650–1100 °C and at constant true strain rates in the range 10−3–10 s−1, to a total true strain of 0.7. The flow stress curves can be categorized into two distinct types, i.e. with/without the presence of steady-state flow following peak stress behavior. A novel constitutive model comprising the strain effect on the activation energy of DRX and other material constants has been established to predict the constitutive flow behavior of the IF steel in both α and γ phase regions, separately. Predicted flow stress seems to correlate well with the experimental data both in γ and α phase regions with a high correlation coefficient (0.982 and 0.936, respectively) and low average absolute relative error (7 and 11%, respectively) showing excellent fitting. A detailed analysis of the flow stress, activation energy of DRX and stress exponent in accord with the modelled equations suggests that dislocation glide controlled by dislocation climb is the dominant mechanism for the DRX, as confirmed by the transmission electron microscopy analysis. Graphic Abstract

Present study investigates microstructural stability and mechanical properties of spark plasma sintered Fe-Cr-Nb alloys for its nuclear first wall reactor application. The nanostructured alloys were prepared by mechanical alloying (MA) for 25 h in argon atmosphere (purity

In the present study, at first flow behavior of Nb–Ti microalloyed and interstitial-free (IF) steels was investigated to know the effects of processing parameters on their microstructural evolution. Then, innovative 3-steps multiphase control rolling schedules have been designed to yield submicron size uniform grains structure and successfully achieved ultrafine ferrite+martensite (0.69–0.78 μm) and ferritic structure (0.83–0.88 μm), respectively, in microalloyed and IF steels. The good combination of yield strength and ductility was achieved for the microalloyed (924 MPa, 13.6% elongation) and IF steel (621 MPa, 19.4% elongation) after rolling as per the designed 3-steps multiphase control deformation schedules. Deformation induced ferrite transformation followed by continuous dynamic recrystallization of the dynamically transformed ferrite is found to be the key mechanism for the formation of the ultrafine grained structure. Due to application of high amount of strains specifically within α+γ phase regime, the α-phase subdivided into several subgrains. These α-subgrains are strongly pinned by the γ/α grain boundaries and thereby restrict the dynamic recovery of the ferrite through reknitting and unravelling subgrain boundaries. On the application of further straining, the misorientation angle between these subgrain boundaries increases continuously through accumulation of the dislocations and finally, ultrafine ferrite grain structure is developed through continuous dynamic recrystallization. Keywords: Low C microalloyed/interstitial-free steels, Flow stress, Deformation induced ferrite transformation, Continuous dynamic recrystallization, Ultrafine ferrite grains, EBSD/TEM analysis

Friction stir processing (FSP) of heat treatable aluminum alloys is reported to deteriorate strength and hardness properties in spite of huge grain size refinement. The improvement in strength due to the grain size refinement after FSP could not be able to offset the reduction in strength due to coarsening or dissolution of the strengthening precipitate(s). To overcome this, the present work investigates an effective way of heat rejection from the FSP zone to control the precipitate evolution besides the grain size refinement during FSP of Al7075 alloy. The FSP was carried out with standardized process parameters (i.e. 720 rpm, 65 mm/min traverse speed) under two different cooling environments: (i) at normal air cooling and (ii) by rushing a chilled mixture (−30 °C) of liquid nitrogen and methanol through the bottom of the backing plate. Microstructural evolution was examined through optical, scanning and transmission electron microscopy and electron backscatter diffraction to confirm the grain size and morphology of precipitate(s) formed. The in-process cryocooling during the FSP led to 65 and 72% improvement in the Vickers hardness and tensile strength, respectively, in comparison to the solutionized base metal. The results have been analyzed in terms of precipitate interface characteristics and its strengthening effect, Hall–Petch strengthening due to grain size reduction and tradeoff between them.

In the present study, the influence of cooling rates (low to ultrafast) on diffusion controlled and displacive transformation of Ti-Nb IF and microalloyed steels has been thoroughly investigated. Mechanisms of nucleation and formation of non-equiaxed ferrite morphologies (i.e., acicular ferrite and bainitic ferrite) have been analyzed in details. The continuous cooling transformation behavior has been studied in a thermomechanical simulator (Gleeble 3800) using the cooling rates of 1-150 °C/s. On the basis of the dilatometric analysis of each cooling rate, continuous cooling transformation (CCT) diagrams have been constructed for both the steels to correlate the microstructural features at each cooling rate in different critical zones. In the case of the IF steel, massive ferrite grains along with granular bainite structures have been developed at cooling rates > 120 °C/s. On the other hand, a mixture of lath bainitic and lath martensite structures has been formed at a cooling rate of ~ 80 °C/s in the microalloyed steel. A strong dependence of the cooling rates and C content on the microstructures and mechanical properties has been established. The steel samples that were fast cooled to a mixture of bainite ferrite and martensite showed a significant improvement of impact toughness and hardness (157 J, for IF steel and 174 J for microalloyed steel) as compared to that of the as-received specimens (133 J for IF steel and 116 J for microalloyed steel). Thus, it can be concluded that the hardness and impact toughness properties are correlated well with the microstructural constituents as indicated by the CCT diagram. Transformation mechanisms and kinetics of austenitic transformation to different phase morphologies at various cooling rates have been discussed in details to correlate microstructural evolution and mechanical properties.

An effort has been made to examine the high cycle fatigue (HCF) properties including crack propagation characteristics and related fracture mechanisms of submicron-grained (SG) Nb + Ti stabilized low C steel processed through advanced multiphase-controlled rolling (MCR) and multiaxial forging (MAF). The HCF and other mechanical properties have been correlated with microstructural features characterized by light optical (LOM), transmission electron (TEM) and scanning electron microscopy (SEM), aided with electron backscatter diffraction (EBSD). TEM analysis near the fracture zones of the fatigue tested samples and corresponding fractographic analysis corroborated well in explaining the improved fatigue life of the SG steel. The fatigue strength was found to have a linear relationship with the tensile strength in both types of processed samples. The fatigue strength of the forged specimens was estimated to be nearly twice than that of the untreated annealed steel, demonstrating significantly different fracture characteristics. Intergranular fracture is found to be dominant in the rolled/forged specimens, in comparison to the transgranular fracture observed in the as-received steel. Such variances in fatigue strength and fracture characteristics have been endorsed to their microstructural constituents. Superior combinations of yield strength (YS), tensile strength (UTS), elongation (% El.) and high cycle fatigue strength (σf) (YS = 1027 MPa, %El. = 8.3%, σf = 355 MPa) were obtained in multiphase-controlled 15-cycle multiaxially forged (MAFed) specimens (processed in intercritical α+γ phase regime). An enhancement of the fatigue strength can be ascribed to the formation of evenly dispersed nano-sized fragmented cementite (Fe3C) particles (~35 nm size) present in the SG ferritic matrix (average ~280 nm size). The fine dislocation substructures/cells together with the nano-sized Fe3C particles could efficiently block the initiation and propagation of cracks thereby enhancing the fatigue endurance limit of the steel. Superior mechanical properties together with high fatigue resistance in the SG material render the present steels highly beneficial for high-strength structural applications.

Present work investigates the microstructural stability during spark plasma sintering (SPS) of Fe-Cr-Y alloys, its mechanical properties and corrosion behavior for its possible applications in nuclear power plant and petrochemical industries. The SPS was carried out for the Fe-7Cr-1Y and Fe-15Cr-1Y alloys at 800 °C, 900 °C, and 1000 °C due to their superior thermal stability as reported in Muthaiah et al. [Mater Charact 114:43–53, 2016]. Microstructural analysis through TEM and electron back scattered diffraction confirmed that the grain sizes of the sintered samples depicted a dual size grain distribution with >50 pct grains within a range of 200 nm and remaining grains in the range 200 nm to 2 µm. The best combination of hardness, wear resistance, and corrosion behavior was achieved for the samples sintered at 1000 °C. The high hardness (9.6 GPa), minimum coefficient of friction (0.25), and extremely low wear volume (0.00277 × 10−2 mm3) and low corrosion rate (3.43 mpy) are discussed in the light of solid solution strengthening, grain size strengthening, grain boundary segregation, excellent densification due to diffusion bonding, and precipitation hardening due to uniformly distributed nanosize Fe17Y2 phase in the alloy matrix. The SEM analysis of the worn surface and corroded features corroborated well with the wear resistance and corrosion behavior of the corresponding samples.

Present work investigates the mechanisms of simultaneous improvement of tensile properties, wear properties and corrosion resistance of stir cast Al7075–2 wt.% SiC micro- and nanocomposites through microstructural refinement by friction stir processing (FSP). Optical, scanning and transmission electron microscopy were used to investigate the microstructural evolution. After the FSP, the nanoparticles reinforced composite showed better mechanical properties than that of the microparticles reinforced composite. Tensile strength ( >3 times) and wear resistance were found to increase significantly with simultaneous enhancement of the ductility (10 times). The improvement is ascribed to the grain size reduction, distribution of SiC nanoparticles uniformly within the matrix, increase particle-matrix interface characteristics and elimination of casting defects such as porosity after the FSP. The corrosion potentials of the as-cast composites were found to shift towards noble direction after the FSP. Enhancement of corrosion resistance after the FSP is attributed to the decrease in the heterogeneity on the surface and uniform dispersion of the reinforced particles, which reduced the effective active surface area exposed to the corrosive solution. All such beneficial effects were found to be superior for the nanoparticles reinforced composite due to the improved particle/matrix interface characteristics and dispersion strengthening.

In the present study, effect of Nb, Y and Zr (having low equilibrium solid solubility in Al) in the formation of nanocrystalline Al-4.5 wt.% Cu-1at.% X (X = Nb, Y & Zr) solid solutions by mechanical alloying and their thermal stability have been investigated. The mechanically alloyed samples were annealed in batches for 1 h at different temperatures ranging from 150 to 550 °C. The thermal stability has been studied through X-ray diffraction (XRD) phase analysis and variation of microhardness as a function of temperature. Transmission electron microscopy (TEM) and SAED analysis showed that 1 at.% Y and 1 at.% Nb could be dissolved into Al-4.5% Cu solid solutions after mechanical alloying up to 8 h of milling. The spontaneity of formation of Al-4.5%Cu Y, Al-4.5%Cu Nb and Al-4.5%Cu Zr solid solutions has been explained from the change of Gibbs free energy as per Miedema's and Toop's models. It demonstrated that the summative energy due to the reduction in crystallite size and accumulation of dislocation density exceeds the theoretical energy barrier required for the formation of the Al-4.5%Cu Y, Al-4.5%Cu Nb disordered solid solutions. The grain sizes were found to retain Cu Nb, Al3Y and Al3Y5 in Al Cu Y and Cu10Zr7, Al3Zr and Cu5Zr in Al Cu Zr alloys.

Aim of the present study is to devise simple and economical heat treatment methods to develop ultrahigh strength cast-grade microalloyed steel for industrial applications. Typical heat treatment cycles were designed on the basis of critical temperatures, Ac 3 & Ac 1 (obtained through Thermo-Calc software) and TTT/CCT curves (constructed using JMatPro software). In this work, homogenized annealed samples have been subjected to typical heat treatment cycles that consisted of repeated short-duration holding at different critical temperatures followed by forced air cooling/ice-brine quenching. The two cycles heat treatment (i.e., holding for 6 min at Ac 3 + 50 °C followed by forced air cooling + holding at intercritical zone for 20 min followed by ice-brine quenching) has resulted a significant increase in the strength with a significant amount of ductility (UTS = 1545 MPa, hardness = 446 HV, 9% elongation). This could be attributed to the development of a typical complex microstructure consisted of degenerate pearlite, fragmented cementite and finely dispersed NbC/VC in the matrix of spider network like martensite. Development of such typical microstructure has been enlightened through the analysis of divorced eutectoid transformation and diffusion controlled mechanisms. Therefore, these heat treatments techniques could be extremely useful to develop ultrahigh strength cast grade steels economically for structural/industrial applications.

An attempt has been made to investigate the high cycles fatigue behavior of the ultrafine grained (UFG) low SFE Cu-Zn and Cu-Al alloys processed through cryorolling/cryoforging followed by short-annealing. The samples (CRed Cu-9.6% Zn, CFed Cu-9.6% Zn & Cu-4.5% Al, and CRed+short-annealed (225 °C) Cu-4.5% Al), which showed better combinations of YS, ductility and fracture toughness, have been selected to investigate their fatigue characteristics. High cycle fatigue strength of the UFG alloys was found to be consistent in accordance with the static mechanical properties of the corresponding sample. For example, the CFed Cu-4.5% Al alloy samples showed superior fatigue strength along with the best combination of YS, ductility and fracture toughness as compared to that of the other samples. The improved fatigue strength of the alloys has been discussed in the light of effect of low SFE, complex and unique microstructural constituents developed by cryodeformation and role of static mechanical properties of the corresponding sample. TEM analysis after fatigue tests and fractography analysis near to fracture zones corroborated well in explaining the improvement of fatigue life of the UFG alloys.

Aim of the present study is to investigate mechanical properties and fracture toughness of low SFE ultrafine grained (UFG) Cu-4.5 wt% 1 Al alloy processed through multiaxial cryoforging followed by short-annealing. The homogenized annealed samples (800 °C, 4 h) were cryoforged for 5, 9 and 11 cycles. The 11-cycles cryoforged sample showed more than 10 times improvement in the yield strength (YS) (818 MPa) as compared to that of the homogenized annealed sample (76 MPa). However, the ductility decreased to 10.1% elonagtion due to suppression of dynamic recovery and recrystallization during cryoforging. Cryoforging followed by short-annealing (225–300 °C for 20 min) was found to enhance the ductility without affecting its YS. Fracture toughness study has been carried out through analysis of apparent fracture toughness ( K Q ) using linear elastic fracture mechanics, equivalent energy fracture toughness ( K ee ) and J -integral values using elastic plastic fracture mechanics from 3-point bend test data. The improvement of the YS with significant ductility as well as fracture toughness is ascribed to the formation of ultrafine grains & rhombic blocks (due to the intersection of nanotwins close to 90°), mutual crossing & misorientation of microshear bands and multisystem deformation twins. The low SFE of the alloy is found to play the pivotal role to develop such microstructural features.

Aim of the present study is to investigate the effect of process parameters on microstructural evolution, mechanical properties and corrosion behavior of an age-hardenable Al 7075 alloy. The alloy plates (6 mm thickness) were friction stir processed (FSPed) at various traverse speed, namely 25, 45, 65, 85, 100 and 150 mm/min at 2 different rpm of 508 and 720. The optimized result in terms of defect-free processed zone with refined microstructure was obtained only at a rotational speed of 720 rpm for a traverse speed of 25, 45, 65 and 85 mm/min. The microstructural evolution was investigated using optical, scanning and transmission electron microscopy. The grain size of the nugget zone was found to decrease with increase in the traverse speed from 25 to 85 mm/min at a constant rpm of 720. The mechanical properties were evaluated by Vickers hardness measurements, tensile and wear testing. Yield strength was found to be the maximum (~366 MPa) for the FSPed sample processed at 85 mm/min. The hardness values also followed the similar increasing trend with increase in the traverse speed. The wear volume loss decreased by 38% for the sample processed at a traverse speed of 85 mm/min as compared to that of the sample processed at 25 mm/min. The friction coefficient was found to substantiate well with the wear track morphology. The improvement in mechanical properties is ascertained to the refinement of grain size at higher traverse speed (due to less heat input). The FSPed samples showed inferior corrosion resistance in contrast to that of the base metal. This is possibly due to the coarsening of precipitates and depletion of solutes in the matrix. The morphology of the corroded samples corroborated well with the corrosion behavior of the corresponding specimen.

Aim of the present study is to examine the possibilities of achieving a better combination of yield strength (YS), ductility and formability with a better corrosion resistance of a Ti-Nb stabilize IF steel through microstructural refinement by simple phase controlled thermomechanical rolling. The phase controlled thermomechanical rolling could be an industrially reliable method for the production of hot strips, which can substitute cold rolled steel sheets. Accordingly, the phase controlled multi-pass rolling was performed in 3 selected phase regimes (γ-recrystallization, γ→α transition & α-regions) on the basis of Ac 3 /Ar 3 , Ac 1 /Ar 1 (obtained through Thermo-Calc & Gleeble-3800) and T nr (from Boratto equation) followed by air cooling. The volume fraction of precipitate(s) correspond to the deformation temperature was estimated using Thermo-Calc Software and the morphology of the precipitates was analyzed by TEM. The strain induced phase transformation of unstable γ occurred during rolling at a high reduction of ~80%, ɛ=1.6 at γ→α transition region. Thus, dynamically recovered stable bimodal equiaxed ferrite structures (fine ferrite ~5 µm embedded with larger size ferrite grains ~32 µm) were obtained after air cooling to room temperature. In case of the rolling at α-region, improvement of the YS (>3-fold) is attributed to the formation of ultrafine ferrite grains (1–3 µm) through subgrain structures, strain-induced precipitation of nanosize NbC and/or TiC and micro-shear bands. Very short-annealing (~100 s) at 850 °C followed by forced air cooling was employed in order to simulate continuous annealing process and was found to improve the formability without much affecting its YS. The avoidance of FeTiP phase formation (which deteriorates to form {111} recrystallization texture) and nucleation of ferrite grains within the deformation bands (studied through EBSD study) by a short-annealing treatment are accountable for regaining the formability. The role of strain hardening exponent ( n ) and plastic-strain-ratio ( r ) on the deformation characteristics of the thermomechanically treated IF steel were also investigated to correlate the YS and uniform elongation. Furthermore, the rolled (at α-region) + short-annealed samples showed an excellent corrosion resistance due to the formation of dense oxide film on the surface. This is attributed to the dissolution of Fe(Ti+Nb)P precipitates (which are the potential sites for initiation of pits), and formation of fine grains (which facilitate to form dense oxide film on the large surface area).

The present work aims to investigate (i) the feasibility of formation of Fe-7Cr-xZr and Fe-15Cr-xZr (x = 0.25, 0.5 and 1 at%) disordered solid solutions by mechanical alloying (MA) and (ii) their thermal stability and mechanical properties of spark plasma sintered (SPSed) samples. Effect of Zr addition on phase evolution in Fe-Cr-Zr alloy and its influence on their thermal stability and mechanical properties (microhardness and compression strength) have been studied in detail. Transmission electron microscopy-selected area diffraction pattern (TEM-SAED) and X-ray diffraction (XRD) phase analysis confirm the formation of complete solid solution after 25 h of MA. Grain size was observed to stabilize significantly within nanometer range after annealing at 600–1200 °C. The SPSed (at 1000 °C) samples showed the best combination of hardness (9.4 GPa) and compressive strength (ultimate compressive strength-2200 MPa and yield strength-1800 MPa) corresponding to an average grain size of

The present research aims to develop high strength Al7075–SiC nanocomposites by integration of stir casting and cryo-FSP. Al7075 alloy based nanocomposites reinforced with varying wt.% (2%, 3% and 5%) of nanosize SiC particles were produced through the bottom pouring stir casting technique. The cryo-FSP was successfully carried out using optimized parameters (i.e. 1025 rpm and 65 mm/min traverse speed) on as-cast samples. To understand the fundamentals of the deformation characteristics and microstructural evolution, a systematic study on microstructural evolution has been performed by SEM with EBSD and TEM. The mechanical properties were evaluated by Vickers hardness and tensile testing. Improvement in the mechanical properties is found to be the maximum for the Al7075-3 wt% SiC nanocomposite sample obtained through cryo-FSP (TS = 552 MPa, and %El. = 17%) as compared to other samples studied (Al7075 alloy/composites). Secondary processing through cryo-FSP not only refined the matrix grain size to 1–3 μm range, but also suppressed the coarsening of the strengthening η′ phase due to higher cooling rate. Moreover, the reinforced nanosize SiC particles distributed uniformly within the matrix thereby enhancing the particle/matrix interface characteristics. These modifications in microstructure drastically enhance the strength (YS, hardness) and ductility of the material simultaneously. The detailed strengthening mechanisms have been analyzed and correlated with the mechanical properties.

In the present study, a series of Al-x% Zr (x = 1–10 at.%) compositions were prepared by mechanical alloying (MA) to investigate the solid solubility extension of Zr in Al and its thermal stability. The elemental powder blends were mechanically alloyed under high purity argon atmosphere in stainless steel grinding media using SPEX 8000 M high energy ball mill. The milling was carried out for 8 h at room temperature and the ball to powder ratio was maintained at 10:1. Formation of disordered solid solutions is validated using Miedema’s semi-empirical model. X-ray diffraction (XRD) analysis confirms the formation of disordered solid solution up to 1at.% Zr; whereas, Al3Zr and Al9.83Zr0.17 intermetallic phases were found to form as per the XRD pattern of 2–10% Zr alloys. Variation of lattice parameter confirmed the formation of Al-1% Zr solid solution. Crystallite size was estimated to be 41 and 30 nm, respectively, for the as-milled 1 and 10% Zr alloys. The matrix grains were found to be stabilized after annealing at 550 °C, and the XRD crystallite size of Al-10% Zr retained at ∼58 nm. TEM and AFM analysis confirmed that the grain size of the as-milled as well as annealed samples was retained in the nanometre range (

Aim of the present study is to investigate mechanical properties and fracture mechanisms of low stacking fault energy ultrafine grained Cu-9.6 wt% Zn brass processed by multiaxial forging (MAF) at cryogenic temperature. The alloy was developed by metal mold casting followed by homogenized annealing at 800 °C for 4 h before cryoforging. The forging was carried out for 5, 9 and 12 cycles which are equivalent to cumulative strains of 3.0, 5.4 and 7.2, respectively. The brass deformed with a cumulative strain of 7.2 showed more than 9 times improvement in the yield strength (YS=680 MPa) compared to that of the homogenized annealed sample (70 MPa). However, the ductility decreased to a very low range after deforming to 12 cycles because of extreme work hardening at the cryogenic temperature due to suppression of dynamic recovery. Cryoforging followed by short-annealing (225–300 °C for 20 min) was found to be effective to enhance the ductility without much affecting its YS. Formation of ultrafine grains along with nanotwins (confirmed by TEM) results a simultaneous improvement in the YS and ductility in the cryoforged+short-annealed samples. Macroscopic tensile fracture mechanisms and fractography analysis were carried out for the cryoforged and cryoforged+short-annealed samples. The unified tensile fracture criterion (Ellipse criterion) in terms of modification of the stress state in the fracture zone suggested that reduction of necking angle is resulted from the geometrical hardening effect due to the change of stress state. As number of forging cycles increases, which correspondingly increases critical normal stress (σo) and shear stress (τo), the tensile shear fracture angle decreases indicating in the increase of shear mode fracture. This is ascertained to the grain size refinement, which increases the hardness of the material. Fractography analysis also highly corroborated with the analysis of the unified tensile fracture criterion and %elongation. The dimple size as well as %elongation was found to gradually decrease with decrease in the shear fracture angle, which is equivalent to increase in the cumulative strain.

The effect of Zr (up to 1 at.%) addition on the formation of Fe–Zr metastable alloys and their thermal stability were investigated for their possible nuclear applications. Fe–xZr (x = 0.25, 0.5, 1%) alloys were synthesised by mechanical alloying under a high-purity argon atmosphere using stainless steel grinding media in a SPEX 8000M high energy mill. The milling was conducted for 20 h with a ball-to-powder weight ratio of 10:1. The formation of metastable solid solutions after milling was confirmed from the change in the Gibbs free energy analysis as per Miedema’s model. The microstructural characterisation was carried out by analysis of X-ray diffraction, atomic force microscopy and transmission electron microscopy. The effect of Zr on the thermal stability of Fe–Zr alloys was investigated by extensive annealing experiments followed by microstructural analysis and microhardness measurements. The stabilisation was found to occur at 800 °C and thereafter, no significant change in the crystallite s...

Many attempts have been made to model and optimize performance parameters of Electrical Discharge Machining (EDM) process. In this investigation, the experimental design includes L9 orthogonal array based on the Taguchi methodology. The corresponding effect of input current, duty cycle and spark on time, on surface integrity, has been studied on AISI D2 tool steel. The residual stress measurement is done using X-ray diffraction method. The obtained output results regarding material removal rate, surface roughness, crack density, and residual stress have been modeled and optimized by regression equation and Genetic algorithm respectively. In this investigation, a metallurgical approach has been introduced towards the explanation of the resulted optimized response parameters.

Cu-2% Al and Cu-4.5% Al alloys were developed by metal mould casting followed by homogenization at 800 °C for cryorolling. The homogenized samples were cryorolled (CR) up to the maximum possible reduction in area (~ 75%). Activation energy for recrystallization of the CR samples was estimated by isoconversion methods using differential scanning calorimetry data; whereas, stored strain energy was determined by X-ray diffraction analysis. The activation energy for recrystallization correlated well with the stored energy of the corresponding sample. Microstructural evolution was analyzed by optical and transmission electron microscopy. The CR + annealed (225 °C) sample showed an improved yield strength (YS) of 810 MPa with a reasonable ductility of 5.1%. The YS found to be 10 times higher than that of the cast + homogenized sample (76 MPa). This is attributed to the recovery of low angle grain boundaries, increasing grain boundary spacing, formation of nanotwins and decrease in the dislocation density without any recrystallization. The YS obtained by analytical modeling of the active strengthening mechanisms is highly corroborated with the experimental YS of the annealed sample. The Hall-Petch strengthening and dislocation strengthening found to play the pivotal role in the improvement of mechanical properties.

In the present study, evolution of microstructure during multiaxial forging (MAF) of a Nb–Ti stabilized IF steel and its mechanical properties have been investigated. The forging schedule was designed on the basis of critical temperatures Ar3, Ar1 (evaluated from dilatometric curve through thermomechanical simulator) and recrystallization stop temperature, Tnr (determined from Boratto equation). MAF was performed for 5 cycles in 3 different phase regimes; in pure γ-region (1050 °C), γ → α transformation zone (800 °C) and pure α-region (650 °C). The deformed samples were cooled by normal air cooling. EBSD and optical microscopy investigation confirmed the formation of fine ferrite grains (~5 μm) due to strain induced transformation of unstable γ at 800 °C and ultrafine ferrites (~1 μm) through subgrains formation at pure α-ferritic region at 650 °C. The specimen forged in pure α-region showed a 4-fold improvement of yield strength (YS) compared to that of the starting material (141 MPa) without much interfering its ductility (25%). This is ascertained to the development of bimodal grain structures and formation of ultrafine carbide precipitates which were confirmed by EBSD and TEM analysis. The theoretical YS was estimated through analysis of different strengthening mechanisms and found to be highly corroborated with the experimentally obtained result. Keywords: Interstitial free steel, Thermomechanical simulation, Critical temperatures, Multiaxial forging, Mechanical properties, Transmission electron microscopy (TEM)

Aim of the present study is to investigate the feasibility of formation of Fe-Cr-Y disordered solid solutions by mechanical alloying and effect of Y on the thermal stability and mechanical properties of such nanocrystalline alloys. Thermodynamic analysis by Miedema's and Toop's models confirms that the energy barrier required to form the disordered solid solutions has been overcome by the stored energy due to strain dislocations and grain boundary defects. Although limited grain growth was observed during annealing of metastable Fe-15Cr-1Y alloy, the grains size found to stabilize at ~ 53 nm after annealing at 1000 °C; and the corresponding hardness value measured to be also quite high (8 GPa). The grain size analysis by TEM and AFM is well-corroborated with the XRD crystallite size. The high thermal stability and large strengthening effect have been discussed in the light of grain boundary pinning by solute segregation, solute drag effect and Zener pinning due to intermetallic phase(s).

The aim of the present study is to simultaneous improvement of strength and ductility as well as corrosion resistance of ultrafine grained 2024 Al-alloy processed by multiaxial cryoforging (MAF) and cryorolling followed by ageing. The evolution of ultrafine grained microstructure during MAF followed by ageing is investigated using optical and transmission electron microscopy. Both multiaxially forged (MAFed) and cryorolled (CRed) samples showed an improvement in yield strength (YS) with a corresponding decrease in the ductility. Aging treatment not only improved the YS, but also its ductility. Improvement in the ductility after ageing is confirmed by the fractography analysis. Corrosion resistance of the MAFed+aged samples found to be higher compared to that of the MAFed and coarse grained counterpart. The corrosion behavior has been analyzed in the light of open circuit potential (OCP), solutionizing, grain size and precipitation strengthening mechanisms. SEM images of the corroded samples also corroborated the corrosion test results.

Deformation characteristics of a Ti + Nb stabilized IF steel have been investigated in the present study to clarify the associated strain hardening and/or dynamic restoration mechanisms. Single-hit hot compression experiments were performed in a Gleeble thermomechanical simulator in the temperature range 650–1100 °C at various constant true strain rates in the range 10−3-10 s−1 to a true strain of 0.7. The ‘safe’ and ‘unsafe’ deterministic domains were identified through the construction of processing maps and verification of mechanisms through the examination of evolved microstructures, regardless of the phase type (austenite and/or ferrite). Among different materials models employed, the ‘safe’ workable region at austenite and ferrite phase, predicted by a modified dynamic materials model (DMM) superimposed with Poletti instability criteria has been identified to be appropriate in accord with the experimental data. Development of a fully recrystallized fine equiaxed ferrite grain structure is ascribed to the occurrence of dynamic recrystallization (DRX). The characteristics of the onset of DRX mechanism have been further analyzed in terms of critical strain and stress required. Moreover, a kinetic model has been established to evaluate the fractional transformation through DRX. In addition, industrial optimization of hot working schedule of the IF steel could be ascertained from the processing map.

Si-Mo alloy was infiltrated into porous graphite to develop a short-term low-ablative SiMo/graphite composite material. 19.4 vol.% residual Si was retained in the composites and significantly improved the short-term (less than 60 s) ablation resistance without sacrificing long-term (∼180 s) ablation resistance. The excellent short-term low-ablative performance is ascribed to the melt and evaporation of Si, which reduces the rate of temperature rise and supplies the consumption of SiO2 in the ablation surface. With the progress of ablation time, the ablation surface forms a SiO2-MoSi2 double layer, which acts as a barrier between the oxyacetylene flame and the ablation surface.

Aim of the current study is to analyze the fracture toughness values along with other mechanical properties and correlating the microstructures of ultrafine grained (UFG) microalloyed and interstitial free (IF) steels produced through advanced 3-steps control multiphase rolling. The analysis of fracture toughness was carried out through computing KQ (conditional fracture toughness), J-integral (crack initiation energy) and Kee (equivalent energy fracture toughness) values from 3-point bend test data of rolled specimens. Microstructural analysis was performed through transmission electron microscopy (TEM) along with selected area electron diffraction (SAED) and Electron backscatter diffraction (EBSD). The quantitative measurement of low and high angle grain boundaries and their distribution in the deformed state were determined through EBSD analysis. The good combinations of fracture toughness, yield strength (YS) and percent elongation (%El.) (i.e. ductility) were achieved through innovative 3-phase control rolling (microalloyed steel: Kee = 68.9MPa√m, J = 81.4 kJ/m2, YS = 923MPa, %El. = 13.6; IF steel: Kee = 72MPa√m, J = 87.7 kJ/m2, YS = 623Mpa and %El. = 19). This is ascribed to the development of homogeneously distributed submicron size (0.69μm) ferritic + martensitic structure in the microalloyed steel and submicron size (0.83μm) ferritic grains along with high density dislocation substructure in the IF steel. These dislocation cells and substructures could effectively block the crack initiation and propagation. The development of UFG microstructure has been analyzed in the light of deformation induced ferrite transformation (DIFT) and dynamic recrystallization (DRX) mechanisms. Superior fracture toughness of the UFG steels along with better combination of mechanical properties is very demanding for high strength structural applications.

Objective of the current study is to enhance the mechanical properties, with a special emphasis on fracture toughness, of Ti + Nb stabilized interstitial free and microalloyed steels through microstructural modification by single-phase controlled multiaxial forging at large cumulative strains. Analysis of fracture toughness was executed through calculating KQ (conditional fracture toughness), Kee (equivalent energy fracture toughness) and J-integral (crack initiation energy) values from single-edge bend test data of the forged specimens. The effect of strain hardening rate and strain hardening exponent on deformation behavior were examined to correlate the yield strength (YS) and uniform elongation. Also, theoretically calculated YS (obtained from analysis of strengthening mechanisms) was correlated well with the experimentally obtained results. The quantitative measurement of grain size, low- and high-angle grain boundaries and their distribution in the deformed state were investigated through EBSD/TEM analysis. Superior combinations of the YS, ductility (%El.) and fracture toughness were obtained through intercritical (α+γ) phase regime (~Ar1) control 15 cycles multiaxially forged (MAFed) microalloyed steel (YS = 1027 MPa, %El. = 8.3% and Kee = 90 MPa√m) and pure α-ferritic region (

In the present study, contribution of individual strengthening mechanisms has been analyzed to corroborate the improvement of the mechanical properties of cryorolled brasses of Cu–3.8% Zn and Cu–9.6% 1 Zn compositions. The samples were cryorolled up to the maximum possible reduction in area after homogenized annealing (at 800 °C for 4 h). After the cryodeformation, a remarkable ~ 12 times improvement of yield strength (YS) was observed for both the alloys compared to that of the homogenized samples. The improvement of the YS is found to be much higher compared to that reported in our earlier work [Mater. Des 67 (2015) 637–643], although alloying content is much less in the present study. This is attributed to the change in rolling design, which allowed deformation of the samples under LN 2 during rolling. The short time ageing (20 min, 225–300 °C) followed by cryorolling was found to enhance ductility without affecting the YS significantly. The solid solution strengthening by Zn in Cu leads to the decrease in the stacking fault energy which prevents the dynamic recovery and restricts the process of cross slip. Thereby, it helps to increase twining activity for further deformation. The TEM investigation confirmed that the formation of subgrains and nanotwins is responsible for the simultaneous improvement of strength and ductility. The analysis of different strengthening mechanisms revealed that the grain size refinement played the pivotal role in the improvement of the mechanical properties.

Aim of the present study is to investigate the effect of deformation temperature and cooling rate on microstructural features and mechanical properties of Nb–Ti stabilized microalloyed steel. Rolling schedule in 3 different phase regimes (γ-recrystallization region at Tnr+50 °C, γ-nonrecrystallization region at Tnr−50 °C and (α+γ) region) was designed on the basis of critical temperatures, Ar3 and Ar1 (obtained from dilatometric study, Gleeble-3800), and Tnr (determined from Boratto equation). The combination of high yield strength (YS) and ductility of the forced air-cooled (FAC) and quenched specimens rolled in (α+γ) region is attributed to the high misorientation angles of matrix, formation of subgrain ferrite (~2 µm)+larger ferrite (~35 µm) and precipitation of NbC (

In the present study, an attempt has been made to synthesize copper based disordered solid solutions by mechanical alloying (MA) of non-equilibrium compositions. The blended compositions of Cu–1% Y, Cu–3% Y, Cu–5% Y and Cu–7.5% Y (at.%) (all the compositions will be addressed as % only hereafter until unless it is mentioned) were ball-milled for 8 h, and then annealed at different temperatures (200–800 °C) for different length of duration (1–5 h) under high purity argon + 2 vol.% H 2 atmosphere. X-ray diffraction (XRD) analysis and Gibbs free energy change calculation confirm the formation of disordered solid solution (up to 7.5%) of Y in Cu after milling at a room temperature for 8 h. The XRD grain size was calculated to be as low as 7 nm for 7.5% Y and 22 nm for 1% Y alloy. The grain size was retained within 35 nm even after annealing for 1 h at 800 °C. Transmission electron microscopy (TEM) analysis substantiates the formation of ultra-fine grained nanostructures after milling. Microhardness value of the as-milled samples was quite high (3.0–4.75 GPa) compared to that of pure Cu. The hardness value increased with increasing annealing temperatures up to 400 °C for the alloys containing 3–7.5% Y, and thereafter it showed a decreasing trend. The increase in the hardness after annealing is attributed to the formation of uniformly distributed ultrafine intermetallic phases in the nanocrystalline grains. The stabilization effect is achieved due to segregation of Y to reduce the grain boundary energy to zero and hindrance of dislocation movement due to precipitation of intermetallic phases. The tensile yield strength ( σ y ) of the HPT consolidated nc Cu–1% Y and Cu–3% Y alloys was found to be at least one order of magnitude higher than that of the course grained-Cu counterpart, and the corresponding UTS ( σ u ) value was more than four times higher. The strengthening effect is discussed in the light of the grain size refinement, solid solution strengthening and strain hardening.

The mechanical properties and microstructural characteristics of ultrafine grained low stacking faulty energy (SFE) brass processed by cryorolling were investigated in the present work. The commercial brass with 18 wt.% Zn was subjected to cryorolling to obtain specimens with different percentage of reduction in area (RA). Short time post-processing annealing was carried out for the specimens with maximum RA (90%) to enhance their ductility. The mechanical properties of all the specimens were assessed by tensile tests and hardness measurements. Microstructural analysis was carried out by optical microscopy, X-ray diffraction (XRD), atomic force microscopy (AFM) and electron microscopy (EM). The maximum yield strength (YS) of 600 MPa with 2.1% ductility was obtained for the cryorolled samples with 90% RA. The YS decreased to 452 MPa with a corresponding increase in the ductility (10%) after annealing at 225 °C. The YS of the cryorolled + annealed sample is found to be 465% higher compared to that of the as-received specimens (YS = 80 MPa). Fractography analysis of the 90% rolled specimens showed a brittle fracture; while, presence of dimples marks on the fractured surface of the annealed specimens indicated a ductile failure. The low SFE of the alloy plays a vital role on the deformation mechanisms during cryorolling and simultaneous improvement of the YS and ductility. Hence, improvement in the mechanical properties has been discussed in the light of refinement of microstructure, formation of sub-grains and nano-twins driven by the low SFE.

A dense SiC–Si coating with average thickness of 580 μm was prepared on graphite by pack cementation. The surface morphology evolution and ablation mechanism of the coating at about 1780 °C were investigated by oxy-acetylene flame. After applying SiC–Si coating, the ablation surface temperature dropped about 300 °C. The ablation products SiO2 presented high viscosity and provided good protection in the early-to-middle stage of the ablation. The mechanical denudation gradually became severe with the ablation time increasing. Finally, the convection between the gases formed in the subsurface and the ablative gases turned out to be the primary ablation barrier.

In the present investigation, we report a simple and new technique to synthesize silicon carbide nanowires by high energy ball milling of silicon and carbon powders followed by annealing at elevated temperatures. The detailed structural analysis was carried out by X-ray diffraction, scanning and transmission electron microscopy, and FT-IR analysis. Nanocrystalline silicon particles were detected to be covered by amorphous carbon after milling. This typical structure was transformed to β-SiC nanowires during annealing by new growth mechanism. The β-SiC nanoparticles were found to be self-assembled one by one and formed a curly chain, and then rotated gradually in accordance to the same crystal orientation along the β-SiC [1 1 1] direction.

The blend compositions of Cu 99 Cr 1 (All the compositions are in atom% until otherwise mentioned.), Cu 94 Cr 6 , Cu 99 Cr 1 –4 wt.% SiC (average particle size ∼30 nm) and Cu 94 Cr 6 –4 wt.% SiC were ball-milled for 50 h in a stainless steel grinding media. The structural investigation and phase evolution during milling of the same compositions have already been reported in elsewhere ( [28] ). In the present study, we are reporting effect of microwave sintering on mechanical properties and electrical conductivity over vacuum and conventional sintering carried out at 900 °C. Relative densification and grain coarsening characteristics were investigated and compared. The microstructural features were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The mechanical properties, namely, Vickers hardness and wear resistance, and electrical conductivity were studied to investigate the suitability of the materials for electrical contact applications. The best combination of mechanical properties and electrical conductivity was obtained for microwave sintered specimens. This is possibly due to the enhanced densification and better grain size distribution achieved in microwave sintering technique. Difference in the properties is discussed in the light of electron scattering factor and densification in presence of ultra fine SiC particles in the nanocomposites.

Nanocrystalline copper (Cu) was generated by cryogenic, high-energy ball milling. Antimony (Sb) was added to investigate its utility in stabilizing the grain structure during annealing up to a maximum temperature of 1073 K (800 °C). When alloyed with Sb in quantities up to 1 at. pct, thermal stability was maintained up to 673 K (400 °C). Cu and Sb have very different molar volumes which can drive segregation of the solute due to the elastic strain energy and hence stabilize the grain size by reducing grain boundary energy. The elastic mismatch of Sb in Cu is calculated to be quite large (113 kJ/mol) when molar volume is used, but when an equivalent equation using atomic radius is applied, the driving force is nearly an order of magnitude lower (~12 kJ/mol). The low elastic mismatch is corroborated by the large equilibrium solubility of Sb in Cu. The results for the Cu-Sb system are compared to the nanocrystalline Ni-W system and the large amount of equilibrium solubility of the solute in both cases is thought to hinder thermal stabilization since segregation is not strongly favored.