A single phase fcc based nanocrystalline solid solution in equiatomic AlCoCrCuFeNi high-entropy alloy (HEA) has been synthesized using ball milling. The milled powders were of “plate-like” morphology ...and possessed a precise lattice parameter of 3.641 Å. Compaction of ball milled powders into bulk components using spark plasma sintering (SPS) at 1023 K led to the precipitation of ordered bcc (B2). Detailed structural and microstructural investigations on the sintered alloy indicate the presence of bimodal grain size distribution with average grain sizes of 112 ± 46 nm and 1550 ± 500 nm, solid solutions (fcc and B2 phases), dislocations and twin boundaries. A high hardness value of 6.5 ± 0.1 GPa was measured for the sample sintered at 1023 K/15 min using Vickers microindentation. Comprehensive analysis on probable strengthening mechanisms suggests that frictional stress, Taylor hardening, Hall-Petch strengthening, solid solution strengthening and twin boundary strengthening mechanisms are responsible. The Taylor hardening arising from intersection of dislocations and grain boundary (Hall-Petch) strengthening arising from grain boundary-dislocation interactions together account for 85% of the observed flow stress. The Tabor's ratio, (H/σflow) attained a value of 2.7 which is in close agreement with that for conventional polycrystalline materials. Nanoindentation at a peak force of 8000 μN yielded a high hardness value of 8.13 ± 0.15 GPa and an elastic modulus of 172 ± 10 GPa. A low strain rate sensitivity of 0.0084 and an activation volume of 13 b3 (b is 0.23 nm) were measured, suggesting that grain boundaries, twin boundaries and interphase boundaries (fcc/B2) are influential in governing the deformation kinetics.
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In this paper, we report a remarkably high hardness in fully-dense nanocrystalline multi-phase MoNbTaTiW based refractory high-entropy alloy which was prepared by ball milling and spark plasma ...sintering. A single phase BCC structure was realized after 30 h of milling whereas sintering led to the decomposition of the as-milled complex lattice into a BCC and two FCC phases. Vickers microindentation performed at various loads led to the observation of pronounced indentation size effect with hardness in the range 18.87-13.89 GPa. It appears that Ti and Fe (from processing media) are responsible for the multi-phase structure realized as well as extraordinarily high absolute hardness (13.89 GPa at 500 g load) and “density-normalized hardness”. These values are the highest reported so far in the family of MoNbTaW alloys. Comprehensive analysis on strengthening mechanisms responsible for the extraordinarily high hardness observed in this alloy indicates that solid solution strengthening and grain boundary (Hall-Petch) strengthening are the dominant factors while lattice frictional stress and Taylor hardening also contribute to some extent. The strain gradient plasticity appears to follow the considerations based on the concept of geometrically necessary dislocations and the characteristic multi-phase structure of this alloy. The back stress that would develop in order to have compatible deformation during the indentation at low loads is also expected to contribute to higher hardness values observed at low indentation loads. The depth-independent hardness of 11.80 GPa and a characteristic length scale of 1.07 μm were realized from the analysis. Achieving of ultra-high hardness in this alloy suggests that careful microstructural engineering as well as optimal addition of alloying elements would be beneficial towards development of novel structural materials based on multi-principal element approach.
Reduced Activation Ferritic-Martensitic (RAFM) steels are currently being considered for test blanket modules of International Thermonuclear Experimental Reactor. This study is aimed at understanding ...the microstructure and mechanical properties of Indian RAFM steel during friction stir welding (FSW) of 6 mm thick plates. The full penetration bead-on-plate welds were fabricated employing PcBN tool at a rotational speed of 200 rpm. The FSW joint consisted of stir zone, thermomechanically affected zone and BM. Microstructure, hardness and tensile properties were evaluated in as-received (base metal, BM), as-welded and post weld normalised + tempered states. The as-received microstructure was composed of Cr-rich M23C6 on prior austenite grain and tempered lath martensite boundaries with intra-lath V-and Ta -rich monocarbides. Substantial changes occurred during welding leading to destruction and dissolution of M23C6 and precipitation of Fe3C in SZ. The microstructure was restored to the BM level by giving a post weld normalising and tempering treatment. Comparative study on tensile properties has been carried out at room and elevated temperatures (up to 550 °C) in all the three states and the observed variations have been explained on the basis of initial microstructure and evolving substructures. The as-welded samples showed higher strength and low percentage elongation values. The ductility was minimum in all the states at an intermediate temperature and ascribed to the dynamic strain ageing. All the states revealed falling strength with increasing temperature up to 550 °C. Failure occurred in AW and PWNT states at the interface between TMAZ and BM. Irrespective of material state and tensile test temperature, the transgranular ductile fracture prevailed. The contribution of the different strengthening mechanisms to the yield strength of the SZ region has been estimated theoretically and it matches with the experimental results. The influence of low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB) on the tensile behaviour of the SZ was well explained using the Electron Back Scattered Diffraction (EBSD) maps of these regions.
The microstructure and mechanical properties of electron beam welded joints of reduced activation ferritic-martensitic steel in the as-welded and post-weld heat treatment (PWHT) states have been ...explored. The as-received base metal (BM) was in normalised and tempered condition. The PWHTs employed include post-weld direct tempering (PWDT) at 760°C/90min/air cooling and (ii) re-austenitizing at 980°C/30min/air cooling+ tempering at 760°C/90min/air cooling (PWNT). The BM microstructure was composed of fully tempered lath martensite with prior austenite grain and martensite lath boundaries decorated with M23C6 type carbides whereas intra-lath regions majorly displayed MX type carbides. In the as-welded state, the fusion zone (FZ) contained martensite in coarse grains and small amount of δ-ferrite with no evidence for precipitation of M23C6 and MX either in intra- or inter-granular regions. The heat affected zone (HAZ) was made up of martensite in fine grains without any δ-ferrite and with subtle variations in microstructure across the HAZ. The as-welded joints exhibited high hardness in the FZ and HAZ due to the occurrence of martensite during the weld thermal cycle. The impact toughness of the as-welded joint was inferior compared to that of the BM due to the combined influence of the martensite, coarse grains and presence of δ-ferrite in the weld zone. Tensile strength of as-welded joint was higher than that of BM. PWHTs were beneficial in decreasing the hardness in the FZ and HAZ. PWDT could not fully eliminate the pronounced variation of hardness observed in the transverse section of welded joint. Though the impact toughness of the weld joint was improved marginally compared to as-welded state after PWDT, it was much lower than that recorded in the case of BM. PWNT treatment minimised the variation in hardness across the transverse section of weld joint and the impact toughness surpassed than that achieved in BM. The tensile properties of BM, welded joints in as-welded and in PWHT conditions were determined at room temperature and correlated with the prevailing microstructures.
This paper presents the results on the physical metallurgy studies in 9Cr Oxide Dispersion Strengthened (ODS) and Reduced Activation Ferritic/Martensitic (RAFM) steels. Yttria strengthened ODS alloy ...was synthesized through several stages, like mechanical milling of alloy powders and yttria, canning and consolidation by hot extrusion. During characterization of the ODS alloy, it was observed that yttria particles possessed an affinity for Ti, a small amount of which was also helpful in refining the dispersoid particles containing mixed Y and Ti oxides. The particle size and their distribution in the ferrite matrix, were studied using Analytical and High Resolution Electron Microscopy at various stages. The results showed a distribution of Y
2O
3 particles predominantly in the size range of 5–20
nm. A Reduced Activation Ferritic/Martensitic steel has also been developed with the replacement of Mo and Nb by W and Ta with strict control on the tramp and trace elements (Mo, Nb, B, Cu, Ni, Al, Co, Ti). The transformation temperatures (
A
c1,
A
c3 and
M
s) for this steel have been determined and the transformation behavior of the high temperature austenite phase has been studied. The complete phase domain diagram has been generated which is required for optimization of the processing and fabrication schedules for the steel.
In the postweld heat-treated (PWHT) fusion welded modified 9Cr-1Mo steel joint, a soft zone was identified at the outer edge of the heat-affected zone (HAZ) of the base metal adjacent to the ...deposited weld metal. Hardness and tensile tests were performed on the base metal subjected to soaking for 5 minutes at temperatures below Ac^sub 1^ to above Ac^sub 3^ and tempering at the PWHT condition. These tests indicated that the soft zone in the weld joint corresponds to the intercritical region of HAZ. Creep tests were conducted on the base metal and cross weld joint. At relatively lower stresses and higher test temperatures, the weld joint possessed lower creep rupture life than the base metal, and the difference in creep rupture life increased with the decrease in stress and increase in temperature. Preferential accumulation of creep deformation coupled with extensive creep cavitation in the intercritical region of HAZ led to the premature failure of the weld joint in the intercritical region of the HAZ, commonly known as type IV cracking. The microstructures across the HAZ of the weld joint have been characterized to understand the role of microstructure in promoting type IV cracking. Strength reduction in the intercritical HAZ of the joint resulted from the combined effects of coarsening of dislocation substructures and precipitates. Constrained deformation of the soft intercritical HAZ sandwich between relatively stronger constitutes of the joint induced creep cavitation in the soft zone resulting in premature failure. PUBLICATION ABSTRACT
In the present paper, influence of hold time on the high temperature low cycle fatigue (LCF) behavior of modified 9Cr–1Mo ferritic steel in the normalized and tempered condition is addressed. Total ...axial strain controlled LCF tests at 873
K with hold times at peak strain up to 10
min in tension and compression were carried out employing total strain amplitude of ±0.6%. The alloy in general, showed a gradual and continuous softening regime. Fatigue life was found to decrease with increase in the duration of hold time in both tension and compression. Compression hold was found to be more damaging than the tension hold. The fatigue failure in compression hold tests were marked by extensive crack branching and formation of secondary cracks. Oxidation-assisted crack initiation and propagation contributed to life reduction at high temperatures in hold time tests. Additionally, the substructural changes of the starting microstructure were also found to be responsible for the reduction of the fatigue life. The combined effect of cyclic deformation and elevated temperature on the evolving microstructure have been evaluated. The application of hold during cycling at elevated temperature accelerates the conversion of initial heavily dislocated lath structure to equiaxed cells with low dislocation density and coarse carbides.
•Effects of various sacrificial layers on the surface characteristics of SS316LN are examined.•Influence of laser energy and pulse width have been comprehensively studied.•Laser induced texturing in ...111 direction is observed.•Absorbent tape results in no surface damage while simultaneously inducing required surface residual stresses.•XRD and TEM results suggest increased dislocation densities in the laser peened surfaces contributing to lattice microstrain.
Laser shock peening (LSP) has been utilized to modify the surface characteristics of SS316LN plates of 6mm thickness. Laser pulse widths employed are 30ps and 7ns and the laser energy was varied in the range 5–90mJ. Peening was performed in direct ablation mode as well as with various sacrificial layers such as black paint, transparent adhesive tape and absorbing adhesive tape. The surface characteristics were greatly influenced by the type of sacrificial layer employed. The average surface roughness values are about 0.4μm when the black paint and transparent adhesive tape were used as sacrificial layers. In contrast to this, using absorbent adhesive tape as a sacrificial layer has resulted in an average surface roughness of about 0.04μm. Irrespective of pulse durations (30ps or 7ns), absorbent adhesive tape has always resulted in compressive residual stresses whereas other layers appear to be not that effective. In case of 30ps pulse, as the laser energy was increased from 5mJ to 25mJ, there was a texture observed in (111) reflection of X-ray diffractograms and the center of the peak has also gradually shifted to left. X-ray line profile analysis suggests that with the increase in laser energy, lattice microstrain also has increased. This lattice microstrain appears to be resulting from the increased dislocation density in the peened sample as evidenced during transmission electron microscopic investigations. Cross-sectional scanning electron microscopy performed on peened samples suggests that absorbing adhesive tape brings no surface damage to the samples whereas other sacrificial layers have resulted in some surface damage. Based on all these structural and microstructural details, it is recommended that absorbent tape could be used as a sacrificial layer during LSP process which induces surface residual stresses with no damage to the sample surface.
Bead-on-plate friction stir welding was conducted on 6mm thick plate of Reduced Activation Ferritic–Martensitic Steel employing polycrystalline cubic boron nitride tool with rotational speeds of 200, ...300, 500 and 700rpm and traverse speed of 30mm/min. The interface temperature between shoulder bottom and top surface of the plate was monitored by non-contact in-line thermography which served to identify the peak temperature attained in the stir zone (SZ). This temperature for 200, 300 and 500, and 700rpm was respectively below Ac1, between Ac1 and Ac3, and above Ac3. In the base metal (BM), the prior austenite grain and martensite lath boundaries were decorated with chromium and tungsten rich M23C6 precipitates while intra-lath regions revealed Ta and V rich MX type carbides. Rotational speeds greater than 300rpm led to martensite formation and simultaneous recovery, recrystallization and grain growth in SZs with wide distribution in grain size whereas SZ of 200rpm and BM possessed similar distribution. The grain boundary M23C6 dissolved and very fine needles of Fe3C precipitated in all SZs. The hardness of all SZs was unacceptably higher compared to the BM. The 200rpm weld exhibited higher impact toughness in the absence of martensite in SZ.
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•Effects of rotational speed during friction stir welding of a ferritic–martensitic steel were investigated.•Distribution of grain size in stir zones varied as a function of rotational speed.•Grain boundary M23C6 precipitates were dissolved while Fe3C formed in stir zones at all rotational speeds.•High rotational speeds promoted martensite occurrence in the stir zones with a drastic reduction in impact toughness.•Peak temperatures below Ac1 in the stir zone enabled impact toughness matching with the base metal.
