Interdendritic Regions Research Articles

Effect of heat treatment on the microstructure and tensile properties of a new superalloy designed for additive manufacturing

The microstructures and tensile properties of a novel superalloy especially designed for additive manufacturing (AM) were investigated under four states: (1) AD (as-deposited), the state of part after LMD (laser metal deposition); (2) HT1, heat treated at 870 °C for 16 h + air cooling; (3) HT2, heat treated at 1080 °C for 4 h + air cooling; (4) HT3, heat treated at 1080 °C for 4 h + air cooling, then heat treated at 870 °C for 16 h + air cooling (HT2+HT1). Each of them were extracted from the vertical (V) and horizontal (H) section of the part, respectively. Methods such as OM, SEM, TEM, XRD and unidirectional tensile tests were used for the investigation. The AD sample displays cellular dendrite microstructures with <001> crystal direction parallel to the building direction. The major phases are γ/γ'; while the minor phases are fine MC and Laves with no more than 1% in volume fraction. The size of primary γ′ ranges from 100 nm to 150 nm, the secondary γ′ particles are no more than 30 nm. With the aid of calculation, the processing HT1 and HT2 are confirmed as ageing and sub-solution treatment, respectively. The former raised the volume fraction of γ′ from 30.93% (AD) to 48.57% (HT1), and raised the size of γ′ at the expense of dissolving the secondary γ′ particles. The latter didn't change the volume fraction of γ′ strikingly, but refined the γ′ with the exception of a small amount of extraordinary large particles in the interdendritic region (IR). HT3 is a two-stage processing (HT2+HT1) which increased the volume fraction of γ′ slightly but raised the size apparently. The value of ultimate tensile strength (UTS) and yield strength (YS) of vertical samples are superior to that of the horizontal. The HT1-V shows the best UTS and poor Elongation (UTS, 1082 MPa; EL, 35.28%), while the AD-V has excellent strength and ductility match (UTS, 1054 MPa; YS, 744 MPa; EL, 50.45%); The best ductility is found among samples of HT3 (EL, 62.1% for HT3-H). The samples of HT2-H are inferior in both strength and ductility, and the obtained properties are scattered. These investigation results provide important fundamental support for optimizing the procedure, microstructure and properties of the new alloy in the future application research.

Comparative study on autogenous diode laser, CO2 laser-MIG hybrid and multi-pass TIG welding of 10-mm thick Inconel 617 superalloy

The present study investigated comparative welding performance of single-pass autogenous diode-laser (DLW) and CO2+ MIG laser-hybrid welding (LHW) processes with multi-pass conventional TIG welding in 10-mm thick Inconel 617 with comprehensive correlation of bead profile, microstructure with mechanical performance of weldments employing FESEM, XRD, high-speed Nano Indentation, uniaxial tensile and impact testing. Results indicated that welds produced by LHW and DLW constituted ‘wine-cup’ and ‘Y” shaped profiles respectively with reduced fusion zone (FZ) and heat-affected-zone (HAZ), high depth-to-width aspect ratio and low distortion as compared to that of ‘bath-tub’ shaped TIG weld on account of low heat input and high-coupling efficiency of laser. Indeed, inhomogeneous microstructures were observed in both LHW and TIG welds due to dual heat sources in LHW (laser + MIG) and multiple passes in TIG as compared to that of DLW, wherein, refined microstructure with fine secondary-dendritic arm spacing and low levels of micro-segregation in interdendritic regions of FZ. XRD confirms presence of higher amounts of M23C6 and M6C carbides in TIG FZ as compared to that of LHW and DLW counterparts and attributed to enhanced micro-segregation due to reduced cooling rate and repetitive thermal cycling effect of multiple passes. Micro and Nano-Indentation hardness profiling indicated that the hardness of FZ of TIG welds is comparatively lower to that of DLW and LHW on account of enhanced segregation and grain coarsening effect. Weld joint efficiency obtained in DLW and LHW are slightly higher than TIG weldments with highest elongation observed in DLW. Overall, joint efficiency of LHW and DLW was found adequate with vast reduction in distortion and improved adoptability for applications in thermal power plant.

Investigation of dendrite growth, Nb segregation during laser IN718 deposition via volume of fluid-phase field modeling

In this work, a macro-micro VOF-PF (Volume of Fluid-Phase field) model is established to simulate the heat transfer, dendrite growth and Nb segregation during the laser IN718 deposition. According to calculated heat transfers, G increases, while Vp decreases, and the corresponding G×Vp decreases from top to bottom in the molten pool. The calculated the primary dendrite arm spacing (PDAS) using the PF model increases from top to bottom. Compared with other models, the calculated PDAS using the PF model considering the liquid-metal flow is closest to experiment measurements. During the dendrite growth, Nb is enriched in the interdendritic region, and finally is distributed in the droplet shape. The calculated actual solute distribution coefficient Cmin/Cmax decreases from 0.584 to 5.548 with the decrease of G×Vp, which follows the equilibrium solidification theory. According to dc/dy=G/ml, the liquidus slope ml is calculated as 0.05322wt.%/μm very close to the theoretical value 0.05394wt.%/μm. The simulated Nb concentrations in the matrix γ and Laves phases are 3.97wt.%, 13.3wt.% respectively, which are in good agreements with experimental 3.7wt.% and 13wt.%.

Microstructure and Mechanical Properties of Hybrid AZ91 Magnesium Matrix Composite with Ti and SiC Particles.

In this paper, a new hybrid metal matrix composite, named AZ91/(SiC + Ti)p, is presented. The commercial AZ91 magnesium alloy was chosen as the matrix. The composite was reinforced with both SiC and Ti particles. The investigated material was successfully fabricated using stir casting methods. Microstructure analyses were carried out by digital and scanning electron microscopy with an energy-dispersive X-ray spectrometer (SEM + EDX). Detailed investigations disclosed the presence (besides the reinforced particles) of primary dendrites of the α phase, α + γ eutectic and some part of discontinuous precipitates of the γ phase in the composite microstructure. The composite was characterised by uniform distribution of the Ti particles, whereas the SiC particles were revealed inside the primary dendrites of the α phase, on the Ti particles and in the interdendritic regions as a mixture with the α + γ eutectic. Both the tensile and compression strength as well as the yield strength of the composite were examined in both uniaxial tensile and compression tests at room temperature. The fabricated AZ91/(SiC + Ti)p hybrid composite exhibited higher mechanical properties of all those investigated in comparison with the unreinforced AZ91 matrix alloy (cast in the same conditions). Additionally, analyses of the fracture surfaces of the AZ91/(SiC + Ti)p hybrid composite carried out using scanning electron microscopy (SEM + EDX) were presented.

Residual stress analysis, microstructural characterization, and mechanical properties of tungsten inert gas-welded P92/AISI 304L dissimilar steel joints

In the present work, the creep strength-enhanced ferritic martensitic P92 steel was welded with 304L austenitic stainless steel. The dissimilar joining of these two different grades of material was performed by using tungsten inert gas welding process. The nickel-based ERNiCr-3 welding consumable was used. The influence of post-weld heat treatment (760 °C, 2h), called as tempering, was also investigated. The secondary electron and optical image confirmed that ERNiCr-3 weld metal is accompanied by an equiaxed austenitic microstructure with Ni weight percentage of 68.29%. The energy-dispersive X-ray spectroscopy and electron probe microanalyzer results of the weld fusion zone revealed the Nb, Cr, and Ti segregation in the inter-dendritic region. The elemental mapping of the carbon-depleted zone, and the interface was performed using an electron probe microanalyzer, which revealed the diffusion of Ni, Cr, and Fe across the interface of P92 steel and ERNiCr-3 filler weld. The tensile test result indicated that part of the dissimilar weld joints (DWJs) with relatively weak ultimate tensile strength was ERNiCr-3 weld metal. The ultimate tensile strength value of the DWJs was observed as 610 MPa and 580 MPa in the as-weld and post-weld heat treatment situations, respectively. The Charpy V-notch impact energy of the ERNiCr-3 weld fusion zone was achieved as 135 ± 2 J and 140 ± 2 J in as-weld and post-weld heat treatment situations, respectively. The low impact toughness of the ERNiCr-3 weld metal was attributed to the presence of niobium carbide (NbC) and titanium carbide (TiC) particles. The peak longitudinal residual stress of 490 MPa and transverse residual stress of 442 MPa were noted in the ERNiCr-3 weld fusion zone at a depth of 3 mm from the top surface. The tempering heat treatment exhibited a significant drop in the residual stresses value for the weld fusion zone and heat affected zone (HAZ).

Comparative Study on the Surface Remelting of Mo-Si-B Alloys with Laser and Electron Beam

The Mo-12Si-8.5B alloy was surface-remelted by laser and electron beam, and the microstructure of its melt pool and substrate regions were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy spectrometry (EDS) techniques. It was found that the composition of the surface phases in the Mo-12Si-8.5B alloy did not change by the high-energy beam surface remelting process, but the microstructure of the molten pool region was significantly different from that of the substrate region, and its phase distribution was more uniform. Dendrites appeared on the surface of the material under the action of both processes, and the Si- and B-rich phases were mainly gathered in the interdendritic region. In the melt pool of the laser-remelted specimens, the α-Mo phase was continuously distributed with an average dendrite length of 70 µm, while the α-Mo phase distribution in the melt pool of the electron beam remelted specimens were relatively concentrated, with a larger dendrite size and an average dendrite length of 120 µm. The dendrite size in the melt pool of the laser remelted material was smaller, and the distribution of the elements was relatively uniform. Using a laser beam as the heat source was more favorable for the next step of the additive manufacturing of the core parts of hypersonic vehicles.

Effects of non-equilibrium microstructures on microstructure evolution and mechanical properties of laser powder bed fusion IN625 Ni-based superalloy during long-term thermal exposure at 700 °C and 750 °C

The effect of non-equilibrium microstructures on microstructure evolution and mechanical properties of laser powder bed fusion (LPBF) IN625 alloy during long-term thermal exposure at 700 °C and 750 °C was studied in this study. The as-built (AB) LPBF IN625 alloy had a typical non-equilibrium microstructure consisting of fine dendritic and cellular microstructures, Laves phase and high-density tangled dislocations. The non-equilibrium microstructure transformed into uniform recrystallized microstructure after solution treatment (ST). During long-term thermal exposure, δ phase in AB sample preferentially nucleated in inter-dendritic regions with high-density dislocations, while δ phase in ST specimen firstly nucleated at grain boundaries (GBs) and then grew into grains. The coarsening rate of δ phase in AB sample was lower and the size of δ phase was smaller than those in ST sample. The coarsening of Laves phase at GBs led to the depleted-δ zone in AB sample, which was not found in ST sample. For AB and ST samples, the ultimate tensile strength (UTS) and yield strength (YS) increased with the increase of thermal exposure time, while the elongation (EI) decreased. The tensile strength of AB sample was higher than that of ST sample during the thermal exposure at 700 °C, while their tensile strength was close during the thermal exposure at 750 °C. In addition, the anisotropy of mechanical properties of AB LPBF IN625 alloy was eliminated gradually during long-term thermal exposure. The microstructure evolution of LPBF IN625 alloy during long-term thermal exposure was explained by a newly proposed mold.

Effect of heat treatment on microstructure, corrosion resistance, and interfacial characteristics of Inconel 625 laser cladding layer

In this study, the changes in the microstructures and properties of an Inconel 625 cladding layer and its heat-affected zone (HAZ) upon heat treatment at 650 °C for 1 h were investigated. The interface characteristics were studied by removing the substrate through chemical and physical stripping. The results indicate that heat treatment can reduce the enrichment of Mo and Nb in the interdendritic region of the cladding layer, thereby distributing them more evenly. After heat treatment, the HAZ microstructure changed noticeably. Further, the martensite decomposed, ferrites formed, grains grew, and the microhardness decreased observably. The corrosion resistance of the cladding layer after heat treatment increased because the passivation film became more difficult to destroy. The interface maintained the microstructural characteristics of both the substrate and cladding layer. A “dendrite skeleton” was distributed in the grain of the sample before heat treatment; however, the dendrite characteristics became less prominent because of diffusion after heat treatment. Thus, the interface mainly retained the characteristics of the substrate. The bonding interface fractures showed a mixed fracture mode regardless of the heat treatment; however, the dimple morphology of the heat-treated sample differed substantially from that of the non-heat-treated sample, and no dendritic characteristics were observed after heat treatment.

The phase evolution and stability of Al0.5CoCrFeNi high-entropy alloy at 600 °C

The Al0.5CoCrFeNi alloy has been shown to possess a high potential for applicability by virtue of its mechanical properties. Its duplex nature proved to be stable after heating cycles to near melting and after aging treatments. In the as-cast state, this alloy contains ∼ 90 vol% of dendrite core (DC) regions having FCC structure, while the remainder consists of inter-dendritic (ID) regions with B2/BCC mixture, characterized by its typical ‘Chinese letter’ morphology. At 600 °C, ordered FCC (L12) nano-precipitations have been shown to form uniformly within the DC regions. These precipitates can potentially increase the DC hardness due to precipitation hardening during aging treatments. This article investigates the phase evolution and changes in the L12 phase morphology during a variety of aging treatments at 600 °C. The effect of aging on the alloy’s hardness was also evaluated. It was found that during aging, the L12 phase changed its morphology from individual particles to a ‘tweed-like’ morphology, which caused a sharp increase in hardness (600 °C/17d: 388.5 ± 11.4HV) compared to a non-aged sample (600 °C/q: 218.7 ± 5.4HV). With further aging, the L12 symmetry evolved into orthorhombic symmetry before finally transiting into a lamellar B2 morphology at the over-aged state, associated with a decrease in hardness. The increase in the DC hardness during aging seems to cause a delay in the formation of a tetragonal (σ) Cr-Fe rich phase, which tends to precipitate along the DC/ID boundaries in this alloy. In the ID region, the phase evolution starts with an HCP precipitation that remains stable as long as the B2/BCC mixture is stable, but once the BCC phase is transformed to FCC, the HCP particles become extinct. The experimental results were compared with the predicted phase content using the TCHEA4 thermodynamic database, and the differences are discussed.

Effect of Laser Energy Density on the Microstructure and Microhardness of Inconel 718 Alloy Fabricated by Selective Laser Melting

This work focused on the effects of laser energy density on the relative density, microstructure, and microhardness of Inconel 718 alloy manufactured by selective laser melting (SLM). The microstructural architectures, element segregation behavior in the interdendritic region and the evolution of laves phases of the as-SLMed IN718 samples were analyzed by optical metallography (OM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and electron probe microanalysis (EPMA). The results show that with an increase in the laser volume energy density, the relative density and the microhardness firstly increased and then decreased slightly. It also facilitates the precipitation of Laves phase. The variation of mechanical properties of the alloy can be related to the densification degree, microstructure uniformity, and precipitation phase content of Inconel 718 alloy.

Effect of electrostatic field on microstructure and mechanical properties of the 316L stainless steel modified layer fabricated by laser cladding

In the present research, the laser cladding 316 L stainless steel layer with and without exerting electrostatic field were investigated. The microstructure and mechanical properties analyses reveal that the high solidification rate and specific convection result in the grain coarsening by stages and the crystal tends to grow paralleling with the laser scanning direction. Moreover, the rapid solidification also leads to the micro-segregation inside grain. The exerting of electrostatic field opposite to laser scanning direction promotes the elements diffusion and convection in the molten pool, which benefits the directional solidification of the laser cladding layer and produces the columnar grains. In addition, the accelerated diffusion eliminates the micro-segregation but induces the formation of stacking faults inside grains. The exerting of electrostatic field consistent with laser scanning direction restricts the elements diffusion and convection in the molten pool, which results in the ordered mushy solidification and ultrafine grain structure in whole cladding layer. Moreover, the existence of electrostatic field consistent with laser scanning direction promotes the micro-segregation and precipitation of small MC carbides. Compared with the laser cladding layer, the exerting of electrostatic field could decrease the porosity and improve the modulus and hardness of the laser cladding layer except the hardness of dendritic region. The interdendritic region in laser cladding layer with exerting of electrostatic field opposite to laser scanning direction obtains the highest modulus and hardness.

Investigation on influence of Ta on microstructure evolution of directionally solidified Ni-based superalloys

The microstructure evolution in directionally solidified Ni-based superalloys DZ411 with different Tantalum (Ta) compositions (2.72, 3.10, and 4.00 wt%) was investigated to develop Ta-containing hot corrosion superalloys. It is found that the addition of Ta promote the element segregation in the interdendritic region, thus, leading to the increase in volume fraction of MC carbides and eutectics, while the morphology of them are not obviously changed. In addition, both the volume fraction of regions increases with increasing Ta composition, which is attributed to the reason that Ta is the main forming element of γ' phase. Besides, the previous controversy on the dependence of both the primary and secondary dendrite arm spacing on Ta composition was solved. The increase of dendrite arm spacing with increasing Ta composition can be attributed to increasing solidification range of these superalloys through both JMatPro and Different Scanning Calorimetry (DSC) analysis. Furthermore, the atomic arrangement of γ and γ' phases in alloys was characterized by spherical aberration corrected transmission electron microscopy. Both the saturation of refractory elements in γ matrix and the lattice constant of γ matrix was found to increase with increasing Ta composition, leading to the increase of lattice mismatch of γ/γ' phase and more regular shape of γ' phase which has also been confirmed by JMatPro analysis.

Large elastocaloric effect in as-cast Ni-Mn-Sn-Fe ferromagnetic shape memory alloys

The Ni-Mn-Sn ferromagnetic shape memory alloys exhibit excellent both magnetic field-induced magnetocaloric effect and uniaxial stress-induced elastocaloric effect. However, the intrinsic brittleness and narrow refrigeration temperature regions hinder their practical applications in elastocaloric and magnetocaloric as well other fields. In this work, the as-cast Ni44-xFexMn46Sn10 (x = 1, 2, 3) alloys are prepared directly by cooling solidification. The as-cast alloys improve the mechanical properties and broaden the martensitic transformation temperature region due to the presence of dendrites. The dendrites prevent cracking formation and propagation along the grain boundaries, and the addition of Fe-atoms enriched in the interdendritic region further optimizes the mechanical performance. In addition, the as-cast Ni41Fe3Mn46Sn10 alloy exhibits a large adiabatic temperature variation up to −10.3 K at a moderate stress of 350 MPa. Accordingly, direct cooling solidification of as-cast alloys could be a novel and feasible method for studying elastocaloric cooling technology.

Probing interdendritic flow and hot tearing during solidification using real time X-ray imaging and droplet tracking

Despite the well-known importance of controlling shrinkage-induced liquid flow in alloy castings to avoid the formation of catastrophic hot tears during the final stages of solidification, there has been little direct experimental measurement of liquid metal flow and hot tear formation under practical conditions. We use synchrotron X-rays to obtain radiographic video sequences of the solidification of monotectic Al-Pb alloys in which Pb droplets form as a fine-scale emulsion. We track and measure the velocity of thousands of Pb droplets as they move through interdendritic regions due to the effect of liquid to solid shrinkage during the final stages of solidification, up to the point of hot tear formation. Based on the droplet velocities, we present an analysis to estimate the interdendritic liquid velocity as solid fraction increases, and thus the shrinkage pressure drop driving the flow. The analysis is applied for video sequences obtained for both equiaxed and columnar microstructures, each under a range of cooling rates. Our measurements of the critical shrinkage-induced pressure for hot tear formation agree well with prior model-based and theoretical suggestions. The limitations and prospects for droplet tracking measurements of liquid metal flows are discussed.

Effect of ultrasonic melt processing and Al-Ti-B on the microstructural refinement of recycled Al alloys

Refining the α-Al grain size and controlling the morphology of intermetallic phases during solidification of Al alloys using ultrasonic melt processing (USMP) and Al-Ti-B have been extensively used in academic and industry. While, their synergy effect on the formation of these phases has not yet clearly demonstrated. In this paper, the influence of USMP and Al-Ti-B on the solidified microstructure of multicomponent Al-4.5Cu-0.5Mn-0.5Mg-0.2Si-xFe alloys (x = 0.7, and 1.2 wt%) has been comparatively studied. The results show that the USMP + Al-Ti-B method produce a more profound refinement effect than the individual methods. In addition, the area of single Fe-rich phases in both alloys with USMP + Al-Ti-B are also refined compared with conventional methods. A mechanism is proposed for the refinement, which are the deagglomerated TiB2 parties induced by USMP providing more effective nucleation sites for α-Al, and the refined interdendritic regions limited the growth of Fe-rich phases in the following eutectic reaction. Finally, the application of combined USMP + Al-Ti-B methods is feasible in microstructural refinement, resulting in the improving the casting soundness and mechanical properties of alloys.

Effects of Cobalt on solidification characteristics and as-cast microstructure of an advanced nickel-based single crystal superalloys

Containing Re and Ru, an advanced Ni-based single crystal superalloys with different cobalt (Co) content (5.9 wt.%, 9.0 wt.% and 12.0 wt.%, respectively) were investigated to study the effects of Co on solidification characteristics and as-cast microstructure. In the aspect of solidification characteristics, the solidus, especially the precipitation temperature of γ′ phase of the alloys decreases gradually with the increase of Co. Furthermore, the dendritic segregation of refractory elements, such as Re, W and Cr, is restrained. With regard to as-cast microstructure, the results show that primary dendrite spacing shorten significantly since Co increased from 5.9 to 9.0 wt.%, and the content of γ/γ′ eutectic is lowest in the alloy with 9.0 wt.% Co. The addition of Co decreases the size of γ′ and homogenizes the size of γ′ between the dendrite core and the interdendritic region. A customized heat treatment schedule is implemented for the three alloys based on the research.

Custom-designed heat treatment simultaneously resolves multiple challenges facing 3D-printed single-crystal superalloys

Single-crystal Ni-based superalloys are currently the material of choice for turbine blade applications, especially with the emerging additive manufacturing (AM) that facilitates the manufacture/repair of these single crystals. This promising AM route, however, comes with a dilemma: in the fusion and heat affected zones after e-beam or laser induced melting, one needs a solutionizing annealing to relieve the residual stresses and homogenize the chemical/microstructure. The super-solvus solutionizing temperature is usually adopted from the protocol for the cast superalloys, which almost always causes recrystallization and stray grain growth, resulting in a polycrystalline microstructure and degrading the high-temperature mechanical performance. Here we demonstrate a custom-designed post-printing heat treatment to replace the conventional super-solvus one. The recovery and relatively low temperature diminish the driving force for recrystallization and the movement of stray grain boundaries, without suffocating the chemical/microstructural homogenization thanks to the narrow dendrite width and short element segregation distance. The optimal duration of the heat treatment is proposed to achieve atomic-diffusion mediated chemical homogenization while limiting γ′-particle coarsening in the interdendritic regions. Our strategy makes it practically feasible to resolve several bottleneck problems with one processing/treatment, removing a seemingly formidable obstacle to effective additive manufacturing of superalloy single crystal products.

Influence of Mo on the dynamic recrystallization behavior of Al–Cu–Mg–Ag alloy

High-throughput double cone specimens were used to study the influence of Mo on the microstructure evolution of Al–Cu–Mg–Ag alloy under a wide range of strain along the axis. The microstructure evolution was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Results demonstrated that Al6(Fe, Mn, Mo) dispersoids formed in the Al–Cu–Mg–Ag alloy with the addition of 0.2% Mo during double-step homogenization. Meanwhile, rod-like Al20Cu2Mn3 and disk-like Al6(Fe, Mn, Mo) dispersoids segregated to opposite distribution in interdendritic regions and dendrite cores, respectively, which obviously eliminated the dispersoid-free zones and intragranular sparse regions. With different effective strains along the axis, Al6(Fe, Mn, Mo) dispersoids obviously reduced the transformation from Low-angle grain boundaries to High-angle grain boundaries. The nucleation and growth of dynamic recrystallization grains were effectively inhibited by the coupling of Al20Cu2Mn3 and Al6(Fe, Mn, Mo) dispersoids due to the obstruction of dislocation movement.

Strengthening and deformation behavior of as-cast CoCrCu1.5MnNi high entropy alloy with micro-/nanoscale precipitation

The microstructure of as-cast CoCrCu1.5MnNi high entropy alloy (HEA) exhibited the dual fcc phase structure with Co–Cr rich dendritic and Cu–Mn rich interdendritic regions. Long submicron-scale needle-shaped Co–Cr precipitates were observed in Cu–Mn rich interdendritic region whereas much smaller nanoscale Cu–Mn rich needle-shaped precipitates were found in Co–Cr rich dendrites. The compositionally inverse relationship between the precipitate and the matrix in dendritic and interdendritic regions was induced by the decreasing solubility of the elements in two phases due to the reduced contribution of entropy during cooling. Excellent as-cast yield strength/ductility combinations at both room (431.5 MPa/55%) and cryogenic (600.2 MPa/67%) temperatures are attributed to the precipitation strengthening and quasi-linear strain hardening in the as-cast CoCrCu1.5MnNi. The quasi-linear strain hardening in the as-cast CoCrCu1.5MnNi is associated with the accumulation of deformation-induced defects such as planar array of dislocations, extended stacking faults and deformation twins in the matrix with closely spaced precipitates with the inter-precipitate spacing smaller than the critical cross-slip annihilation distances. The predicted yield strengths of CoCrCu1.5MnNi with nanoscale and submicron-scale precipitates strengthening in dual fcc phase structure at RT and 77 K were in good agreement with the experimental strengths.

The effect of filler metal on microstructure and mechanical performance of GTAW inconel 625 at room and elevated temperature

The aim of this study was evaluating the influence of ER321 and ERNiCr-3 filler metals on the mechanical performance of the Inconel 625 gas tungsten arc welded joints at room and elevated temperature and correlating with the microstructure. Optical and scanning electron microscopy was conducted to characterize microstructure features at the fusion and the interfaces. Microhardness measurements, tensile test, Charpy impact test, and creep rupture test were applied to the weldments to investigate the effect of filler metal on mechanical properties of the joints. Creep rupture test was performed at 600 °C under load stresses of 150, 200 and 300 MPa. At high stress levels (300 MPa), nucleation and coalescence of voids accelerates and a ductile fracture with dimple morphology for both filler metals was seen. At lower stress level of 150 MPa, the creep deformation becomes dominant for both weldments and the voids which are mainly micro-creep voids get smaller in size and are located near the precipitates. A 100% increase in stress level (from 150 MPa to 300 MPa), reduces the creep rupture life of the ERNiCr-3 and ER321 welds by about 11 and 30 times, respectively. The results show that stringers of δ-Ferrite in inter-dendritic region of austenite appear in ER321 weldments along with some precipitates which are highly enriched in Ti and C. The formation of the unmixed zone with a different concentration gradient of the elements is observed at the Inconel 625/ER321 stainless steel interface. XRD pattern reveals the formation of (Fe2Mo) Laves phase in ER321 weld metal where the amount of Fe in the weld increases and Mo solubility decreases. ERNiCr-3 weldments show fully austenitic structure and some NbC precipitates. EDS analysis of ERNiCr-3 weld metal confirms high amount of Mo and Nb at the inter-dendritic region, whereas the dendritic core is depleted from Mo and Nb. By applying different filler metals, different fracture morphology was revealed under different loads. A non-uniform hardness profile and the minimum microhardness value of 120 HV were seen in the wide unmixed zone of ER321 weld metal due to its heterogeneous structure and also due to lack of weld metal dilution. It was shown that ER321filler metal does not give an appreciable tensile and impact properties comparing to ERNiCr-3 weldments. The tensile elongation was greater for ERNiCr-3 weldments (13.1%) than ER321 joints (10.2%) andthe average impact toughness of ERNiCr-3 weldments was about two times higher than ER321.