Incipient Melting Research Articles

Phosphorous behavior and its effect on secondary phase formation in high refractory content powder-processed Ni-based superalloys

The behavior of phosphorous and its effect on the microstructure of a powder-processed, high refractory content, Ni-based superalloy was studied and compared to a P-free variant. Transmission electron microscopy and atom probe tomography were used to identify the crystal structure of the five topologically close packed (TCP) and geometrically close packed (GCP) phases that formed, assess phase compositions, and determine any elemental segregation to phase boundaries. The results revealed strong phosphorous segregation to grain boundaries, moderate solubility in TCP and MC phases, limited solubility in the γ matrix and no solubility in the γ′ phase or any other Ni3X type GCP phases. The phosphorus segregation to the grain boundaries of the P-doped alloy lowered the incipient melting temperature, thereby eliminating the processing window, and led to the formation of (Ni,Co,Cr,P)2(Nb,Mo) laves phases along grain boundaries during supersolvus heat treatment. Interestingly, APT revealed segregation of P to Al2O3 oxide surfaces, explaining the frequent observation of laves precipitates nucleating on such impurities. Due to the low stability of the C14–Ni2Nb laves phase, it was prone to phase separation, and served as the nucleation point for μ, intergrown η/δ, and Nb3P phosphide phases.

Effect of Transition Metals Addition on the Microstructure and Incipient Melting of 354-Based Alloys

Thermal analysis of various 354 alloy melts was carried out to determine the sequence of reactions and phases formed during solidification under close-to-equilibrium cooling conditions. The main reactions observed comprised formation of the α-Al dendritic network at 598 °C followed by precipitation of the Al–Si eutectic and post-eutectic β-Al5FeSi phase at 560 °C; Mg2Si phase and transformation of the β-phase into π-Al8Mg3FeSi6 phase at 540 °C and 525 °C; and lastly, precipitation of Al2Cu and Q-Al5Mg8Cu2Si6 at 498 °C and 488 °C. As a result of the low solidification rate of the thermal analysis castings, all Zr-containing alloys are located in the L + Al3Zr region of the Al–Zr phase diagram during the melting stage. Three main reactions are detected with the addition of Ni, i.e., the formation of AlFeNi, AlCuNi and AlSiNiZr phases. Larger sizes of AlFeNi and AlCuNi phase particles were observed in higher Ni content alloys of 3.6 wt%. Mn addition helps in reducing the detrimental effects of the β-iron phase by replacing it with the less-detrimental Chinese script α-Al15(Fe,Mn)3Si2 phase and sludge particles. With the use of the multi-step solution treatment—involving higher solution temperatures and longer durations, an increased amount of incipient melting is likely to occur.

Ferrous oxide-rich asteroid achondrites

Ferrous oxide (FeO)-rich asteroid achondrites can be defined as asteroid-derived samples that experienced incipient partial melting processes in the early Solar System (>4.5 Ga) leading to melt-residues and cumulate and melt rocks that have high FeO in silicate grains (molar Mg/[Mg + Fe] < 80), implying relatively oxidative conditions (fO2 of IW +1 to +3). These achondrites include olivine-dominated brachinite and brachinite-like achondrite meteorites, ungrouped meteorites including Lewis Cliff 88763, Northwest Africa (NWA) 6693 and NWA 6704, Tafassasset, NWA 011/1296, and the oligoclase-rich meteorites Graves Nunataks (GRA) 06128 and GRA 06129. Ferrous oxide-rich asteroidal achondrites differ from other partially-melted achondrites, including ureilites and acapulcoite-lodranites in that the latter have higher molar Mg/ (Mg + Fe) in silicate grains, and lower fO2 (IW 0 to −2). New mineral chemical, whole-rock major- and trace-element and highly siderophile element (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, Au) abundance data, and O and Os isotope data are presented for FeO-rich achondrite meteorites Allan Hills 84025 (brachinite), Miller Range (MIL) 090206 and MIL 090405 (brachinite-like achondrites), and NWA 6693 (ungrouped). These results, combined with available data for FeO-rich asteroidal achondrites, reveal that these rocks include nearly-pure residues after partial melting, to samples formed by melt-rock reaction and as cumulates, requiring variable to extensive Fe-Ni-S partial melting, and between 1 and 20% silicate partial melting. The FeO-rich asteroidal achondrites originate from at least four distinct parent bodies, based on O-Cr-Ti isotope systematics, and occur for both carbonaceous and non-carbonaceous chondrite precursor sources. The initial water and volatile contents of FeO-rich asteroid achondrites were similar to carbonaceous chondrite groups, implying both carbonaceous and non-carbonaceous precursor materials generated water-rich partially-melted asteroidal bodies. The existence and recognition of FeO-rich asteroid achondrites explains the otherwise enigmatic nature of some iron meteorite groups (e.g., IVA, IVB) that require segregation from an oxidized asteroid parent body. The internal structure of some asteroid parent bodies was likely to be complex, reflecting early differentiation processes of nascent core formation, Fe-Ni-S melt pooling, variable silicate partial melting, igneous differentiation and the important role of melt-rock reaction, melt refertilization and late-stage C- (reduced bodies) or S-rich (oxidized bodies) fluid and vapor reactions.

Evaluation of microstructural degradation in a failed gas turbine blade due to overheating

Gas turbine blades may undergo overheating, which could cause serious microstructural degradation and even failure of turbine components. Nonetheless, limited published investigations focus on the evaluation of the microstructural degradation in overheated turbine blades. In this study, a high-pressure turbine blade was investigated, which comprised of equiaxed-cast superalloy substrate and an AlSi coating. The blade failed due to material loss at the airfoil tip of the leading edge. A systematic microstructural investigation of the failed blade was conducted, and the results were compared with those of thermally exposed samples. By taking several microstructural degradation parameters as references, the microstructural degradation of the failed blade was evaluated and the equivalent maximum service temperature was estimated. The results indicated that most locations of the failed blade had been operating under normal conditions. However, the airfoil tip of the leading edge had suffered serious overheating, and the overheating temperature was almost 200 °C above the normal service temperature. The overheating led to incipient melting of the substrate and material loss of the serviced blade. This study provides a guidance on the evaluation of microstructural degradation and failure analysis in conventionally cast turbine blades.

Thermo‐mechanical interaction of a large impact melt sheet with adjacent target rock, Sudbury impact structure, Canada

AbstractThe 1.85 Ga Sudbury Igneous Complex (SIC) and its thermal aureole are unique on Earth with regard to unraveling the effects of a large impact melt sheet on adjacent target rocks. Notably, the formation of Footwall Breccia, lining the basal SIC, remains controversial and has been attributed to impact, cratering, and postcratering processes. Based on detailed field mapping and microstructural analysis of thermal aureole rocks, we identified three distinct zones characterized by static recrystallization, incipient melting, and crystallization textures. The temperature gradient in the thermal aureole increases toward the SIC and culminates in a zone of partial melting, which correlates spatially with the Footwall Breccia. We therefore conclude that assimilation of target rock into initially superheated impact melt and simultaneous deformation after cratering strongly contributed to breccia formation. Estimated melt fractions of the Footwall Breccia amount to 80 vol% and attest to an extreme loss in mechanical strength and, thus, high mobility of the Breccia during assimilation. Transport of highly mobile Footwall Breccia material into the overlying Sublayer Norite of the SIC and vice versa can be attributed to Raleigh–Taylor instability of both units, long‐term crater modification caused by viscous relaxation of crust underlying the Sudbury impact structure, or both.

Influence of Cu addition on the heat treatment response of A356 foundry alloy

Strength and high-temperature performance are important properties in Al-Si-Mg alloys for power-train applications such as combustion engine cylinder head components in automotive industry. Addition of Copper (Cu) to Al-Si-Mg alloys is a common practice to obtain high strength as well as high temperature stability. However, Cu addition alters precipitation kinetics and precipitate types during artificial ageing, and consequently properties of final cast products. In this study, the effects of Cu on tensile properties in connection with precipitation and precipitate types, as observed through aging curves and TEM analysis, were investigated in Al-Si-Mg alloys having three different Cu contents in order to optimize Cu contents and heat treatment parameters. Higher solutionizing temperature enabled a significant increased Cu content in solid solution without any incipient melting of Cu-rich phases. A refinement of the precipitate microstructures was observed in the Cu-containing alloys, accompanied with a phase-change of the hardening needle precipitates. different precipitation kinetics and double aging peaks were observed during artificial aging in higher Cu containing alloys, which were correlated to higher strength than in the Cu-free alloy in underaged and peak aged conditions. Optimized Cu contents and heat treatment parameters were found with respect to a combination of strength and elongation.

Analysis of precipitation, dissolution and incipient melting of Al2Cu in B206 Al alloy using in-situ neutron diffraction

The B206 Al alloy is a commonly used automotive alloy for powertrain, suspension and chassis components. Heat treatment is an important post-casting process used to improve the mechanical properties of cast components. Improvement of the heat treatment process requires a better understanding of phase evolution of Al2Cu as a function of temperature. In this study, a novel application of in-situ neutron diffraction was used to examine precipitation, dissolution and incipient melting characteristics of Al2Cu as a function of temperature during sample heating. Additional experiments, where samples were quenched at each examined temperature and analyzed in the SEM, were performed to confirm the reason for changes in the amount of Al2Cu at each stage. The results show that neutron diffraction effectively measured Al2Cu precipitation at temperatures between 200 and 440 °C. At temperatures between 460 and 520 °C, the reduction in Al2Cu peak intensity was due to dissolution of precipitates and interdendritic Al2Cu, while above 530 °C, it was primarily due to incipient melting. This research suggests that neutron diffraction is an effective technique in the in-situ analysis of solid-state and solid-to-liquid state phase transformations during heating.

Thoughts on High Performance Superalloy Design and Microstructural Characteristics of a Newly Designed Ni-Cr-Co-W Superalloy Applied above 850°C

With the development of aircraft engine, higher requirement was put forward on turbine disk materials. In the present work, new thoughts on improving high temperature properties of superalloys have been proposed and a newly developed candidate turbine disk material for 850°C-900°C application with a composition of Ni-Co-Cr-W superalloy has been investigated. The results show that W is beneficial for mechanical properties. Microstructural characteristics and hot deformation of this new alloy were studied by optical microscope (OM), field emission scanning electric microscope (FESEM) and energy dispersive X-ray spectrometer (EDX) and differential scanning calorimetry (DSC). The results show that the main precipitates in the as-cast condition are γ’ phase, primary MC carbides and eutectic phase. The incipient melting temperature, γ’ solvus and MC solvus are 1312°C, 1220°C and 1356°C respectively. Cracks are observed in the tested samples after hot deformed at 1160°C to 1220°C with 30% strain. They initiated at the surface of the samples and propagated along the grain boundaries and also initiated at the interface of carbides and matrix.

Low‐Pand high‐Tmetamorphism of basalts: Insights from the Sudbury impact melt sheet aureole and thermodynamic modelling

AbstractLow‐pressure and high‐temperature (LP–HT) metamorphism of basaltic rocks, which occurs globally and throughout geological time, is rarely constrained by forward phase equilibrium modelling, yet such calculations provide valuable supplementary thermometric information and constraints on anatexis that are not possible to obtain from conventional thermometry. Metabasalts along the southern margin of the Sudbury Igneous Complex (SIC) record evidence of high‐grade contact metamorphism involving partial melting and melt segregation. Peak metamorphic temperatures reached at least ~925°C at ~1–3 kbar near theSICcontact. Preservation of the peak mineral assemblage indicates that most of the generated melt escaped from these rocks leaving a residuum characterized by a plagioclase–orthopyroxene–clinopyroxene–ilmenite‐magnetite±melt assemblage. Peak temperatures reached ~875°C up to 500 m from theSIClower contact, which marks the transition to metabasalts that only experienced incipient partial melting without melt loss. Metabasalts ~500 to 750 m from theSICcontact are characterized by a similar two‐pyroxene mineral assemblage, but typically contain abundant hornblende that overgrew clino‐ and orthopyroxene along an isobaric cooling path. Metabasalts ~750 to 1,000 m from theSICcontact are characterized by a hornblende–plagioclase–quartz–ilmenite assemblage indicating temperatures up to ~680°C. Mass balance and phase equilibria calculations indicate that anatexis resulted in 10–20% melt generation in the inner ~500 m of the aureole, with even higher degrees of melting towards the contact. Comparison of multiple models, experiments, and natural samples indicates that modelling in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2(NCFMASHTO) system results in the most reliable predictions for the temperature of the solidus. Incorporation of K2O in the most recent amphibole solution model now successfully predicts dehydration melting by the coexistence of high‐Ca amphibole and silicate melt at relatively low pressures (~1.5 kbar). However, inclusion of K2O as a system component results in prediction of the solidus at too low a temperature. Although there are discrepancies between modelling predictions and experimental results, this study demonstrates that the pseudosection approach to mafic rocks is an invaluable tool to constrain metamorphic processes atLP–HTconditions.

The effect of phosphorus on the formation of grain boundary laves phase in high-refractory content Ni-based superalloys

The effect of phosphorous on the microstructure of a powder-processed, high Nb-content, Ni-based superalloy was studied and compared to a P-free variant. The P-doped sample had a lower incipient melting temperature, forming a (Ni, Co, Cr, P)2(Nb, Mo) laves phase along the grain boundaries. Transmission electron microscopy and atom probe tomography were used to identify the crystal structure, assess phase compositions, and determine any elemental segregation to phase boundaries. The results revealed strong partitioning of P to the laves phase and a slight Mo gradient towards the γ′ phrase. Finally, a simplified laves phase was successfully modeled via density functional theory calculations.

Direct laser metal deposition of Inconel 738

Inconel 738 is one of the widely used nickel-based superalloys in high-temperature applications, especially in land-based and aerospace gas turbine engines. This paper reports the feasibility of direct laser metal deposition (LMD) of Inconel 738. Cracks evolved during deposition at the substrate/deposit interface and within the deposit along high angle grain boundary for scanning speed of 6 and 12 mm/s due to the intense residual stress and incipient melting. Results showed liquation cracking due to low melting crack boundary γ′ and significant micro-segregation of Al and Ti along the crack boundaries. By maximizing the energy density and by reducing the scanning speed to 3 mm/s, crack-free single wall specimens were successfully manufactured. Microstructural evolution of primary, secondary, grain boundary γ′, MC carbides, and M2B borides in the as-deposited and heat-treat specimens are discussed. Mechanical properties and microstructural development were investigated using tensile testing and scanning electron microscopy. Energy dispersive spectroscopy confirmed significant micro-segregation on various elements along the interdendritic and grain boundaries. X-ray diffraction validated the presence of the observed carbides and boride in the as-deposited and heat-treated samples.

Development of strong, oxidation and corrosion resistant nickel-based superalloys: critical review of challenges, progress and prospects

ABSTRACTA comprehensive review of alloying effects in nickel-based single-crystal superalloys for turbine blades and vanes operating in a very aggressive environment of stress, oxidation and corrosion is presented. Exceptionally strong single-crystal superalloys have been developed containing increasing amounts of rhenium and decreasing amounts of chromium resulting in reduced environmental resistance. Interactions in a superalloy containing up to 15 alloying elements are complex and poorly understood. The superalloys can form brittle topologically closed-packed (TCP) phases unless the alloying additions are carefully selected. Development of superalloys with a required balance of strength and environmental resistance has been very challenging. Aluminium, chromium and tantalum are essential alloying elements for strength and environmental protection. Aluminium beyond an upper limit can lead to incipient melting during heat treatment necessary for achieving an optimum size and distribution of the γ′ phase. Rhenium and ruthenium additions contribute significantly to strength, while considerably degrading environmental resistance. Hafnium and yttrium singularly or in combinations improve oxidation and corrosion resistance. Progress in modelling based on thermodynamics, kinetics and regression analysis of prior data to simultaneously predict strength and environmental resistance has been limited since the strengthening and environmental degradation are distinctly separate mechanisms. The paper presents a critical review of alloying studies and provides an insight into future developments.

Effect of extrusion speeds on the microstructure, texture and mechanical properties of high-speed extrudable Mg[sbnd]Zn[sbnd]Sn[sbnd]Mn[sbnd]Ca alloy

A new type of low-cost Mg-3.36Zn-1.06Sn-0.33Mn-0.27Ca (wt.%) (named as ZTMX3100) alloy ingot with good extrudability was prepared by semi-continuous casting. Two-stage homogenization treatment and hot extrusion were performed on the ingot. The effects of extrusion speeds (0.2–10 mm/s) on the extrudability, microstructure and mechanical properties of the alloy were investigated. The excellent extrudability of the alloy is attributed to the high incipient melting temperature of 572 °C resulting from the formation of thermally stable phase CaMgSn. CaMgSn phases distribute along the extrusion direction at each extrusion speed. The average grain size increases gradually and distribution uniformity of grain size firstly increases and then decreases. The as-extruded alloys exhibit textures with (0001) basal planes parallel to extrusion direction. Besides, the alloy exhibits a <101¯0> or <101¯0> − <112¯0> fiber texture depending on extrusion speed. The alloy extruded at 0.2 mm/s presents the highest mechanical property, which is attributed to the fine grain structure, strengthening effect from second phase particles, and weak basal texture. The alloy extruded at high speed of 10 mm/s (die-exit speed 18 m/min) still maintains a relatively high strength and elongation, which is mainly due to the combined effect of moderate alloying content and thermally stable phase CaMgSn.

Influence of minor boron on the microstructures of a second generation Ni-based single crystal superalloy

Boron is added into single crystal superalloys as a micro-alloying element to strengthen low angle grain boundaries. However, systematic investigations on the effect of boron on microstructures of single crystal superalloys are limitedly reported. The effect of boron on as-cast and heat-treated microstructures was investigated in two experimental Ni-based single crystal superalloys containing 3 wt% Re. The current results indicated that the volume fraction of (γ + γ′) eutectic and M3B2 borides was evidently increased, while the number of micropores was evidently decreased with the addition of 0.02 wt% boron. The (γ + γ′) eutectic could not be dissolved completely due to the lower incipient melting temperature caused by the formation of M3B2 borides. Meanwhile, the M3B2 borides were found to be enriched with indispensable strengthening elements Cr, Mo, W and Re, and this may lower the strengthening effect and cause stress concentration during high temperature creep.

An investigation into the cracking of platinum aluminide coated directionally solidified CM247 LC high pressure nozzle guide vanes of an aero engine

Abstract In service inspection of RT-22 platinum aluminide coated directionally solidified CM247LC high pressure turbine nozzle guide vanes of an aero engine which underwent 134 runs as an assembly set revealed cracking in 23 out of the total 34 vanes(17 pairs). A detailed investigation was launched on one of the cracked vanes which revealed that thermomechanical fatigue was the root cause of the failure. Incipient melting, in addition, was observed in platinum aluminide coating and at the interior of the vanes. A relook at the engine shut down procedure and an operating temperature not exceeding safe maximum limit were recommended to protect the vanes against these degradations.

Microstructures and high-temperature mechanical properties of a directionally solidified Ni-based superalloy: Influence of boron content

The effects of boron content on the microstructure and mechanical properties of a directionally solidified Ni-based superalloy were investigated. The increase of boron content significantly increases the amount of γ/γ′ eutectic, impels the formation of multi-phase eutectic-like constituent, and promotes the morphological transformation of MC carbides from rod-like to blocky. After standard heat treatment, the incipient melting occurs in the high boron alloy, which can lower the dissolving temperature of γ/γ′ eutectic. With increasing boron content, the volume fraction of secondary γ′ phase decreases significantly and the homogeneity and alignment of secondary γ′ show a decrease, and the tensile strengths at 900 °C decrease slightly mainly due to the strengthening effect of much more tertiary γ′ phases. The wide γ matrix channel facilitates the movement of dislocations, which is somewhat beneficial to the elongation, while the incipiently melted region (IMR), the decreased volume fraction of secondary γ′ phase and the less homogeneity of secondary γ′ phase are rather harmful to the elongation. Consequently, the elongation is drastically reduced. The existence of a large amount of IMRs is principally responsible for the reduction of the stress rupture property. Tertiary γ′ phases re-dissolving in γ matrix channel facilitates the movement of dislocations between large secondary γ′ phases, which is beneficial to the elongation of the alloys. In addition, the IMRs provide masses of crack sources, and consume the precipitation strengthening elements (Ti, Ta) and the solid solution strengthening elements (Cr, W and Mo), which plays the major role in the degradation of the high-temperature tensile or stress rupture properties.

Pargasite in fluid inclusions of mantle xenoliths from northeast Australia (Mt. Quincan): evidence of interaction with asthenospheric fluid

Three spinel lherzolite xenoliths from Mt. Quincan (Queensland, northeastern Australia) were studied with special attention to their enclosed fluid inclusions. The xenoliths are deformed, have porphyroclastic textures and overall show very similar petrographic features. The only significant difference is manifested in the abundance of fluid inclusions in the samples, mostly in orthopyroxene porphyroclasts. Xenolith JMTQ11 is fluid inclusion-free, whereas xenolith JMTQ20 shows a high abundance of fluid inclusions (fluid inclusion-rich). Xenolith JMTQ45 represents a transitional state between the previous two, as it contains only a small amount of fluid inclusions (fluid inclusion-bearing). Previous studies revealed that these xenoliths and the entrapped fluid inclusions represent a former addition of a MORB-type fluid to the pre-existing lithosphere, resulting from asthenosphere upwelling. There is a progressive enrichment in LREE, Nb, Sr and Ti from the fluid inclusion-free xenolith through the fluid inclusion-bearing one to the fluid inclusion-rich lherzolite. This suggests an increase in the extent of the interaction between the fluid-rich melt and the lherzolite wallrock. In addition, the same interaction is considered to be responsible for the formation of pargasitic amphibole as well.The presence of fluid inclusions indicates fluid migration at mantle depth, and their association with exsolution lamellae in orthopyroxene suggests fluid entrapment following the continental rifting (thermal relaxation) during cooling. A series of analyses, including microthermometry coupled with Raman spectroscopy, FTIR hyperspectral imaging, and Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) was carried out on the fluid inclusions. Based on the results, the entrapped high-density fluid is composed of 75–89 mol% CO2, 9–18 mol% H2O, 0.1–1.7 mol% N2 and ≤0.5 mol% H2S with dissolved trace elements (melt component).Our findings suggest that the metasomatic fluid phase could have been either a fluid/fluid-rich silicate melt released from the deeper asthenosphere, or a coexisting incipient fluid-rich silicate melt. Further cooling, possibly due to thermal relaxation and the upward migration of the fluid phase, caused the investigated lherzolites to reach pargasite stability conditions. We conclude that pargasite, even if only present in very limited modal proportions, can be a common phase at spinel lherzolite stability in the lithospheric upper mantle in continental rift – back-arc settings. Studies of fluid inclusions indicate that significant CO2 release from the asthenosphere in a continental rifting environment is resulting from asthenosphere upwelling and its addition to the lithospheric mantle together with fluid-rich melt – lherzolite interaction that leaves a CO2-rich fluid behind.

Influence of solution treatment on microstructure and stress rupture properties of K439 nickel-base superalloy

The isochronal solution treatment (4h) in the 1080-1190°C range with air cooling are performed to investigate the effects of solution temperature on the microstructure and stress rupture properties of K439 alloy. The detailed as-cast and solution treated microstructures were analyzed through optical microscopy (OM), scanning electron microscopy (SEM) and differential scanning calorimeter (DSC). The experimental results show that the as-cast alloy exhibited a typical dendritic structure with five phases of γ, γ´, γ/γ´, η and MC. After solution heat treated at 1080°C, although all of the γ´ particles in the dendritic cores were dissolved, there were still some of these precipitates in the interdendritic regions beside η phase. With the solution temperature increasing to 1150°C, all of the γ´ and η solutioned and a uniform microstructure was observed. Furthermore, increasing the temperature to 1210°C, a small amount of incipient melting occurred in the alloy. The stress rupture life of K439 alloy at 760°C/530MPa increased with the rising of solution temperature and reached to the top value under the solution temperature of 1150°C. The optimum solution treatment considered to be 1150 C/4 h followed by air cooling.

Microstructural evaluation of welded fresh-to-aged reformer tubes used in hydrogen production plants

Abstract Heat resistant reformer tubes comprise a significant fraction of petrochemical reforming plants cost considering their high alloy content (i.e. 25Cr-35Ni-1Nb-0.1Ti). The bottom portion of tubes experiences the highest temperatures in the furnaces leading to microstructural changes, creep damage, and loss of elongation over their service life which in this case is twenty years. There is a cost- and time-driven motivation to only replace this portion of tubes by welding in contrast with replacing entire set of tubes which is the common industrial practice. However, welding new to aged tubes may lead to reliability issues due to difference in mechanical properties as a result of microstructural differences. In the current study, the microstructure and tensile properties of aged and new tubes have been evaluated in an effort to qualify the mechanical integrity of weldments. Welding trials are carried out to investigate the microstructure of the aged-to-new weldments and correlate it with the tensile properties (particularly elongation). Findings reveal that the heat affected zone of aged tubes is prone to micro-cracking of bulky primary carbides and incipient melting particularly at the inner surface where the root pass is applied. Adopting preheating for the root pass is effective in reducing carbide micro-cracking by decreasing cooling rate which assists in the accommodation of stresses generated by thermal contraction. Despite presence of carbide micro-cracks, tensile elongation is not severely affected as aged-to-new welds exhibit comparable and slightly higher elongation than aged base metals (above 4%). It is proposed that this is partially due to the orientation of micro-cracks in carbides. Further microstructural and tensile property results are presented and discussed.

Immiscible sulfide melts in primitive oceanic magmas: Evidence and implications from picrite lavas (Eastern Kamchatka, Russia)

Silicate-sulfide liquid immiscibility in mantle-derived magmas has important control on the budget of siderophile and chalcophile metals, and is considered to be instrumental in the origin orthomagmatic sulfide deposits. Data on primitive sulfide melts in natural samples, even those representing most voluminous magmatism in oceanic rifts, are very scarce due to the small size and poor preservation of incipient sulfide melt globules. Here we present the first detailed report of the crystallized sulfides melts in the oceanic picrites of the (presumably) Cretaceous age Kamchatsky Mys ophiolite complex in Eastern Kamchatka (Far East Russia). Sulfide melts are present in three forms; (1) as inclusions in olivine (87.1–89.6 mol% Fo), (2) interstitial to the groundmass minerals (clinopyroxene, plagioclase, and Ti-magnetite) of studied picrites, and (3) as daughter phases in silicate melt inclusions hosted by olivine and Cr-spinel phenocrysts. The sulfide melt inclusions in olivine and the groundmass of studied rocks are composed of several sulfide phases that correspond to the monosulfide (Fe–Ni; Mss) and intermediate (Fe–Cu–Ni; Iss) solid solutions. Several 0.5], continuously evolve with crystallization of (e.g., increasing Cu/Ni and Au/PGE) and demonstrate metal fractionation between Mss and Iss. Although the compositional systematics found in this study are consistent with those previously recorded, the compositions of individual sulfide phases are strongly affected by the noble metal (PGE, Au) “nuggets” that exsolve at subsolidus temperatures and form during serpentinization of the rocks. We conclude that the budget of noble metals in the studied picrites is controlled by sulfides, but the abundances of Pt and Au are influenced by mobility in post-magmatic alteration. Our data can be also used for modeling sulfide saturation at crustal pressures and understanding behavior of the noble metals in primitive oceanic magmas.