Improved High-Temperature Microstructural Stability and Creep Property of Novel Co-Base Single-Crystal Alloys Containing Ta and Ti

The influence of Ta and Ti additions on microstructural stability and creep behavior in novel Co-Al-W base single-crystal alloys has been investigated. Compared to the ternary alloy, the γ′ solvus temperature and γ′ volume fraction were raised by individual additions of Ta and Ti, and increased further in the quinary alloy containing both alloying additions. In contrast to ternary and quaternary alloys, an improved microstructural stability with the stable γ–γ′ two-phase microstructure and more than 60% γ′ volume fraction existed in the quinary alloy after prolonged aging treatment at 1050°C for 1000 h. The creep behavior at 900°C revealed lower creep rates and longer rupture lives in the quaternary alloys compared to the ternary alloy, whereas the quinary alloy exhibited even better creep resistance. When the creep temperature was elevated to about 1000°C, the creep resistance of the quinary alloy exceeded the previously reported Co-Al-W-base alloys and first-generation Ni-base single-crystal superalloys. The improved creep resistance at approximately 1000°C was considered to be associated with high γ′ volume fraction, γ′ directional coarsening, and dislocation substructure, which included γ–γ′ interfacial dislocation networks and the sheared γ′ precipitates containing stacking faults and anti-phase boundaries.     

Influence of cooling rate on secondary phase precipitation and proeutectoid phase transformation of micro-alloyed steel containing vanadium

During continuous casting process of low carbon micro-alloyed steel containing vanadium, the evolution of strand surface microstructure and the precipitation of secondary phase particles (mainly V(C, N)) are significantly influenced by cooling rate. In this paper, influence of cooling rate on the precipitation behavior of proeutectoid α-ferrite at the γ-austenite grain boundary and in the steel matrix are in situ observed and analyzed through high temperature confocal laser scanning microscopy. The relationship between cooling rate and precipitation of V(C, N) from steel continuous casting bloom surface microstructure is further studied by scanning electron microscopy and electron dispersive spectrometer. Relative results have shown the effect of V(C, N) precipitation on α-ferrite phase transformation is mainly revealed in two aspects: (i) Precipitated V(C, N) particles act as inoculant particles to promote proeutectoid ferrite nucleation. (ii) Local carbon concentration along the γ-austenite grain boundaries is decreased with the precipitation of V(C, N), which in turn promotes α-ferrite precipitation.     

Effect of Spheroidizing Annealing on Microstructure and Mechanical Properties of High-Carbon Martensitic Stainless Steel 8Cr13MoV

The effects of holding time during both austenitizing and spheroidizing on microstructure and mechanical properties of high-carbon martensitic stainless steel 8Cr13MoV were experimentally studied. The results showed that the amount of carbides and the proportion of fine carbides decrease first and then increase with the increase in austenitizing time (t ₁) in the case of short spheroidizing time (t ₂), whereas the amount of the lamellar carbides increases. In the case of long t ₂, both the amount of carbides and the proportion of fine carbides decrease, and the amount of the lamellar carbides did not increase. The hardness of the steel decreases first and then increases with the increase of t ₁. Under the conditions of different t ₁, the change in the size of carbides and hardness of the steel show a same trend with the variation of t ₂. The size of spheroidized carbides increases, whereas the hardness of the steel decreases with increasing t ₂. The longer the holding time of austenitizing, the higher is the spheroidizing rate at the earlier stage. However, the spheroidizing rate shows an opposite trend with t ₁ at the later stage of spheroidizing. The effect of cooling rate on microstructure is similar with t ₂. With increasing cooling rate, the dimension of carbides became smaller, and the amount of lamellar carbides increased. The elongation of the sample fracture exhibits no corresponding relationship with holding time, whereas it is closely related to the precipitation of secondary carbides caused by the alloying elements segregation.     

A Study of Tensile Flow and Work-Hardening Behavior of Alloy 617

The simple power relationship σ?=?Κε _p ⁿ satisfactorily expresses the tensile flow behavior of many metals and alloys in their uniform plastic strain regime. However, many FCC materials with low stacking fault energy have opposed such power law relationship. Alloy 617, an age-hardenable Ni-based superalloy is also observed not to obey the simple power law relationship neither in its solution-treated nor in its aged conditions. Various flow relationships were used to obtain the best fit for the tensile data, and different relationships were identified for the different aged conditions. The work-hardening rate (θ) demonstrates three distinct regions for all aged conditions, and there is an obvious change in the trend of θ versus σ. In the initial portion, θ decreases rapidly followed by a gradual increase in the second stage and again a decrease in its third stage is perceived in the Alloy 617. These three-stage characteristics are attributed to a commonly known precipitate, γ′: Ni₃(Ti, Al) which evolves during aging treatment and well recognized under transmission electron microscopy (TEM) observation. TEM results also reveal a slight degree of coarsening in γ′ over aging. The tensile flow and the work-hardening behavior are well correlated with other microstructural evolution during the aging treatments.     

Phase Equilibria of the Ternary Al-Cu-Zn Alloys on Al-Zn Rich Side

Phase equilibria of the Al-Cu-Zn system on Al-Zn rich side was experimentally determined with 16 alloys annealed at 360 °C. The annealed alloys were examined by means of x-ray diffraction, electron probe microanalysis and differential scanning calorimetry. Five single-phase regions and seven two-phase regions as well as three three-phase regions, i.e. α-(Al)?+?θ-Al₂Cu?+?τ′-Al₄Cu₃Zn, α-(Al)?+?τ′-Al₄Cu₃Zn?+?ε-CuZn₄ and α-(Al)?+?ε-CuZn₄?+?(Zn), were determined. The partial isothermal section of the Al-Cu-Zn system on Al-Zn rich side at 360 °C was constructed based on the obtained experimental data in this work. It was observed that the solid solution phase α-(Al) would easily decompose into ε-CuZn₄, (Zn) and α′-(Al) at the ambient temperature in the early stages. The ternary phase τ′-Al₄Cu₃Zn would form and ε-CuZn₄ would disappear gradually along with the extension of aging time.     

Effect of Cold Rolling and Heat Treatment on Microstructure and Mechanical Properties of Ti-4Al-1Mn Titanium Alloy

Mechanical behavior of Ti-4Al-1Mn titanium alloy has been studied in annealed, cold-rolled and heat-treated conditions. Room temperature tensile strength as well as % elongation has been found to be low with increasing amount of cold rolling. Lowering of strength in cold worked condition is attributed to premature failure. However, the same has been mitigated after heat treatment. Significant effect of cooling media (air and water) from heat treatment temperature on microstructure was not found except for the degree of fineness of α plates. Optimum properties (strength as well as ductility) were exhibited by samples subjected to 15% cold rolling and heat treatment below β transus temperature, which can be attributed to presence of recrystallized microstructure. In cold worked condition, the microstructure shows fine fragmented α plates/Widmanstätten morphology with high dislocation density along with a large amount of strain fields and twinning, which gets transformed to recrystallized equiaxed microstructure and with plate-like morphology after near β heat treatment. Prior cold work is found to have a significant effect on mechanical properties supported by evolution of microstructure. Twinning is found to be assisting in deformation as well as in recrystallization through the formation of deformation and annealing twins during cold working and heat treatment. Fracture analysis of the tested sample with prior cold work and heat-treated condition revealed quasi-ductile failure as compared to only ductile failure features seen for samples heat treated without prior cold work.     

Phase Equilibria and Microhardness of As-Cast and Annealed Ni-Al-Os Alloys in Ni-Rich Region

In this paper, the isothermal section at 1273 K and liquidus projection of ternary Ni-Al-Os system in Ni-rich region were firstly measured by using 6 annealed and 6 as-cast Ni-Al-Os alloys with 65 at.% Ni in combination with x-ray diffraction, optical microscopy and electron probe microanalysis techniques. For the determined partial isothermal section at 1273 K, 2 single-phase, 4 two-phase and 2 three-phase regions were observed. The solubilities of Os in both γ and γ′ phases were also determined. For the proposed liquidus projection, four primary surfaces of γ′, γ, β and δ, and two invariant reactions were identified. Secondly, the microhardness of both as-cast and annealed alloys were measured. The evolution trend of microhardness in both as-cast and annealed alloys with Os addition generally increases first, and then decreases. Thirdly, the further comprehensive discussion on possible substitution of Re by Os in new-generation nickel-based single-crystal superalloys were performed in terms of strengthening degree, high-temperature creep resistance, and possibility for formation of harmful topologically close-packed phases. It was finally concluded that Os may be used as a new additional element to replace or partly replace Re in Ni-based single crystal superalloys.     

Creep Properties of Directionally Solidified Nb-Modified Ni-Base Superalloy,Mar-M247

The Ni-base superalloy Mar-M247 is widely used for elevated-temperature applications in both equiaxed and directionally solidified forms. The alloy contains about 3 wt.% Ta. Due to the cost and density of Ta, an atom-for-atom substitution of Nb for Ta was investigated. The creep properties in the temperature range of 750-950?°C and stresses ranging from 200 to 1000 MPa were determined. In general, the creep properties of the Nb-modified Mar-M247 material were very similar to those of Mar-M247 samples at 750 and 850?°C. However, the Mar-M247 samples exhibited superior creep properties at 950?°C. The lower creep properties of the Nb-modified Mar-M247 are likely due to the finer starting microstructure, more rapid coarsening rate of the γ′ and reduced γ′ strengthening effect (i.e., APB energy) of Nb, compared to Ta. No evidence of TCP phases was observed in either alloy.     

Analysis of factors responsible for the accelerated creep rupture of 12% Cr martensitic steel weld joints

In the process of the investigation of the heat resistance of a 0.07C–12Cr–Ni–Mo–V–Nb steel of the martensitic–ferritic class, a reduction was revealed in the long-term strength of its welded joints to below the level of the strength of the base metal. To establish the causes for the accelerated failure of the welded joints, an imitation of the thermal cycles was carried out that produce the structure of the heataffected zone using a dilatometer. In the samples with the structure that corresponds to that of the heataffected zone, a local zone of softening was revealed. The investigations of the metal structure using transmission electron microscopy have shown that the reduction in the creep rupture strength was caused by structural changes under the conditions of the thermal cycle of welding upon the staying of the steel in the temperature range between the Ac₁ and Ac₃ points.     

Dynamic Recrystallization Kinetics and Microstructural Evolution for LZ50 Steel During Hot Deformation

The dynamic recrystallization (DRX) behavior of LZ50 steel was investigated using hot compression tests at a deformation temperature of 870-1170 °C and a strain rate of 0.05-3 s^?1. The effects of deformation temperature, strain, strain rate, and initial austenite grain size on the microstructural evolution during DRX were studied in detail. The austenite grain size of DRX was refined with increasing strain rate and decreasing temperature, whereas the initial grain size had no influence on DRX grain size. A model based on the Avrami equation was proposed to estimate the kinetics of the DRX under different deformation conditions. A DRX map, which was derived from the DRX kinetics, the recrystallized microstructure, and the flow stress analysis, can be used to identify optimal deformation conditions. The initiation of DRX was lower than Z _c (critical Zener-Hollomon parameter) and higher than ε_c (critical strain). The relationship between the DRX microstructure and the Z parameter was analyzed. Fine DRX grain sizes can be achieved with a moderate Z value, which can be used to identify suitable deformation parameters.     

Evolution of grain–subgrain structure and carbide subsystem upon annealing of a low-carbon low-alloy steel subjected to high-pressure torsion

The effect of annealing on the evolution of an ultrafine-grain structure and carbides in a 06MBF steel (Fe–0.1Mo–0.6Mn–0.8Cr–0.2Ni–0.3Si–0.2Cu–0.1V–0.03Ti–0.06Nb–0.09C, wt %) has been studied. The grain–subgrain structure (d = 102 ± 55 nm) formed by high-pressure torsion and stabilized by dispersed (MC, M₃C, d = 3–4 nm) and relatively coarse carbides (M₃C, d = 15–20 nm) is stable up to a temperature of 500°C (1 h) (d = 112 ± 64 nm). Annealing at a temperature of 500°C is accompanied by the formation in regions with a subgrain structure of recrystallized grains, the size of which is close to the size of subgrains formed by high-pressure torsion. The average size and distribution of dispersed particles change weakly. The precipitation hardening and the increase in the fraction of high-angle boundaries in the structure cause an increase in the values of the microhardness to 6.4 ± 0.2 GPa after annealing at 500°C as compared to the deformed state (6.0 ± 0.1 GPa). After 1-h annealing at 600 and 700°C, the microcrystal size (d = 390 ± 270 nm and 1.7 ± 0.7 μm, respectively) increases; the coarse M₃C (≈ 50 nm) and dispersed carbides grow by 5 and 8 nm, respectively. The value of the activation energy for grain growth Q = 516 ± 31 kJ/mol upon annealing of the ultrafine-grained steel 06MBF produced by high-pressure torsion exceeds the values determined in the 06MBF steel with a submicrocrystalline structure formed by equal-channel angular pressing and in the nanocrystalline α iron.     

Mechanical and Tribological Properties of HVOF-Sprayed (Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-NiCr+Ni) Composite Coating on Ductile Cast Iron

The aim of the investigations was to compare the microstructure, mechanical, and wear properties of Cr₃C₂-NiCr+Ni and Cr₃C₂-NiCr coatings deposited by HVOF technique (the high-velocity oxygen fuel spray process) on ductile cast iron. The effect of nickel particles added to the chromium carbide coating on mechanical and wear behavior in the system of Cr ₃ C ₂ -NiCr+Ni/ductile cast iron was analyzed in order to improve the lifetime of coated materials. The structure with particular emphasis of characteristic of the interface in the system of composite coating (Cr ₃ C ₂ -NiCr+Ni)/ductile cast iron was studied using the optical, scanning, and transmission electron microscopes, as well as the analysis of chemical and phase composition in microareas. Experimental results show that HVOF-sprayed Cr₃C₂-NiCr+Ni composite coating exhibits low porosity, high hardness, dense structure with large, partially molten Ni particles and very fine Cr₃C₂ and Cr₇C₃ particles embedded in NiCr alloy matrix, coming to the size of nanocrystalline. The results were discussed in reference to examination of bending strength considering cracking and delamination in the system of composite coating (Cr ₃ C ₂ -NiCr+Ni)/ductile cast iron as well as hardness and wear resistance of the coating. The composite structure of the coating provides the relatively good plasticity of the coating, which in turn has a positive effect on the adhesion of coating to the substrate and cohesion of the composite coating (Cr₃C₂-NiCr+Ni) in wear conditions.     

Comparative study of Oswald ripening and trans-interface diffusion-controlled theory models: Coarsening of γ′ precipitates affected by elastic strain along a concentration gradient

According to Lifshitz, Slyozov, and Wagner (LSW) and Trans-Interface Diffusion-Controlled (TICD) theoretical models, this paper reports the microstructure and its coarsening behavior of γ′ metastable-coherent precipitates in concentration gradient of Ni-13.75Ti (at%)/Ni generated by diffusion couple. The coarsening of precipitates was evaluated in two different Ti contents (R1-11.4Ti (at%) and R2-13Ti (at%)) generated along the concentration gradient and includes average size, size distributions and growth rate. The solvus and metastable-coherent bimodal lines as determined at 850 °C of 9.16 (at%) and 9.92Ti (at%) respectively by scanning electron microscopy. This paper suggests that elastic strains produced by the matrix/precipitate lattice mismatch caused significant deviations between the experimental results and those predicted by the LSW or TIDC theories. Activation energies for TIDC (Q _i ) and LSW (Q _r ) are Q _r : 219.69 and 172.61 kJ mol^-1 for R1 and R2 regions, respectively, and Q _i : 218.46 and 164.56 kJmol^-1 for R1 and R2 regions, respectively. A concentration gradient allows the study of various alloys with different concentration and volume-fraction in a single sample.     

Quantification of Microtexture at Weld Nugget of Friction Stir-Welded Carbon Steel

Friction stir welding of C-Mn steel was carried out under ~800-1400 rpm tool rotation. Tool traversing speed of ~50 mm/min remained same for all joints. Effect of thermal state and deformation on texture and microstructure at weld nugget was investigated. Weld nugget consisted of ferrite + bainite/Widmanstatten ferrite with different matrix grain sizes depending on peak temperature. A texture around (? ₂ = 0°, φ = 30°, ? ₂ = 45°) was developed at weld nugget. Grain boundary misorientation at weld nugget indicated that continuous dynamic recrystallization influenced the development of fine equiaxed grain structure. Pole figures and orientation distribution function were used to determine crystallographic texture at weld nugget and base metal. Shear texture components D₁, D₂ and F were present at weld nugget. D₁ shear texture was more prominent among all. Large number of high-angle grain boundaries (~60-70%) was observed at weld nugget and was the resultant of accumulation of high amount of dislocation, followed by subgrain formation.     

A Thermodynamic Study of a Constitutional Diagram for Duplex Stainless Steels

Arc welding involves joining two metal plates by a welding pool, which is kept molten for short periods due to an electric arc. This weld metal has, sometimes, a different composition from the plates, due to a filler metal. Arc welding is a non-equilibrium process. Technological approach, nevertheless, uses empirical tools to predict important features of the joints. In stainless steel welding, the Schaeffler diagram, and other constitutive diagrams, as the WRC 1992 diagram, are used to predict the resultant microstructure in the welded joint. The composition input is calculated by Chromium and Nickel equivalents expressions. Schaeffler diagram is used to evaluate weldability of stainless steels. Some people adapt the W and N alloying elements in Schaeffler diagram to predict the weldability of duplex stainless steels (DSS). Present work shows how these technological tools can be understood as results of phase equilibrium in key isotherms of the Fe-Cr-Ni system, used in all constitutive diagrams. The validity and limitation of the Chromium and Nickel equivalents expressions were discussed based on the effect of up to two alloying components (X₁ = Mo or W and X₂ = N) on the alpha/gamma phase equilibrium, as estimated by calculations in system Fe-Cr-Ni-X₁-X₂ that are typical DSS alloying elements. This allows extending these Chromium and Nickel equivalents expressions based on thermodynamics and to identify vulnerabilities for two important problems in DSS welding: Chromium nitride and sigma phase precipitation. Originally, Schaeffler’s diagram does not consider the effect of W and N in the (γ/α + γ) equilibrium field.     

Microstructure and Room-Temperature Mechanical Properties of FeCrMoVTi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> High-Entropy Alloys

FeCrMoVTi _x (x values represent the molar ratio, where x = 0, 0.5, 1.0, 1.5, and 2.0) high-entropy alloys were prepared by a vacuum arc melting method. The effects of Ti element on the microstructure and room-temperature mechanical properties of the as-cast FeCrMoVTi _x alloys were investigated. The results show that the prepared alloys exhibited typical dendritic microstructure and the size of the microstructure became fine with increasing Ti content. The FeCrMoV alloy exhibited a single body-centered cubic structure (BCC1) and the alloys prepared with Ti element exhibited BCC1 + BCC2 mixed structure. The new BCC2 phase is considered as (Fe, Ti)-rich phase and was distributed in the dendrite region. With the increase of Ti content, the volume fraction of the BCC2 phase increased and its shape changed from a long strip to a network. For the FeCrMoV alloy, the fracture strength, plastic strain, and hardness reached as high as 2231 MPa, 28.2%, and 720 HV, respectively. The maximum hardness of 887 HV was obtained in the FeCrMoVTi alloy. However, the fracture strength, yield stress, and plastic strain of the alloys decreased continuously as Ti content increased. In the room-temperature compressive test, the alloys showed typical brittle fracture characteristics.     

Influence of Zinc on Coarsening of δ-Ni<Subscript>2</Subscript>Si Particles,Aging Behavior and Hardness in a Cu-Ni-Si Alloy

In the present paper, the aging precipitation and coarsening of disk-like δ-Ni₂Si particles in Cu and Cu-10Zn alloys aged at 450 °C have been investigated by hardness, electric resistivity measurement and transmission electron microscopy observation. The coarsening dynamics of the average diameter of the δ-Ni₂Si particles coincides with the t ^1/3 time law for both alloys. The coarsening of the diminution of supersaturation related to aging time t coincides with the t ^?1/3 time rule. Adding Zn to the Cu-Ni-Si alloy increases the growth and coarsening rate of the particles mainly because of the increased diffusivity D of the δ-Ni₂Si particles in the matrix. The value of D of the δ-Ni₂Si particles in the Cu-xZn (x = 0, 10 wt.%) matrix and the Cu/δ-Ni₂Si interfacial energy γ are independently calculated by using the Lifshitz–Slyozov–Wagner theory which was extended to include disk-like particles by Boyd and Nicholson. The values of D and γ increase from 0.77 × 10^?19 to 2.21 × 10^?19 m²/s and 0.19 to 0.63 J/m², respectively, when Zn is added to the Cu-Ni-Si alloy. These calculations and the analysis show that the properties of Cu-Ni-Si-Zn alloy can significantly be enhanced by reducing the aging temperature.     

Analysis of Plastic Flow Instability During Superplastic Deformation of the Zn-Al Eutectoid Alloy Modified with 2 wt.% Cu

The aim of this work is to analyze the plastic flow instability in Zn-21Al-2Cu alloy deformed under 10^?3 s^?1 and 513 K, which are optimum conditions for inducing superplastic behavior in this alloy. An evaluation using the Hart and Wilkinson–Caceres criteria showed that the limited stability of plastic flow observed in this alloy is related to low values of the strain-rate sensitivity index (m) and the strain-hardening coefficient (γ), combined with the tendency of these parameters to decrease depending on true strain (ε). The reduction in m and γ values could be associated with the early onset of plastic instability and with microstructural changes observed as function of the strain. Grain growth induced by deformation seems to be important during the first stage of deformation of this alloy. However, when ε > 0.4 this growth is accompanied by other microstructural rearrangements. These results suggest that in this alloy, a grain boundary sliding mechanism acts to allow a steady superplastic flow only for ε < 0.4. For ε values between 0.4 and 0.7, observed occurrences of microstructural changes and severe neck formation lead to the supposition that there is a transition in the deformation mechanism. These changes are more evident when ε > 0.7 as another mechanism is thought to take over.     

Multicomponent Phase Diagram of Lean Duplex Stainless Steel UNS S82441 and Its Application to Evaluate the Microstructure in the Heat Affected Zone

Duplex stainless steels are Fe-Cr-Ni-Mo-N alloys with nitrogen replacing some nickel to improve its properties. These steels have a balanced microstructure, which means that ferrite (α) and austenite (γ) are in equal proportions of approximately 50% by volume. These alloys have a tendency to form intermetallic precipitates such as chromium nitrides and sigma phase (σ), depending on their chemical composition and welding thermal history. Precipitation of these intermetallic phases can impair mechanical and corrosion properties. In addition, material processes such as welding and heat treatment can change the balanced microstructure. The objective of this work was to compare phases formed during heat treatment and welding of duplex stainless steel UNS S82441 with the equilibrium phases predicted by Thermo-calc^® software. In general, the results showed good agreement between predicted and measured phases, as well as, agreement between the measured volume fraction of austenite in the heat-affected zone and austenite predicted by Thermo-calc^® software.     

Investigation of Interface Bonding Mechanism of an Explosively Welded Tri-Metal Titanium/Aluminum/Magnesium Plate by Nanoindentation

A tri-metal titanium/aluminum/magnesium (Ti/Al/Mg) cladding plate, with an aluminum alloy interlayer plate, was fabricated for the first time by explosive welding. Nanoindentation tests and associated microstructure analysis were conducted to investigate the interface bonding mechanisms of the Ti/Al/Mg cladding plate. A periodic wavy bonding interface (with an amplitude of approximately 30 μm and a wavelength of approximately 160 μm) without a molten zone was formed between the Ti and Al plates. The bonding interface between the Al and the Mg demonstrated a similar wavy shape, but the wave at this location was much larger with an amplitude of approximately 390 μm and a wavelength of approximately 1580 μm, and some localized melted zones also existed at this location. The formation of the wavy interface was found to result from a severe deformation at the interface, which was caused by the strong impact or collision. The nanoindentation tests showed that the material hardness decreased with increasing distance from the bonding interface. Material hardness at a location was found to be correlated with the degree of plastic deformation at that site. A larger plastic deformation was correlated with an increase in hardness.