Impression creep behavior of Al–1.9%Ni–1.6%Mn–1%Mg alloy

In the present study, microstructure and creep behavior of an Al–1.9%Ni–1.6%Mn–1%Mg alloy were studied at temperature ranging from 493 to 513 K and under stresses between 420 and 530 MPa. The creep test was carried out by impression creep technique in which a flat ended cylindrical indenter was impressed on the specimens. The results showed that microstructure of the alloy is composed of primary α(Al) phase covered by a mantle of α(Al)+Ni₃Al intermetallic compound. Mn segregated into Al_xMn_yNi_z or Al₆Mn phases distributed inside the matrix phase. It was found that the stress exponent, n, decreases from 5.2 to 3.6 with increasing temperature. Creep activation energies between 115 kJ/mol and 151 kJ/mol were estimated for the alloy and it decreases with rising stress. According to the stress exponent and creep activation energies, the lattice and pipe diffusion- climb controlled dislocation creep were the dominant creep mechanism.     

Microstructure evolution and flow behavior of hot-rolled aluminum – 5% B4C composite

Differential strain rate compression tests were conducted to study flow behavior of hot rolled Al–5 wt% B₄C composite as a function of sample orientation (longitudinal and transverse) over the temperature and strain rate ranges of 25–500 °C and 10⁻⁴ to 1 s⁻¹, respectively. The longitudinal samples are found to show lower flow stress than that shown by the transverse samples in the temperature range of 25–200 °C. The reverse becomes true at higher temperatures of 300–500 °C. The values of stress exponent (n) and activation energy for deformation (Q), based on applied stress, ranged from 10 to 46 and 307–416 kJ/mol, respectively. However, by considering effective stress, these values were reduced to n = 8 and Q = 126–190 kJ/mol. This stress exponent ofn = 8 is further reduced to n = 5 by considering substructural evolution, which suggests the dislocation climb creep mechanism to be favorable for deformation.     

Impression creep behavior of a cast MRI153 magnesium alloy

Creep behavior of a cast MRI153 magnesium alloy was investigated using impression creep technique. The tests were carried out under constant punching stress in the range of 360–600 MPa at temperatures between 425 and 490 K. Microstructure of the alloy was composed of α(Mg) matrix phase besides Mg₁₇Al₁₂ and Al₂Ca intermetallic compounds. Stress exponent of minimum creep rate, n, was found to vary between 6.45 and 7. Calculation of the activation energy showed a slight decrease with increasing stress such that the creep activation energy of 115.2 kJ/mol under σ_imp/G = 0.030 decreased to 99.5 kJ/mol under σ_imp/G = 0.040. The obtained stress exponent and activation energy data suggested that the pipe diffusion dislocation climb controlled creep as the dominant mechanism during the creep test.     

Mechanical properties and phase composition of potential biodegradable Mg–Zn–Mn–base alloys with addition of rare earth elements

Mechanical properties and creep resistance of the MgY4Zn1Mn1 alloy in the as cast as well as in the T5 condition were compared to those of the MgCe4Zn1Mn1 alloy in the same conditions. Yield tensile stress and ultimate tensile strength of the MgY4Zn1Mn1 alloy are slightly better in the temperature range 20 °C–400 °C than these of the MgCe4Zn1Mn1 alloy. Better thermal stability of ultimate tensile strength was observed in the T5 treated MgCe4Zn1Mn1 alloy than in this material in the as cast condition. An outstanding creep resistance at 225 °C–350 °C found in the MgY4Zn1Mn1 alloy is due to the existence of the 18R long period stacking structure persisting in this alloy even a long heat treatment of 500 °C/32 h. No similar stacking effects happen when Ce substitutes Y in approximately the same concentration. The creep resistance deteriorates considerably in the MgCe4Zn1Mn1 alloy. Rectangular particles of the equilibrium Mg₁₂Ce phase dominate in the microstructure of as cast as well as of high temperature heat-treated MgCe4Zn1Mn1 alloy. A population of small oval particles containing Mg and Zn develops additionally during annealing of this alloy. These particles pin effectively dislocations and can be responsible for the better thermal stability of the T5 treated material.     

The effect of heterogeneous deformation on the hot deformation of WE54 magnesium alloy

The high temperature forming behavior of WE54 magnesium alloy is studied by means of compression and tension tests. Metallographic investigation was performed to evaluate the heterogeneous deformation of the compression samples at high temperature. Dynamic recrystallization was found to be related to the amount of deformation in the various regions of the compression sample. The compression data allowed determination of the Garofalo equation describing the hot deformation behavior. The parameters n and Q, stress exponent and activation energy, of this equation were 4.4 and 237 kJ/mol respectively. This equation was used to predict the formability behavior for the hot rolling process and also to determine the maximum forming efficiency and stability of the alloy. The optimum rolling temperature was found to be 520 °C.     

The hot deformation behavior and processing map of Ti–47.5Al–Cr–V alloy

The high temperature flow behavior of as-extruded Ti–47.5Al–Cr–V alloy has been investigated at the temperature between 1100 °C and 1250 °C and the strain rate range from 0.001 s^− 1 to 1 s^− 1 by hot compression tests. The results showed that the flow stress of this alloy had a positive dependence on strain rate and a negative dependence on deformation temperature. The activation energy Q was calculated to be 409 kJ/mol and the constitutive model of this material was established. By combining the power dissipation map with instability map, the processing map was established to optimize the deformation parameters. The optimum deformation parameter was at 1150 °C–1200 °C and 0.001 s^− 1–0.03 s^− 1 for this alloy. The microstructure of specimens deformed at different conditions was analyzed and connected with the processing map. The material underwent instability deformation at the strain rate of 1 s^− 1, which was predicted by the instability map. The surface fracture was observed to be the identification of the instability.     

Effects of d.c. current on the phase transformation in 7050 alloy during homogenization

Evolutions of properties and morphologies of secondary phases in 7050 alloy homogenized with direct electric current were studied in detail by conductivity measurement, tensile test and energy dispersive X-ray microanalysis. With increasing temperature, the conductivity decreases, and while the ultimate tensile strength, yield strength and elongation of 7050 alloy increase in alloy homogenized with direct electric current, and reaches the saturation values at 440 °C/2 h. During homogenization, the brighter white AlZnMgCu phase having (weight ratio %) 12.1–15.2Mg, 27.2–32.1Al, 20.9–26.2Cu and 31.4–34.4Zn gradually transforms to the gray phase having (weight ratio %) 10.3–15.1Mg, 41.4–53.7Al, 22.9–35.2Cu and 5.8–13.1Zn and then to the dark gray phase having (weight ratio %) 10.6–14.2 Mg, 38.2–54.9Al, 23.3–44Cu, and 3.6–9.2Zn. With the application of direct electric current, the elemental diffusion network becomes profuse, the amount of gray phase increases, the diffusion of Zn accelerates, the apparent activation of the transformation from AlZnMgCu to Al₂MgCu decreases from 125.52 kJ/mol for the alloy homogenized without direct electric current to 118.82 kJ/mol, and the area fraction of secondary phase decreases by 38% at 450 °C.     

Hot deformation behavior of the new Al–Mg–Si–Cu aluminum alloy during compression at elevated temperatures

The hot deformation behavior of the new Al–Mg–Si–Cu aluminum alloy was investigated by compression tests in the temperature range 350 °C–550 °C and strain rate range 0.005 s^− 1–5 s^− 1 using Gleeble-1500 system, and the associated structural changes were studied by observations of metallographic and TEM. The results show that the true stress–true strain curves exhibit a peak stress at a small strain (< 0.15), after which the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener-Hollomon parameter in an exponent-type equation with the hot deformation activation energy Q of 236 kJ/mol. The substructure in the deformed specimens consists of very small amount and fine precipitates with equaixed polygonized subgrains in the elongated grains and developed serrations in the grain boundaries, indicating that the dynamic flow softening is mainly as the result of dynamic recovery (DR) and recrystallization (RDX).     

Effect of overheating temperature on the microstructure and creep behavior of HP40Nb alloy

HP40Nb alloy has been widely used as a high temperature material in petrochemical plants. However, overheating or local temperature excursion occurs occasionally in service and leads to serious damage on the material. Effect of temperature on the microstructure and creep performance of the HP40Nb alloy is investigated in the present work. Several specimens are cut from serviced components of the alloy and heat-treated at different temperatures from 900 °C to 1250 °C for its possible working conditions, in which the temperature of 1200 or 1250 °C is used to simulate the overheating condition of HP40Nb tubes. The microstructure of specimens is examined by scanning electron microscope (SEM) and transmission electron microscope (TEM). The creep behavior is evaluated through using impression creep tests with a flat-ended cylindrical indenter. The content of inter- and intra-dendritic carbides in the specimens is represented by the surface fraction of each phase, which has been estimated by image processing method. The results show that the total of the surface fraction of inter- and intra-dendritic carbides in the HP40Nb alloy does not significantly change at the temperature lower than 1100 °C. However, the surface fraction of inter-dendritic carbides reaches the maximum at 1100 °C. A maximal steady state impression rate is also observed at 1100 °C. The results suggest that the content of inter-dendritic carbides is the main influencing factor on the creep performance of HP40Nb alloys comparing with that of the intra-dendritic carbides.     

Effect of Nb and C on the hot flow behavior of Nb microalloyed steels

The compressive deformation behaviors of a C–Mn steel (0.36C–1.42Mn) and two Nb microalloyed steels (0.35C–1.41Mn–0.044Nb and 0.055C–1.42Mn–0.036Nb) were investigated at the temperatures from 900 °C to 1100 °C and strain rates from 0.005 s⁻¹ to 10 s⁻¹ on Gleeble-1500 thermo-mechanical simulator. It was found that the flow stress of the C–Mn steel is the lowest among the experimental steels, indicating that Nb microalloying in HSLA steels can effectively increase the hot deformation flow stress, and the 0.055C–1.42Mn–0.036Nb steel has a higher flow stress than that of the 0.35C–1.41Mn–0.044Nb steel, indicating that C addition generates a softening effect. The flow stress constitutive equations of hot deformation were developed for the experimental steels, the activation energy Q about 360 kJ/mol for the 0.055C–1.42Mn–0.036Nb steel was higher than that for the 0.35C–1.41Mn–0.044Nb steel (347 kJ/mol) and the C–Mn steel (278 kJ/mol). Characteristic points of flow stress for the three steels were analyzed. The results showed that Nb addition can effectively increase the peak strain and the steady state strain of steels, thus delay distinctly the occurrence of dynamic recrystallization, while C addition can reduce the peak strain and the steady state strain of Nb microalloyed steels, thus promote the occurrence of dynamic recrystallization.     

Dynamic strain ageing of P91 grade steels of varied silicon content

Plate materials of modified 9Cr–1Mo steel (P91) containing four levels of silicon (Si) exhibited dynamic strain ageing (DSA) phenomenon characterized by gradually reduced failure strain and serrations in the engineering stress–strain diagrams under tensile loading within a temperature range of ambient to 400 °C. Since parameters including dislocation density (ρ), activation energy (Q) and work-hardening index (n) can play important roles in developing a mechanistic understanding of DSA, efforts have been made to evaluate them. A maximum value of ρ was calculated at 400 °C due to the formation of critical sized precipitates near grain boundaries, thus impairing the dislocation motion. The average value of Q ranged between 64 and 80 kJ/mole depending on Si content. The magnitude of n was gradually enhanced with increasing temperature up to 400 °C, followed by significant drop at 550 °C. Except for the alloy with 2.0 wt.% Si, the other tested materials exhibited a combination of intergranular cracking and cleavage failures at the primary fracture surface.     

Hot deformation and processing map of an as-extruded Mg–Zn–Mn–Y alloy containing I and W phases

The hot deformation characteristics of an as-extruded ZM31 (Mg–Zn–Mn) magnesium alloy with an addition of 3.2 wt.% Y, namely ZM31 + 3.2Y, have been studied via isothermal compression testing in a temperature range of 300–400 °C and a strain rate range of 0.001–1 s^− 1. A constitutive model based on hyperbolic-sine equation along with processing maps was used to describe the dependence of flow stress on the strain, strain rate, and deformation temperature. The flow stress was observed to decrease with increasing deformation temperature and decreasing strain rate. The deformation activation energy of this alloy was obtained to be 241 kJ/mol. The processing maps at true strains of 0.1, 0.2, 0.3 and 0.4 were generated to determine the region of hot workability of the alloy, with the optimum hot working parameters being identified as deformation temperatures of 340–500 °C and strain rates of 0.001–0.03 s^− 1. EBSD examinations revealed that the dynamic recrystallization occurred more extensively and the volume fraction of dynamic recrystallization increased with increasing deformation temperature. The role of element Y and second-phase particles (I- and W-phases) during hot compressive deformation was discussed.     

Influence of solution treatment on microstructure and mechanical properties of a near β titanium alloy Ti-7333

The effect of solution treatment on the microstructure and mechanical properties of Ti-7333, a newly developed near β titanium alloy, was investigated. Compared to Ti-5553 and Ti-1023, Ti-7333 possesses the slowest α to β dissolution rate, allowing a wider temperature window for processing. The rate of β grain growth decreases with the increase of soaking time and increases with the increase of solution temperature. The β grain growth exponents (n) are 0.30, 0.31, 0.32 and 0.33 for solution treatment temperature of 860 °C, 910 °C, 960 °C and 1010 °C, respectively. The activation energy (Q_g) for β grain growth is 395.6 kJ/mol. Water cooling or air cooling after solution treatment have no significant influence on microstructure, which offers large heat treatment cooling window. However, under furnace cooling, the fraction of α phase increases sharply. α phase maintains strictly the Burgers orientation relation with β phase ({0 0 0 1}_α//{1 1 0}_β and 〈1 1 −2 0〉_α//〈1 1 1〉_β), except the α_p particles formed during forging. The tensile strength decreases with the increase of the solution temperature when only solution treatment is applied, whereas the ductility increases gradually. When aging is applied subsequently, the tensile strength increases with the increase of the solution temperature and the ductility decreases gradually.     

Investigation and improvement on structural stability and stress rupture properties of a Ni–Fe based alloy

The structural stability and stress rupture properties of a Ni–Fe based alloy, considered as boiler materials in 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants, was studied. Investigation on the structural stability of the existing alloy GH984 shows that the most important changes in the alloys are γʹ coarsening, the γʹ to η transformation and the coarsening and agglomeration of grain boundary M₂₃C₆ during thermal exposure. The stress rupture strength was found to be slightly lower than the requirement of 700 °C A-USC. The fracture mode of creep tested specimens was intergranular fracture. Detailed analysis revealed that η phase precipitation is sensitive to Ti/Al ratio and can be suppressed by decreasing Ti/Al ratio. The coarsening behavior of γʹ phase is related to Fe content. Adding B and P was suggested to stabilize M₂₃C₆ and increase grain boundary strength. Based on the research presented and analysis of the data, a modified alloy was developed through changes in composition. For the modified alloy, η phase is not observed and M₂₃C₆ is still blocky and discretely distributes along grain boundary after thermal exposure at 700 °C for 20,000 h. Moreover, the creep strength is comparable to the levels of Ni-based candidate alloys for 700 °C A-USC.     

Characterization of hot compression behavior of a new HIPed nickel-based P/M superalloy using processing maps

Isothermal forging was a critical step process to fabricate the high-performance nickel-based superalloy. The temperature and strain rate served the most critical role in determining its microstructure and mechanical properties. In this article, we employed the hot compression to simulate the isothermal forging process upon the temperature ranging from 1000 °C to 1100 °C in combination with a strain rate of 0.001–1.0 s^− 1 for a new P/M nickel-based alloy. The activation energy was determined as 903.58 kJ/mol and the processing maps at a strain range of 0.4–0.7 were developed. The instability domains were more inclined to occur at strain rates higher than 0.1 s^− 1 and manifested in the form of adiabatic shear bands. The map further demonstrated that the regions with peak efficiency of 55% were located at 1080 °C/0.0015 s^− 1 and 1095 °C/0.014 s^− 1, respectively. Obvious dynamic recrystallization could be detected at the strain rate 0.01 s^− 1 leading to a significant flow stress drop and the grain growth was remarkably triggered under 1100 °C. The findings can shed light on the forging processing optimization of the new nickel-based superalloy.     

Tensile properties of hot rolled Mg–3Sn–1Ca alloy sheets at elevated temperatures

Mechanical behavior of hot rolled Mg–3Sn–1Ca (TX31) magnesium alloy sheets were studied in the temperature range 25–350 °C. The microstructure of the alloy consisted of the eutectic structure of α-Mg + Mg₂Sn and a dispersion of needle-like CaMgSn. The highest room-temperature ductility of 18% was obtained by hot rolling of the cast slabs at 440 °C, followed by annealing at 420 °C. The high temperature tensile deformation of the material was characterized by a decrease in work hardening exponent (n) and an increase in strain rate sensitivity index (m). These variations resulted in respective drops of proof stress and tensile strength from 126.5 MPa and 220 MPa at room temperature to 23.5 MPa and 29 MPa at 350 °C. This was in contrast to the ductility of the alloy which increased from 18% at room temperature to 56% at 350 °C. The observed variations in strength and ductility were ascribed to the activity of non-basal slip systems and dynamic recovery at high temperatures. The TX31 alloy showed lower strength than AZ31 magnesium alloy at low temperatures, while it exhibited superior strength at temperatures higher than 200 °C, mainly due to the presence of thermally stable CaMgSn particles.     

Characterization of hot deformation behavior of a new Ni–Cr–Co based P/M superalloy

The hot deformation behavior of a new Ni–Cr–Co based P/M superalloy was studied in the temperature range of 950–1150 °C and strain rate range of 0.0003–1 s^? 1 using hot compression tests. It was characterized by true stress–true strain curves, constitutive equation, strain rate sensitivity m contour maps, power dissipation η maps and hot processing maps. The microstructural validation of processing maps was also done. The results show that the flow stress decreases with increasing temperature and decreasing strain rate. The hot deformation apparent activation energy of the Ni–Cr–Co based P/M superalloy at peak stress is 805 kJ/mol. The m and η contour maps are similar, and the values of m and η in the peak zones increase with increasing strain. When the strain is 0.5, a domain with its peak η of 40% and peak m of 25% occurs at 1050 °C and 0.0003 s^? 1, which corresponds to dynamic recrystallization and can be as an optimum condition for good workability.     

Hot tensile behavior of an extruded Al–Zn–Mg–Cu alloy in the solid and in the semi-solid state

This is the first reported research into the tensile behavior of as-deformed Al–Zn–Mg–Cu alloy in the semi-solid state. Tensile tests of extruded 7075 aluminium alloy were carried out in the high temperature solid and semi-solid states. Based on the tensile results and microstructural examination, the tensile behavior can be divided into three stages according to the effect of liquid: one behaves in predominantly ductile character between 400 and about 520 °C (f_l ∼ 0.31%), one is governed by both of solid and liquid between 520 and 550 °C (f_l ∼ 2%), and almost completely dominated by liquid above ∼550 °C. A brittle temperature range (519–550 °C) is proposed, in which the as-deformed Al–Zn–Mg–Cu alloy exhibits large crack probability. An equation based on ultimate tensile stress and temperature is proposed.     

Diffusion bonding of titanium alloy to micro-duplex stainless steel using a nickel alloy interlayer: Interface microstructure and strength properties

In the present study, diffusion bonding of titanium alloy and micro-duplex stainless steel with a nickel alloy interlayer was carried out in the temperature range of 800–950 °C for 45 min under the compressive stress of 4 MPa in a vacuum. The bond interfaces were characterised by scanning electron microscopy, electron probe microanalyzer and X-ray diffraction analysis. The layer wise Ni₃Ti, NiTi and NiTi₂ intermetallics were observed at the nickel alloy/titanium alloy interface and irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ was observed in the Ni₃Ti intermetallic layer. At 950 °C processing temperature, black island of β-Ti phase has been observed in the NiTi₂ intermetallics. However, the stainless steel/nickel alloy interface indicates the free of intermetallics phase. Fracture surface observed that, failure takes place through the NiTi₂ phase at the NiA–TiA interface when bonding was processed up to 900 °C, however, failure takes place through NiTi₂ and β-Ti phase mixture for the diffusion joints processed at 950 °C. Joint strength was evaluated and maximum tensile strength of ∼560 MPa and shear strength of ∼415 MPa along with ∼8.3% ductility were obtained for the diffusion couple processed at 900 °C for 45 min.     

Deformation behavior and microstructural evolution of directionally solidified TiAlNb-based alloy during thermo-compression at 1373–1573 K

Alloy Ti44Al6Nb1.0Cr2.0V0.15Y0.1B is newly designed and melted by vacuum consumable melting method, and is then prepared by cold crucible directional solidification (CCDS) technology. Thermo-compression experiment is carried out on CCDS billets at 1373–1573 K. Experimental results show that, when compressing direction (CD) is perpendicular to columnar grains, higher deformation active energy of Q = 689.8 kJ/mol can be acquired which means excellent creep resistance in radial direction; the microstructural evolution with different lnZ parameters is investigated by electron back scattering diffraction and the deformation mechanism is discussed; the ordering temperature of B2/β phase is deduced to be in the range of 1200–1250 °C. When the CD is parallel to the columnar grains, a much higher peak stress is acquired which means more excellent creep resistance in the axial direction; the shearing deformation feature is presented and the schematic of deformation mechanism is given.