Effect of initial temper on the creep behavior of a Mg–Gd–Nd–Zr alloy

The effect of initial temper on the tensile creep behavior of a cast Mg–Gd–Nd–Zr alloy has been investigated. Specimens in unaged, underaged and peak-aged conditions exhibit a sigmoidal creep stage between the primary and steady-state creep stage, while the overaged specimens have no such creep stage. Transmission electron microscope observations revealed that sigmoidal creep stage was induced by the dynamic precipitation in the microstructure, and the rapid formation of β₁-phase and β-phase plates takes responsibility for the softening of material in this stage. Comparative evaluation of creep properties of the specimens showed that alloy in overaged condition had creep resistance superior to those in other conditions. Stress and temperature dependence of the steady-state creep rate were studied over a temperature range of 250–300 °C and stress range of 50–100 MPa, and a dislocation creep mechanism was proposed for the alloy.     

Steady state creep and creep recovery behaviours of pre-aging Al–Si alloys

The creep and creep recovery of pre-aging Al–1 wt.%Si and Al–1 wt.%Si–0.1 wt.%Zr–0.1 wt.%Ti alloys have been investigated at room temperature under different constant stresses. The aging temperature dependence of steady creep rate, _st, and the recovery strain rate, π, show that under the same test conditions first alloy yields creep or creep recovery rates much higher as compared with those of second alloy. The stress exponent n was found to change from 2.5 to 7.43 and 4.57 to 11.99 for two alloys, respectively, characterizing dislocation slipping mechanism. The activation energies of steady state creep of the two alloys were found to be 78.4 kJ/mol and 32.8 kJ/mol for Al–Si and Al–Si–Zr–Ti alloys, respectively. The microstructure of the samples studied was investigated by optical and transmission electron microscopy (TEM).     

Creep behavior of Mg–2 wt.%Nd binary alloy

A binary magnesium alloy, Mg–2 wt.%Nd, has been prepared. Under the condition of temperature between 150 and 250 °C and applied stress between 30 and 110 MPa, the alloy exhibits good creep resistance due to both solution-hardening and especially precipitation-hardening. Tiny precipitates forming dynamically during creep have been observed, which play an important role in restricting dislocation movements. When the creep tests are carried out at the temperature range between 150 and 250 °C, the stress exponents lie in the range of 4.5–7.1 at low stresses, which is consistent with the “five-power-law”. The values of stress exponent increase up to 9.8–29.5 at high stresses indicate power-law breakdown. When the creep tests are carried out under the applied stress between 30 and 90 MPa, the apparent activation energy values vary from 70.0 to 96.0 kJ/mol at low temperatures, but increase to 199.9–246.1 kJ/mol at high temperature range. Dislocations in basal plane are activated in the primary creep stage, but as creep goes on, they are observed in non-basal plane. The creep is mainly controlled by both dislocation-climb and cross-slip.     

The interpretation of cyclic creep deformation mechanism of copper in terms of the anelastic recovery

The cyclic creep deformation behaviour of copper has been studied in the temperature range of 0.4 to 0.5T _m and under the constant stress range of (/E)=4×10^–4 to 10×10^–4. To see the effect of cyclic stress frequency, stress amplitude and the duration of the unloading time in a fixed frequency on cyclic creep behaviour, static and cyclic creep tests were conducted under the conditions mentioned above. The measured activation energies for static and cyclic creep were analysed in terms of the various experimental parameters. Anelastic behaviour during the unloading period was also studied to find out the possible assistance for the positive creep deformation in cyclic creep. Using the concept of anelastic recovery and the activation energy for the anelastic it is hypothesized that the accelerated cyclic creep deformation is controlled by the anelastic recovery during the unloading period.     

Microstructure and superplasticity of laser welded Ti–6Al–4V alloy

In this paper laser beam welding (LBW) was used to join Ti–6Al–4V alloy as a pre-forming operation before superplastic deformation (SPF) process. Superplastic deformation behavior of laser welded Ti–6Al–4V alloy was investigated. The results indicated that the welded Ti–6Al–4V alloy had good superplasticity when deformed at temperature range of 870–920 °C and strain rate range of 10⁻³–10⁻² s⁻¹, and the elongation was 233–397%. The microstructure observation indicated that dynamic recrystallization happened in the weld bead, and the acicular structure of weld bead was transforming into equiaxed grains during tensile process.     

Microstructure and creep properties of a cast intermetallic Ti–46Al–2W–0.5Si alloy for gas turbine applications

The microstructure and creep properties including minimum creep rate, time to 1% creep deformation and creep fracture time of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) were investigated. The creep specimens were prepared from investment-cast plate and two large turbine blades. Constant load creep tests were performed in air at applied stresses ranging from 150 to 400 MPa in the temperature range 973–1073 K. The microstructure of the specimens is characterised by optical, scanning and transmission electron microscopy before and after creep deformation. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of minimum creep rate is n = 7.3 and the apparent activation energy for creep is Q_a = 427 ± 14 kJ/mol. The initial microstructure of the creep specimen is unstable. The ₂(Ti₃Al)-phase transforms to γ(TiAl)-phase and needle-like B2-precipitates during long-term creep testing at all testing temperatures. At lower applied stresses, the creep specimens fail by the growth and coalescence of cavities and small cracks formed along the γ/₂ interfaces. At the highest applied stresses, the specimens fail by nucleation and propagation of cracks.     

Plastic deformation behavior in dual-phase Ni–31Al intermetallics at elevated temperature

Plastic deformation behavior of dual-phase Ni–31Al intermetallics at elevated temperature was examined. It was found that the alloy exhibited good plasticity under an initial strain rate of 1.25 × 10⁻⁴ s⁻¹ to 8 × 10⁻³ s⁻¹ in a temperature range of 950–1075 °C. A maximum elongation of 281.3% was obtained under an initial strain rate of 5 × 10⁻⁴ s⁻¹ at 1000 °C. The strain rate sensitivity, m value was correlated with temperature and initial strain rate, being in the range of 0.241–0.346. During plastic deformation, both the two phases Ni₃Al and NiAl in dual-phase Ni–31Al could co-deform without any void formation or debonding, the initial coarse microstructure became much finer after plastic deformation. Dislocation played an important role during the plastic deformation in dual-phase Ni–31Al alloy, the deformation mechanism in dual-phase Ni–31Al could be explained by continuous dynamic recovery and recrystallization.     

Creep behaviour and creep microstructures of a high-temperature titanium alloy Ti–5.8Al–4.0Sn–3.5Zr–0.7Nb–0.35Si–0.06C (Timetal 834): Part I. Primary and steady-state creep

The tensile creep behaviour of the high-temperature near -Ti alloy Ti–5.8Al–4.0Sn–3.5Zr–0.7Nb–0.35Si–0.06C (Timetal 834) with a duplex microstructure has been extensively investigated in the temperature range from 500°C to 625°C and the stress range from 100 to 550 MPa. Both primary and secondary creep are being considered. The results of the primary creep are analysed in terms of the dependencies of stress on strain (strain hardening) and on strain rate (strain rate sensitivity). It is shown that the strain-hardening exponent depends on temperature, and takes values between 0.5 for 500°C and 0.33 for higher temperatures; this would give a dependence of the primary creep strain of σ² and σ³. The strain rate exponents obtained in both primary and secondary creep have been found to be similar; this is also the case for the activation energies. It is thought that, in the stress and temperature range investigated, creep is controlled by bow-out and climb of dislocation segments pinned at lath boundaries and second-phase particle. Analysis of the dislocation substructure is presented to give some support for this mechanism.     

Creep deformation of Alloys 617 and 276 at 750–950 °C

Alloys 617 and 276 were subjected to time-dependent deformation at elevated temperatures under sustained loading of different magnitudes. The results indicate that Alloy 617 did not exhibit strains exceeding 1 percent (%) in 1000 h at 750, 850 and 950 °C when loaded to 10% of its yield strength (YS) values at these temperatures. However, this alloy was not capable of sustaining higher stresses (0.25YS and 0.35YS) for 1000 h at 850 and 950 °C without excessive deformation. Interestingly, Alloy 617 showed insignificant steady-state creep rate at 750 °C irrespective of the applied stress levels. Alloy 276 almost met the maximum creep deformation criterion when tested at 51 MPa–750 °C. Severe creep deformation of both alloys at 950 °C could be attributed to the dissolution of carbides and intermetallic phases remaining after solution annealing or precipitated during quenching.     

Hot deformation behavior and microstructure evolution of twin-roll-cast Mg–2.9Al–0.9Zn alloy: A study with processing map

The hot deformation behavior and microstructure evolution of twin-roll-cast of Mg–2.9Al–0.9Zn–0.4Mn (AZ31) alloy has been studied using the processing map. The tensile tests were conducted in the temperature range of 150–400 °C and the strain rate range of 0.0004–4 s⁻¹ to establish the processing map. The different efficiency domains and flow instability region corresponding to various microstructural characteristics have been identified as follows: (i) the continuous dynamic recrystallization (CDRX) domain in the range of 200–280 °C/≤0.004 s⁻¹ with fine grains which provides a potential for warm deformation such as deep drawing; (ii) the discontinuous dynamic recrystallization (DDRX) domain around 400 °C at high strain rate (0.4 s⁻¹ and above) with excellent elongation which can be utilized for forging, extrusion and rolling; (iii) the grain boundary sliding (GBS) domain at slow strain rate (below 0.004 s⁻¹) above 350 °C appears abundant of cavities, which result in fracture and reduce the ductility of the adopted material; and (iv) the flow instability region which locates at the upper left of the processing map shows the metallographic feature of flow localization.     

Effect of loading rate on the monotonic tensile behavior and tensile strength of an oxide–oxide ceramic composite at 1200 °C

The influence of loading rate on monotonic tensile behavior and tensile properties of an oxide–oxide ceramic composite was evaluated in laboratory air at 1200 °C. The composite consists of a porous alumina matrix reinforced with woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tensile tests conducted at loading rates of 0.0025 and 25 MPa/s revealed a strong effect of rate on the stress–strain behavior as well as on the ultimate tensile strength (UTS), elastic modulus and failure strain. At 0.0025 MPa/s, increase in stress results in non-monotonic change in strain, with the rate of change of strain reversing its sign at stresses 25 MPa/s. Several samples were subjected to additional heat treatments prior to testing in order to determine whether this unusual stress–strain behavior was an artifact of incomplete processing of fibers in the as-received material. The unusual material response in the 0–30 MPa stress range was further investigated in creep tests conducted with the applied stresses ≤26 MPa. Negative creep (i.e. decrease in strain under constant stress) was observed. Porosity measurements indicate that a decrease in matrix porosity and matrix densification may be taking place in the N720/A composite exposed to 1200 °C at stresses <30 MPa for prolonged periods of time.     

Compressive creep behavior of as-cast and aging-treated Mg–5 wt% Sn alloys

The microstructure and compressive creep behaviors of as-cast and aging-treated Mg–5 wt% Sn alloys are investigated in this paper. The compressive creep resistance of aging-treated Mg–5 wt% Sn alloy is much better than that of as-cast alloy at the applied stresses from 25 MPa to 35 MPa and the temperatures from 423 K to 473 K, which is mainly due to the dispersive distribution of Mg₂Sn phase in the aging-treated Mg–5 wt% Sn alloy. The calculated average values of stress exponent n and activation energy Q_c suggest that dislocation cross slip and dislocation climb happen respectively in as-cast and aging-treated Mg–5 wt% Sn alloys during creep.     

Structural evolution and its influence on luminescence of SiO2–SrF2–ErF3 glass ceramics prepared by sol–gel method

Er³⁺ doped SrF₂–SiO₂ transparent glass ceramics were prepared by sol–gel method and heat treatment. The decomposition of Sr²⁺–CF₃COO⁻ and the formation of SrF₂ nano-crystals were found to proceed synchronously in the xerogel. After crystallization of the xerogel, SrF₂ nano-crystals with 8–10 nm in size distributed homogenously among the glassy matrix, and the microstructure of the glass ceramic was stable under and at the temperature of 800 °C probably due to interfacial interaction between nano-crystals and glassy matrix. When heat-treated at 800 °C, the chemically bonded water in the sample was eliminated, resulting in the appearance of the visible luminescence bands of ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions.     

Thermomechanical processing of Ti–5Al–5Mo–5V–3Cr

The constitutive behaviour and microstructural evolution of the near-β alloy Ti–5Al–5Mo–5V–3Cr in the α + β condition has been characterised during isothermal subtransus forging at a range of temperatures and strain rates. The results indicate that Ti–5Al–5Mo–5V–3Cr has a shallower approach curve, and therefore, offers a more controllable microstructure than the near-β alloy Ti–10V–2Fe–3Al. Flow softening is small in magnitude in both alloys in the α + β condition. The steady state flow stresses obey a Norton–Hoff constitutive law with an activation energy of Q = 183 kJ mol⁻¹, which is similar to the activation energy for self-diffusion in the β phase, suggesting deformation is dominated by dynamic recovery in the β matrix. Good evidence is found for the existence of ω phase after both air cooling and water quenching from above the β transus. In addition, dissolution of the α phase is found to be slow at near-transus temperatures.     

Non-stationary creep simulation with a modified Armstrong–Frederick relation applied to copper canisters

A previously formulated model using back stress to handle non-stationary creep during power-law breakdown is further developed. In particular, the way to integrate the back stress is modified. Usually the Armstrong–Frederick relation has been applied, but it can give unphysical results in the sense that the back stress exceeds the tensile strength of the material. Such a problem can be solved by replacing the back stress term in this relation with the back stress deviator.The creep model is applied to copper canister in waste packages intended for encapsulating spent nuclear fuel. These waste packages will be placed in the bedrock at a depth of about 500 m as a final stage of disposal. During storage, radioactivity-induced thermal evolution raises temperature in repositories and water-saturation generates pressure directly on the copper canister. The thermally activated creep in copper canister occurs readily. To estimate the amount of creep deformation, a finite element model is set up to compute the evolution of creep deformation in copper canister. The creep model takes both stationary and non-stationary creep into account. The computed maximum creep strain is shown to be 7.8% over 10 years, which should not cause failure since measured creep elongations are in the range of 15–40%.     

Preparation and properties of barium aluminosilicate glass-ceramics

Barium aluminosilicate (BAS) glass-ceramics were prepared by hot-pressing BAS glass powder derived via the sol–gel method, and the microstructure and properties were studied. Through control of composition, BAS glass-ceramics with two different types of microstructure were obtained. One is a sub-micrometre celsian surrounded by a thin layer of glassy phase, and the other is a celsian–mullite composite microstructure. BAS glass-ceramics with the celsian–mullite composite microstructure have a strength of 204 MPa and a creep rate of ≤ 1.03 × 10^-4 h^-1 at 1250 °C. After heat treatment at 1250 °C for 100 h, no significant change of the microstructure was found, and the high-temperature strength did not decrease greatly. The BAS glass-ceramics prove to be a potentially good matrix for fibre- or whisker-reinforced composites for high-temperature applications. This revised version was published online in November 2006 with corrections to the Cover Date.     

Improved stability of organic–inorganic composite emitting film with sol–gel glass encapsulated Eu-complex

A high-stability organic–inorganic composite emitting film has been realized via a sol–gel process using an optimized silane alkoxide and Eu-complex. We found that the long-term and thermal stabilities were improved by using a combined starting solution of phenyltrimethoxysilane and tetrametoxysilane as encapsulating agent. The resulting emitting film exhibited sharp red-luminescence under ultraviolet (UV) excitation and a high transparency in the visible wavelength region. In addition, no decrease in photoluminescence (PL) quantum yield was observed after thermal treatment up to 180 °C, and the reduction in PL intensity during UV irradiation was suppressed by encapsulating the Eu-complex within the sol–gel derived silica glass.     

Creep and microstructure in powder metallurgy 15 vol.% SiC<Subscript>p</Subscript>–2009 Al composite

The creep behavior and microstructure of powder metallurgy (PM) 15 vol.% silicon particulate-reinforced 2009 aluminum alloy (SiCp–2009 Al composite) and its matrix PM 2009 Al were investigated over six orders of magnitude of strain rate and at temperatures in the range 618–678 K. The results show that the creep behavior of PM 15% SiC_p–2009 Al composite resembles that of PM 2009 Al with regard to (a) the variations in both the apparent stress exponent and the apparent activation energy for creep due to applied stress, (b) the value of the true stress exponent, (c) the value of the true activation energy for creep, (d) the interpretation of creep in terms of a threshold stress, and (e) the temperature dependence of threshold stress. This resemblance implies that deformation in the matrix governs deformation in the composite. Analysis of the creep data in terms of creep rate against an effective stress shows that the creep behaviors of the composite and unreinforced alloy are consistent with the operation of viscous glide creep at low stresses. A comparison between the creep data of the composite and those of the unreinforced matrix revealed that the composite exhibited more creep-resistant characteristics than its matrix over the entire range of applied stresses.     

Flow behavior and microstructural evolution of Al–Cu–Mg–Ag alloy during hot compression deformation

The flow behavior of Al–Cu–Mg–Ag alloy and its microstructural evolution during hot compression deformation were studied by thermal simulation test. The flow stress increased with increasing the strain rate, and decreased with increasing the deforming temperature, which can be described by a constitutive equation in hyperbolic sine function with the hot deformation activation energy 196.27 kJ/mol, and can also be described by a Zener–Hollomon parameter. The dynamic recrystallization only occurred at low Z values, which must be below or equal to a constant of 5.31 × 10¹³ s⁻¹. With decreasing Z value, the elongated grains coarsed and the tendency of dynamic recrystallization enhanced. Correspondingly, the subgrain size increased and the dislocation density decreased. And the main soften mechanism of the alloy transformed from dynamic recovery to dynamic recrystallization.     

Hot-pressed ZrB2–SiC–YSZ composites with various yttria content: Microstructure and mechanical properties

ZrB₂–10 vol%SiC–20 vol%YSZ composites were prepared by hot-pressed sintering with yttria content ranging from 2 mol% to 8 mol% in YSZ. The phase constitution, microstructure and mechanical properties of the composites were found to be strongly dependent on the yttria content. The average grain size became bigger for the composites with higher yttria content. When the yttria content was below 3 mol%, there is no cubic zirconia in the polished surface of composites, and the flexural strength of the composites was above 740 MPa. With the increase in yttria content, the fracture toughness fell down from 6.4 MPa m^1/2 to 5.6 MPa m^1/2. Vickers’ hardness of the hot-pressed composites varied above 18 GPa without obvious effect of the yttria content.