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.     

Investigation of the compressive creep behavior of AZ91D magnesium alloy

The creep resistance of AZ91D alloy has been studied in uniaxial compression tests at temperature ranges from 275 °C to 325 °C. The initial microstructure of the alloy consists of α phase and β phase precipitated in the grain boundary. The minimum creep rate dependence on applied stress and the temperature is also analyzed in detail. We find that the stress exponent n is close to the theoretical values (3 or 5) and the activation energy Q for creep varies from 121 kJ/mol to 171 kJ/mol. Creep could be controlled by high-temperature climb and cross-slip of dislocation at different temperatures.     

Creep deformation and crack growth rates of a super α2 titanium aluminide alloy

The influences of stress and temperature on creep deformation behavior and the creep crack growth rates of the super α₂ Ti₃Al alloy were investigated with respect to its safe application at high temperatures. In a temperature range of 1033–1093 K at low applied stress levels, the stress exponent was equal to 1.5. At an intermediate stress range (10^?3 < σ/E < 3 × 10^?3), a stress exponent of 3.3 was observed. As the applied stress was increased, the stress exponent changed from 3.3 to 4.4. The high temperature crack growth rate of the Ti₃Al alloy can be correlated with stress intensity factor K rather than C1 at 1033 K due to environmental embrittlement.     

The microstructure and impression creep behavior of cast AZ80 magnesium alloy with yttrium additions

The effects of 0.5, 1.0 and 2.0 wt.% Y additions on the microstructure and creep behavior of the as-cast AZ80 alloy were investigated by impression tests. The tests were performed at temperatures in the range 423–523 K, under punching stress in the range 150–650 MPa. At low temperatures up to 473 K, the AZ80 + 0.5Y alloy had the highest creep resistance among all materials tested, whereas with increasing temperature from 473 K to 523 K, the AZ80 + 1.0Y alloy had a better performance. This can be attributed to the fact that at low temperatures the presences of β-Mg₁₇Al₁₂ and Al₂Y phases together with solid solution hardening effects of Al in the Mg matrix strengthen the AZ80 + 0.5Y alloy. At higher temperatures, AZ80 + 1.0Y with a higher volume fraction of the more thermally stable Al₂Y and lower amounts of the less stable β-Mg₁₇Al₁₂ exhibits better creep behavior. The stress exponents and activation energies were almost the same for all alloy systems studied, 6.0–8.8 and 90–119 kJ/mol, respectively. The observed decreasing trend of creep-activation energy with stress suggests that two parallel mechanisms of lattice and pipe-diffusion-controlled dislocation climb are competing. Climb of dislocations with an additional particle strengthening effect controlled by dislocation pipe diffusion is dominant at high stresses, whereas climb of dislocations is the controlling mechanism at low stresses.     

Creep behavior of nanostructured Mg alloy at ambient and low temperature

Tensile creep test at temperature <0.35 T_m was carried out to investigate the creep behavior in nanostructured Mg alloy with an average grain size of 45 nm consolidated from mechanically alloyed powders using power creep law. The stress exponent is found to be larger than one and with a threshold stress. The activation energy for the creep is measured to be 76 kJ mol⁻¹ smaller than that for grain boundary diffusion in Mg. It is deduced that creep behavior is affected by the presence of impurities and nanovoids inherited from the processing history.     

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.     

Microstructure,tensile properties and creep behavior of Al-12Si-3.5Cu-2Ni-0.8Mg alloy produced by different casting technologies

The relationship between the as-cast microstructure and mechanical properties of the Al-12Si-3.5Cu-2Ni-0.8Mg alloys produced by permanent mold casting (PMC) and high pressure die casting (HPDC) is investigated. The alloys in both PMC and HPDC consist of Al, Si, Al₅Cu₂Mg₈Si₆, Al₃CuNi, and Al₇Cu₄Ni phase. However, the microstructure of the HPDC alloy is significantly refined. Compared to the PMC alloy, the ultimate tensile strength of the HPDC alloy is significantly increased from 244 MPa to 310 MPa, while the elongation shows a reverse trend at room temperature. At low stress and temperature range, slight variations of stress exponent and activation energy indicate that the minimum creep rate is controlled by the grain boundary creep. Then the minimum creep rate is higher for the specimen with the smaller grain size, where grain boundary creep is the dominant creep mechanism. At high stress region, the stress exponent for the PMC alloy and HPDC alloy is 5.18 and 3.07, respectively. The different stress exponents and activation energies measured at high stress and high temperature range indicates that the creep mechanism varies with the casting technologies.     

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.     

Crystallization and spectroscopic properties investigations of Er3+ doped transparent glass ceramics containing CaF2

Transparent oxyfluoride glass ceramics with composition of 45SiO₂–25Al₂O₃–5CaCO₃–10NaF–15CaF₂–0.5ErF₃ (in mol%) were developed through controlled crystallization of melt-quenched glass. Non-isothermal crystallization kinetics investigation showed that the average apparent activation energy E_a and Avrami exponent n are about 283 kJ/mol and 2.22, respectively, indicating the crystallization a three dimensional crystal growth process controlled by the diffusion with a decreasing nucleation rate. X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) observation revealed the precipitation of CaF₂ crystallites sized about 15 nm among the glass matrix after heat-treatment at 650 °C for 2 h. For as-made glass, no upconversion signals were detected when excited with a 30 mW diode laser at 980 nm, while strong upconversion emissions at 545, 660 and 800 nm were obtained for transparent glass ceramic under similar excitation condition.     

Effect of Zn addition,strain rate and deformation temperature on the tensile properties of Sn–3.3 wt.% Ag solder alloy

Stress–strain characteristics of the binary Sn–3.3 wt.% Ag and the tertiary Sn–3.3 wt.% Ag–1 wt.% Zn solder alloys were investigated at various strain rates (SR, ε^·) from 2.6 × 10^− 4 to 1.0 × 10^− 2 s^− 1 and deformation temperatures from 300 to 373 K. Addition of 1 wt.% Zn to the binary alloy increased the yield stress σ_y and the ultimate tensile stress σ_UTS while a decrease of ductility (total elongation ε_T) was observed. Increasing the strain rate (ε^·) increased both σ_y and σ_UTS according to the power law σ = C ε^·m. A normal decrease of ε_T with strain rate was observed according to an empirical equation of the form ε_T = A exp (− λε^·); A and λ are constants. Increasing the deformation temperature decreased both σ_y and σ_UTS in both alloys, and decreased the total elongation ε_T in the Zn-free binary alloy, whereas ε_T was increased in the Zn-containing alloy. The activation energy was determined as 41 and 20 kJ mol^− 1 for these alloys, respectively. The results obtained were interpreted in terms of the variation of the internal microstructure in both alloys. The internal microstructural variations in the present study were evaluated by optical microscopy, electron microscopy and X-ray diffraction. The results show the importance of Zn addition in enhancing the mechanical strength of the Sn–3.3 wt.% Ag base alloy.     

Impression creep behavior of different zones in friction stir welded Mg–Zn–Mn wrought alloy

The friction stir welded joint of wrought ZM21 alloy was divided into five parts, and their localized creep behavior was studied via the impression method. The tests were carried out in the stress range of 300–450 MPa (σ_imp/G ≈ 0.02–0.03) and in the temperature range of 448–523 K. Optical and SEM micrographs and EDS taken before and after the impression tests were used to study the microstructure of various zones of the FS welded joint. Power law was found to satisfactorily relate the stress and strain rates. The steady-state impression velocity was found to vary significantly between the advancing and retreating sides of TMAZ and HAZ. For TMAZ, the creep exponent on the AS was 4.8, and on the RS, it was 7.8. The activation energy on the AS was ~?133 kJ/mol, and on the RS, it was ~?101 kJ/mol. Similarly, for HAZ, the creep exponent on the AS was found to be 5.5 and on the RS, it was 4.9. The activation energy on the AS was ~?86 kJ/mol and on the RS, it was ~?232 kJ/mol. The cross-over of steady-state impression velocity of different zones indicates that the weak zone was temperature and stress dependent. Within the stresses and temperatures studied, the weld zone's creep resistance (i.e., lower minimum impression velocity) was found to be better than the base material. As it is with most magnesium alloys, dislocation climb was found to be the operative mechanism in the FS weldments of ZM21 alloy. The rate-controlling mechanism remains to be identified because the wide variation in n and Q values suggests that different creep mechanisms are in operation in different zones.

Graphical abstract

     

Hot workability of a high carbon high chromium tool steel

In this work, hot tension tests were conducted on as cast and wrought samples of a high carbon high chromium tool steel to study the hot workability under the rolling conditions. The flow curves illustrate the classical shape of dynamic recrystallization (DRX). It is observed that broken carbide nets in the wrought samples result in lower deformation activation energy 398 kJ/mol in comparison to the as cast samples 432 kJ/mol. Necking strains were calculated using the inflection point of the work-hardening (θ) vs. stress curves. Wrought samples show higher hot ductility and lower maximum stress than the as cast ones. It is shown that lower activation energy of deformation and lower stress concentration around the smaller carbides in the wrought samples is responsible for their higher hot workability.     

The glass to NZP crystal transformation in a mixed alkali germano-phosphate glass matrix

A mixed alkali germano-phosphate glass composition (Li_0.5Na_0.5Ge₂(PO₄)₃) has been prepared and characterized. The crystallization mechanism of a NZP-type phase, the nucleation rate and the non-isothermal kinetic parameters have been studied by differential thermal analysis (DTA).The plot of the DTA peak height versus the glass particle size has shown a great tendency for surface crystallization. The nucleation rate type curves have given a maximum at 465 ± 2 °C and confirmed that nucleation was instantaneous at this temperature. Furthermore, the average value of the Avrami exponent (5.4 ± 0.4) has revealed a tri-dimensional growth controlled by the development of the glass–crystal interface with a decreasing nucleation rate in the temperature range explored. From non-isothermal experiments, the apparent activation energy for the overall crystallization phenomenon has been founded in the range 260–273 kJ/mol whereas the activation energy for crystal growth was 488 ± 6 kJ/mol.     

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.     

High-temperature creep behavior and microstructural evolution of an 18Cr9Ni3CuNbVN austenitic stainless steel

The creep properties of an 18Cr9Ni3CuNbVN austenitic stainless steel have been investigated at temperatures ranging from 923 to 1073 K and stresses from 120 to 250 MPa. The rupture lives ranged from 10 to 20,105 h. The stress dependence of the minimum creep rate obeyed a power law, with stress exponents ranging from 6 to 8.6. The activation energy was determined to be 460 to 485 kJ/mol/K. The microstructural evolution during the creep test was investigated. The V-rich Z-phase and metallic Cu precipitates began to precipitate in the middle of the creep deformation, resulting in an increase of the creep strength, while only Nb-based MX precipitates were present from the beginning of the high-temperature creep test.     

Constitutive analysis of Mg–Al–Zn magnesium alloys during hot deformation

The constitutive behaviors of Mg–Al–Zn magnesium alloys during hot deformation were studied over a wide range of Zener–Hollomon parameters by consideration of physically-based material’s parameters. It was demonstrated that the theoretical exponent of 5 and the lattice self-diffusion activation energy of magnesium (135 kJ/mol) can be used in the hyperbolic sine law to describe the flow stress of AZ31, AZ61, AZ80, and AZ91 alloys. The apparent hyperbolic sine exponents of 5.18, 5.06, 5.17, and 5.12, respectively for the AZ31, AZ61, AZ80, and AZ91 alloys by consideration of deformation activation energy of 135 kJ/mol were consistent with the considered theoretical exponent of 5. The influence of Al upon the hot flow stress of Mg–Al–Zn alloys was characterized by the proposed approach, which can be considered as a versatile tool in comparative hot working and alloy development studies. It was also shown that while the consideration of the apparent material’s parameters may result in a better fit to experimental data, but the possibility of elucidating the effects of alloying elements on the hot working behavior based on the constitutive equations will be lost.     

A ferrite stainless steel Cr27Mo6Al3Cu with oxidation resistance

A new ferrite steel Cr27Mo6Al3Cu with high Mo and low Al is developed, exhibiting excellent oxidation resistance, corrosion resistance, and mechanical property. Alloy rods with a diameter of 10 mm were prepared by copper mold suction-casting method and then solution-treated at 1373 K for 2 h. The experimental results show that this alloy maintains a stable monolithic BCC microstructure. After oxidization at 1373 K for 100 h, a dense and thin oxide layer is generated on the surface of the alloy with the weight gain per unit area G⁺ being about 0.4863 g/m². Its corrosion-resistant property in 3.5 wt.% NaCl at 298 K is characterized with the corrosion potential E_corr being about −0.091 V, pitting corrosion potential E_b reaching up to 0.867 V. Its mechanical properties are yield tensile strength σ_0.2 = 523 MPa, ultimate tensile strength σ_b = 637 MPa respectively.     

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523 K and under normal stresses in the range 50–300 MPa for the compression creep and 150–650 MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg₁₇Al₁₂ and Mg₂Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009 < (σ/G) < 0.015 and 0.021 < (σ_imp/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol⁻¹, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141 kJ mol⁻¹ are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.     

Progressive crushing of stitched glass/polyester composite cylindrical shells

This paper examines the effect of mode I interlaminar fracture toughness (G_Ic) on the specific energy absorption of stitched glass/polyester composite cylindrical shells under axial compression. The laminated composite cylindrical shells used as energy absorbers, absorb large amount of impact energy during collision. Since mode I delamination in the thin wall of axially collapsed shell is one of the major energy absorbing modes, contribution of G_Ic to specific energy absorption (SEA) of tubes is significant during collision. The G_Ic values are determined through double cantilever beam (DCB) test with stitched and unstitched planar specimens. The four and six-layered cylindrical tubes of D/t ratios 29.27 and 20, respectively, with G_Ic values ranging from 1.68 to 8.09 kJ/m² are prepared by stitching and are subjected to quasi-static axial compression. Increasing G_Ic up to certain value leads to controlled progressive crushing, which is a good energy absorbing mechanism, beyond which failure is uncontrolled. Cylindrical tubes having G_Ic up to 6.34 kJ/m² leads to 40% increase in SEA for four-layered tubes and 6.6% for six-layered tubes comparing with the corresponding unstitched tubes. When the tubes have G_Ic of 8.09 kJ/m², four-layered tubes undergo unstable failure, but six-layered tubes undergo stable progressive crushing with 22% increase in SEA. Transition from stable to unstable failure depends upon the thickness of tubes. An analytical model is developed based on energy approach to predetermine the steady state mean crush load of cylindrical composite shells under axial compression. The model results are validated by experimental results, and show good agreement.     

Electrical properties of nanostructures (CoFeZr)x + (Al2O3)1−x with use of alternating current

CoFeZr–Al₂O₃ nanocomposite films of 3–5 μm thickness, containing metallic alloy nanoparticles embedded into the dielectric alumina matrix, have been deposited on a glass ceramic substrate using magnetron sputtering of composite target in Ar gas ambient. Measurements of AC conductance and lagging have been performed within the frequency range of 50 Hz–1 MHz at the temperatures from 79 K to 373 K in the initial (as-deposited) samples as well as directly after their isochronous (15 min) annealings within the temperature range from 398 K to 648 K with 25 K step.The observed variations of real part AC electrical conductivity with temperature and frequency σ_real(T, f) in the as-deposited films display transition from dielectric to metallic behaviour when crossing the percolation threshold x_C in the studied nanocomposites. After annealing of the samples below the x_C the σ_real(T, f) progress follows the hopping law of electron conductivity with sigmoidal frequency dependence. The samples being far beyond the percolation threshold revealed transition from metallic to activational σ_real(T) law after high-temperature annealing attributed to the internal oxidation of metallic nanoparticle by excess of oxygen presented in the as-deposited samples.