The precipitation process in Mg–Ca–(Zn) alloys investigated by positron annihilation spectroscopy

Coincidence doppler broadening (CDB) spectroscopy has been applied to study the precipitation process induced by aging in Mg–1.0 wt.% Ca and Mg–1.0 wt.% Ca–1.0 wt.% Zn alloys. In addition positron lifetime experiments and microhardness measurements have been performed. A peak centered at 11.5 × 10⁻³m₀c is found in the CDB ratio spectra of the alloys aged at 473 K. It is attributed to annihilations with the core electrons of Ca. The results indicate the formation of a particle dispersion that hardens the alloys. This dispersion is correlated with the appearance of the peak attributed to Ca atoms. Zn atoms in the Mg matrix inhibit the formation of quenched-in vacancies bound to Ca atoms in the aged ternary alloy producing the dispersion refinement.     

Thermal property characterization of a Ti–4 wt.%Nb–4 wt.%Zr alloy using drop and differential scanning calorimetry

The thermal properties of Ti–4 wt.%Nb–4 wt.%Zr alloy, namely the enthalpy increment and heat capacity have been characterized as a function of temperature using drop and differential scanning calorimetry, respectively. The measured data clearly attested to the presence of a phase change from α (hcp) to β (bcc) phase at about 1100 ± 5 K. In fact, the alloy exhibited a transformation domain in the temperature interval 1100–1170 K. The enthalpy associated with the α → β phase change is estimated to be about 73 (±5%) J g⁻¹. The jump in the specific heat at the transformation temperature is 1714 (±7%) J kg⁻¹ K⁻¹. The drop and differential scanning calorimetry results are consolidated to obtain the first experimental data on the thermodynamic quantities of this alloy.     

Snoek relaxation in a copper-precipitated alloy steel

We studied the effect of thermal embrittlement of a steel containing 1.29 at% copper on the Snoek relaxation. The hardness increased with isothermal aging at 723 K and decreased after showing a maximum. Hardness change is governed by bcc–copper clusters precipitated from the ferrite iron crystal. Using a forced-vibration torsion-pendulum method, we studied the internal-friction spectrum and recovery of the maximum relaxation strength after quenching from 723 K. We observed a broad nonsymmetrical spectrum which apparently consists of three Debye peaks. With higher hardness, the recovery rate increased. We interpreted our results in terms of Nowick’s theory of interstitial/substitutional-solute interactions. For recovery, we used the Cottrell–Bilby–Harper t^2/3 model.     

Early oxidation behaviors of Mg–Y alloys at high temperatures

The early oxidation behaviors of Mg–Y alloys (Y = 0.82, 1.09, 4.31 and 25.00 wt.%) oxidized in pure O₂ have been investigated at high temperatures. The results showed that the oxidation behaviors of the Mg–Y alloys (Y = 4.31 and 25.00 wt.%) obeyed a parabolic law, while that of the Mg–Y (Y = 0.82 and 1.09 wt.%) exhibited both parabolic and linear kinetics depending on the oxidation temperature. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses indicated that an oxide film with a single structure composed of MgO and Y₂O₃ had formed. Moreover, the higher the oxidation temperature was, the thicker the oxide film was. Finally, the corresponding oxidation mechanism has been discussed, and the improved oxidation resistance of the Mg–Y alloys can be due to the formation of a continuous Mg-dissolving Y₂O₃ protective film.     

Influence of prior cold working on the tribological behavior of Cu–0.65 wt.%Cr alloy

The influence of prior cold working on the friction and wear behavior of Cu–0.65 wt.%Cr alloy under dry sliding against a steel disk was investigated on a pin-on-disk wear tester. The worn surfaces and debrises of Cu–Cr alloy were analyzed by scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectrum. The results indicated that prior cold working and aging had an effect on the hardness and wear resistance of Cu–Cr alloy; in other words hardness and wear rate increased with the amount of cold working. At constant aging temperature, the wear rate of Cu–Cr alloys increase with cold working and reached maximum at 50% cold working. At constant amount of cold working aged specimens at 500 °C shows higher wear resistance than 450 °C. Crack initiation and propagation in the tribolayer and at the interface of subsurface and tribolayer was the dominant mechanism during the sliding process.     

Structure evolution of EV31 Mg alloy

Structure modifications and precipitation hardening of EV31 Mg alloy have been monitored by calorimetric measurements (DSC), TEM observations and positron annihilation spectroscopy (PAS). At the early stages of precipitation DSC measurements and TEM observations evidenced the formation of a β″ phase, while the presence of an intermediate β′ is supported by positron data. Overageing is correlated to the formation of an incoherent β₁ phase, which transforms in situ into the equilibrium β precipitate.     

Effects of stirring parameters on rheocast structure of Al–7.1wt.%Si alloy

In this study, effects of stirring time and stirring speed on the microstructure of semisolid rheocast (SSR) Al–7.1 wt.%Si were examined. The results demonstrated that the non-dendritic structure could be formed by a short stirring period below liquidus temperature and further stirring had little impact on the final morphology of the primary particles. Stirring was shown, however, to affect the average particles size mainly during the initial stages of solidification. Although the average shape factor of primary particles was relatively insensitive to large variations in the stirring speed, higher stirring speeds made the shape and size of the primary particles more uniform. Higher stirring speeds also rendered smaller and more rounded agglomerates of primary particles. The results of two stability models employed suggest that, in general, the primary particles generated in the initial stages of solidification can attain growth stability before pouring and maintain this stability during the secondary cooling stage.     

Interface structure and energy in Cu–71.8 wt.% Ag

The as-cast eutectic Cu–71.8 wt.% Ag was prepared. The orientation relationship and interface structure of Cu and Ag phases were investigated using high-resolution transmission electron microscopy. The interface energies and nucleation rates in the formation of different orientation relationships were discussed. The Cu and Ag eutectic lamellae can form the cube-on-cube and hetero-twin orientation relationships and the occurrence probabilities of the two orientation relationships are approximate in the solidification of the eutectic structure. There are misfit dislocations and monolayer steps at the interface between both phases having the cube-on-cube or hetero-twin relationship. The twin fault and lattice mismatch energies at the interface result only in an insignificant effect on the formation of different orientation relationships between both phases of the eutectic laminae.     

Preparation by gravity casting and hydrogen-storage properties of Mg–23.5 wt.%Ni–(5, 10 and 15 wt.%)La

Mg–23.5 wt.%Ni–(5, 10 and 15 wt.%)La alloys were prepared by gravity casting and their hydrogen-storage properties were examined after pulverizing. The gravity cast Mg–23.5Ni–(5, 10 and 15)La alloys consist of α-Mg, Mg₂Ni and Mg₁₇La₂ phases. The activated Mg–23.5Ni–10La alloy has the highest hydrogen-storage capacity of 4.96 wt.%H (from P–C–T curve) and the highest initial hydriding rate (hydrogen content 3.83 wt.%H at 10 min) with an initial hydrogen pressure in the channel of 11 bar H₂ at 573 K. This is attributed to its containing the largest amount of the Mg₁₇La₂ phase, which is easily dissociable during the hydriding reaction.     

Determination of the pre-exponential factor and the activation energy for the low-temperature dislocation relaxation in aluminum

The ultrasonic attenuation and the sound velocity at 10 MHz were measured in deformed Al single crystals in the temperature range between 1.5 and 50 K. In the high purity specimen (RRR=20 000) the change in sound velocity due to deformation shows a step in temperature dependence at 17 K, which corresponds to the 11 K relaxation peak at 50 kHz found by Kosugi and Kino [J. Phys. Soc. Jpn. 58 (1989) 4269], although the change in attenuation due to deformation was masked by the large background attenuation due to phonon–electron interaction. From the present experiment and that of Kosugi and Kino, the activation energy and the attack frequency of the relaxation process are estimated to be 14 meV and 1.0×10¹²/s, respectively. If the kink pair formation process of dislocations on the Peierls potential is responsible for this relaxation, the Peierls stress is 5.4×10⁻⁵G, where G is the shear modulus, and this value does not conflict with the flow stress at low temperatures.     

Influence of plastic deformation on the velocity and ultrasonic attenuation of copper single crystals

High-purity copper single crystal samples were plastically deformed between 0.02% and 1% in the direction. The absolute ultrasonic velocity and attenuation related to the dislocations in copper single crystal were measured as a function of the plastic deformation. The amplitude-independent Swing Model for dislocation resonance was considered to obtain information about the evolution of the dislocation density (Λ) with the degree of plastic deformation (%). The experimental data were interpreted as resulting from the contribution of two kinds of dislocations that lie in a cell structure. The results show a linear increase with the deformation of the density of dislocations (Λ_I) that lies in the cell walls [Λ_I (cm⁻²)≈1.5×10⁹ %]. The dislocation density inside the cell (Λ_II) is about two orders of magnitude lower than Λ_I.     

Influence of flux containing YCl3 additions on purifying effectiveness and properties of Mg–10Gd–3Y–0.5Zr alloy

In order to improve the purifying efficiency of RJ6 flux, 5 wt.% YCl₃ additions were introduced into the flux to refine Mg–10Gd–3Y–0.5Zr (GW103K) alloy. The results show that the RJ6 flux containing 5 wt.% YCl₃ additions exhibits excellent adsorption ability to nonmetallic inclusions. Thermodynamic analysis indicates that the main reason could be attributed to the decrease of the surface tension of the flux. Moreover, the mechanical, corrosion and fluidity properties of the alloy were investigated. It was found that these properties were improved to a certain degree due to the removal of nonmetallic inclusions in the alloy.     

In-situ monitoring of ultrasonic attenuation during rotating bending fatigue of carbon steel with electromagnetic acoustic resonance

Using electromagnetic acoustic resonance (EMAR), we studied the evolution of the surface-shear-wave attenuation and phase velocity during rotating bending fatigue of a 0.45% C steel. In the EMAR method, we used a magnetostrictively-coupled electromagnetic acoustic transducer (EMAT) for the contactless measurements of the axial shear wave that is a surface-shear wave, propagating in a cylinder-specimen circumference direction with the axial polarization. The attenuation coefficient always showed sharp peaks around 90% of the fatigue life, independent of the fatigue-stress amplitude. In addition to the ultrasonic measurements, we made crack-growth observations using replicas and measured recovery of attenuation and velocity after stopping the cyclic loading just before and after the peak. From these results, we concluded that the evolution of the ultrasonic properties is caused by a drastic change in dislocation mobility being accompanied by the crack growth.     

Superplasticity and superplastic forming of Mg–Al–Zn alloy sheets fabricated by strip casting method

Superplastic properties and formability of the AZ31 sheet processed by strip casting and subsequent warm rolling were examined. Microstructure of the AZ31 sheet with thickness of 1.3 mm was uniform and composed of equiaxed grains with an average size of 6.6 μm. The sheet exhibited excellent superplasticity with a maximum elongation of 800% at 673 K and 2 × 10⁻⁴ s⁻¹. A 250 mm × 250 mm size panel with complicated embossing patterns could be successfully formed into near net shape by gas pressure against a single female die. Small grain size and slow grain growth during deformation resulted in the superior superplastic properties at 673 K. The governing deformation mechanisms in any given strain-rate and temperature ranges could be predicted by the deformation mechanism maps for Mg–Al–Zn alloys.     

Nano- and micro-scale free volume in ultrafine grained Cu–1 wt.%Pb alloy deformed by equal channel angular pressing

Nano- and micro-scale free volumes in ultrafine grained Cu–1 wt.%Pb alloy deformed by equal channel angular pressing (ECAP) were examined by a combination of radiotracer diffusion measurements, transmission electron microscopy, depth milling by focused ion beam, and positron annihilation spectroscopy. The positron lifetime spectroscopy revealed the existence of vacancy-type defects, presumably related to grain boundary states, and vacancy clusters. The positron annihilation sites probed by coincident Doppler broadening spectroscopy are surrounded by both Cu and Pb atoms, implying preferred nucleation of nanovoids near small Pb inclusions. Larger free volume defects, such as nano- and submicron-sized pores and submicrometer cracks, were directly observed by the focused ion beam technique. Ultra-fast penetration of liquid tracer solution in the sample bulk was established in the radiotracer diffusion experiments. The results provide an unambiguous proof of the existence of open porosity in ultrafine grained Cu–Pb alloy deformed by ECAP. Under specific deformation conditions, a connected network of open channels, which promotes long-range penetration of a liquid radiotracer solution, was shown to form during the severe deformation. The relative volume fraction of the percolating porosity was estimated at about 2 × 10⁻⁶.     

Theory of dislocation tunneling through a pinning point

We have calculated the transition rate for a string unpinning from a point barrier, using a truncated parabolic potential. In this approximation, it was shown that the result for this N-dimensional system has the one-dimensional form R=ν_effexp (−ΔU/kT_eff), where ν_eff is an effective frequency, ΔU the barrier height, T_eff the effective temperature. There is a crossover temperature T* separating the high temperature classical behavior from the low temperature quantum rate and given by T*=ω_eff/2k. The effective temperature is given by the actual temperature above T*, while below it, is given by the ground state energy, calculated using the effective frequency. The important point is that if one knows the transition rate at high T, then the crossover temperature and the low T transition rate may be calculated. The effective frequency has been calculated for dislocations in the classical regime: ν_eff0.26 (U₀/Gb³) ν_D, where U₀ is the binding energy with a pinning atom, G the shear modulus, b the Burgers vector, and ν_D is the Debye frequency. The predicted crossover temperature of a few tenths Kelvin for an Al crystal is in good agreement with our recent experimental results.     

Retardation of grain growth in Mg–3Al–1Zn alloy processed by strip-casting method

Grain growth behaviors of the two AZ31 alloy sheets processed by slab- and strip-casting methods were examined and compared. Grain growth rate of the strip-casting processed AZ31 alloy was considerably lower than that of the slab-casting processed AZ31 alloy. The result could be ascribed to the presence of finer Al–Mn compound particles more uniformly and densely distributed in matrix of the strip-casting alloy. Low grain growth rate via effective Zener pinning of the Al–Mn particles notably improved tensile ductility of the AZ31 alloy at elevated temperatures.     

Microstructural evolution behavior of Mg–5Si–1Al alloy modified with Sr–Sb during isothermal heat treatment

The microstructural evolution behavior of Mg–5Si–1Al alloys modified with Sr–Sb during isothermal heat treatment was investigated in the present study. Although the morphology of eutectic Mg₂Si phase varied with isothermal holding temperature increasing from 620 to 670 °C, no spheridization occurred for primary Mg₂Si polyhedrons in Sr–Sb-modified alloy even when the temperature reaches 660 or 670 °C. Such an abnormal phenomenon of primary Mg₂Si, during partial remelting isothermal treatment, might be ascribed to both the spheridization restriction effect caused by incorporation of Sb in Mg₂Si and stability of octahedral primary Mg₂Si crystals faced by {1 1 1} planes.