Structure of the grain boundary region in an Al–15 at.% Zn and Al–2.0 at.% Zn–1.3 at.% Mg alloy aged at elevated temperatures

The results of TEM investigations are presented of the effects of various solutions and ageing treatments, cooling and quenching rates as well as of tensile stress at elevated temperatures on the microstructure of the grain boundary (GB) region of the examined Al Zn and Al Zn Mg alloys. A pronounced influence was found of partial coherency of GB stable β(Zn) precipitates with the α matrix in the Al Zn alloy on the asymmetry of Zn depleted GB region and on the shape of GB precipitates. The role of GB's as sources and sinks of vacancies and the effect of stress gradients in GB regions were emphasized in the establishment of differences in the nucleation and decomposition characteristics within the GB region as compared to those within the grain interior. The existence of a negative stress gradient towards the GB has been suggested to lead to the observed gradual retardation of transformation kinetics within the GB region in Al Zn alloys. A rapid quench from annealing to ageing temperature introduced large local stresses close to GB giving rise to “band of precipitates” on subsequent ageing within the former precipitate free region (PFZ) in Al Zn Mg alloy. Similar enhancement of local stresses accompanied by acceleration of decomposition kinetics was also observed in the aged Al Zn sample subjected to a prior tensile creep exposure within the one-phase region.     

Positron lifetime study of helium clustering in Ni and Ni-1 at.% Ti

Helium clustering and bubble growth have been studied in Ni and Ni-1 at % Ti through positron lifetime measurements. Helium pressure in the bubble as well as bubble concentration have been estimated and compared for the cases of Ni and Ni-1 at % Ti. The results seem to indicate enhanced helium trapping due to substitutional Ti impurity in the host Ni.     

On the decomposition behaviour of Al-Zn(4.5 at.%)-Mg(2;3 at.%) alloys during continuous cooling from the temperature range of homogeneity

The decomposition processes taking place in the Al-Zn (4.5)-Mg(2;3) alloys were studied by means of XSAS and resistivity measurements and by TEM investigations after continuous slow cooling to certain temperatures T_i and during an isothermal ageing at T_i following the continuous cooling procedure. During the cooling between 380 °C and 320 °C mostly particles of the η-phase are growing. The formation of particles homogeneously distributed in the matrix starts at 165 °C in the alloy with 2 at.% Mg and at 175 °C in the one with 3 at.% Mg. Below these temperatures new subcritical GP zones are formed and particles already present in the matrix are growing. The dominating process depends on T_i.     

On the Influence of Short R.T. preageing on the formation of the η′-phase in an Al-Zn(4.5 at.%)-Mg(2.5 at.%) alloy

The influence of short R.T. preageing (1 min) after quenching from 480 °C into RT-water on the formation of the η′-phase in an Al-Zn(4.5)-Mg(2.5) alloys has been investigated by means of TEM, XSAS, resistivity, and hardness measurements in the ageing temperature range between T_a = 120 °C comparing samples directly quenched (DQ) to T_a and those quenched into RT-water, kept there for 1 min, and subsequently stored in an oil-bath of T_a (IQ). At T_a ≦ 170 °C both heat treatments result in finely distributed precipitates of the η′-phase accompanied by a fairly high hardness value. But at T_a ≧ 180 °C the DQ leads only to few coarse heterogeneously formed η′- and η-precipitates, whilst the IQ yields qualitatively the same precipitation structure as already described at T_a ≦ 170 °C even till 280 °C. These results are interpreted assuming that. A1) the limitation temperature of homogeneous formation of η′ is located at about T_h = 175°c and A2) during the pre-ageing at RT “nucleation sites” are formed as already suggested by RYUM, which are able to act as sites for heterogeneous formation of η′ even at T_a > T_h. Hints are obtained that the formation of these “nucleation sites” is not so strongly governed by the quenched-in excess vacancies as discussed in the literature.     

The Ln–Ba–Cu–O (Ln = Lanthanoid) systems: Compounds and compatibilities in air at 900–980°C

Comparison of phases observed in air at 900–980°C in the Ln–Ba–Cu–O (Ln = Lanthanoid) systems is reported. On the basis of the occurrence of compounds found in these Ln₂O₃–BaCO₃–CuO systems they must be divided into at least three subgroups: the first is characteristic of La, Pr and Nd, the second specific for Sm and Eu, and the third common to the smaller lanthanoids, the Y-type elements, with some variation within the groups.     

Study of the instability boundaries of plastic flow in Cu + 14 at.% Al single crystals at low temperatures

The authors experimentally revealed the existence of the temperature-strain rate region of jumplike deformation in Cu + 14 at.% Al single crystals between 4.2 and 25 K. For each particular temperature in this temperature range, there are a lower and an upper critical strain rates, between which deformation is jumplike. The character of transients occurring during strain rate variation was investigated by increasing the strain rate to the lower critical value. The experimental data are explained in terms of the thermomechanical instability concept and inertial effects of the sample-machine system.     

The influence of the direct quench and indirect quench to Ta > Th on the decomposition behaviour of an Al-2 at.% Zn-1 at.% Mg alloy

The decomposition behaviour of an Al-2.0 at.% Zn-1.0 at.% Mg alloy after direct quench (DQ) and indirect quench (IQ:T = 23 °C, t) to the ageing temperature T = 160 °C (above the temperature of the rapid homogeneous nucleation, T_h) was investigated by means of TEM and isothermal resistivity measurements. The precipitation density of the η-particles yields a maximum value at pre-ageing times about 10 min at T = 23 °C. To clear up the reason for that phenomenon a loop-formation and clustering model was introduced.     

Effects of the addition of SiC on the crystallization of Al84Ni8Co4Y3Zr1 (at.%) amorphous ribbons

Amorphous alloys present exceptional values of mechanical strength but lack any significant plasticity at room temperature. Deformation of amorphous alloys occurs in shear transformation zones that connect to form shear bands, which are easier to deform than the surrounding matrix, thus facilitating further deformation in the same location of the specimen. However, the presence of particles dispersed in the amorphous matrix can modify such strain softening behavior, resulting in real plastic deformation before fracture. Also, depending on the type of particles and how they are introduced, they can modify the crystallization behavior of the amorphous matrix by acting as heterogeneous nucleation sites. In this context, this paper reports on the effects of the addition of SiC particles on the crystallization of Al₈₄Ni₈Co₄Y₃Zr₁ amorphous ribbons. Pre-alloyed ingots with and without added SiC particles were melt-quenched into amorphous ribbons by the single-roller melt-spinning technique and then selectively and partially crystallized at the first and second crystallization temperatures, as determined by differential scanning calorimetry. Primary crystallization of nanometric-sized fcc-Al crystals was found to occur in both ribbons (with and without added SiC), confirming that crystallization reactions were not altered by the ceramic particles. Aluminum carbide (Al₄C₃) crystals resulting from high-temperature liquid metal/SiC reactions were observed as coatings on the SiC particles and as isolated particles dispersed in the amorphous solid matrix. In both cases, the Al₄C₃ particles also did not change the crystallization behavior of the amorphous Al₈₄Ni₈Co₄Y₃Zr₁ matrix, since no heterogeneous nucleation of fcc-Al crystals was observed.     

Photoelectrical properties of (Sb15As30Se55)100−xTex (0 ⩽ x ⩽ 12.5 at.%) thin films

This paper reports photoelectrical properties of (As₃₀Sb₁₅Se₅₅)_100?xTe_x amorphous chalcogenide films (0 ? x ? 12.5 at.%) through measurements of ‘steady state’ and ‘transient’ photocurrents. The composition dependence of the steady state photocurrent at room temperature shows that the photoconductivity increases while the photosensitivity decreases with increasing Te content. A study of photoconductivity of (As₃₀Sb₁₅Se₅₅)_100?xTe_x at different levels of light intensity reveals that, the photoconductivity increases exponentially with increase in light intensity. The Photocurrent (I_ph) when plotted against light intensity (G) follows a power law (I_ph = G^γ) the exponent γ for (As₃₀Sb₁₅Se₅₅)_100?xTe_x films has been found nearly 0.5 suggesting bimolecular recombination. The transient photoconductivity shows that the lifetime of the carrier decreases with increasing the light intensity. This decrease suggests that the photoconductivity mechanism in our samples was controlled by the transition trapping processes. The increase of Te content results in a monotonic decrease in the band gap and the free carrier life time of (As₃₀Sb₁₅Se₅₅)_100?xTe_x thin films. These results were interpreted on the basis of the chemical-bond approach.     

Elastic and elasto‐optical properties of Rb1–x(NH4)xH2AsO4 mixed crystals studied by Brillouin spectroscopy

The elastic and elasto‐optical properties of Rb_1–x (NH₄)_x H₂AsO₄ mixed crystals were studied by Brillouin spectroscopy at room temperature. The measurements were made on single crystals obtained from the aqueous solution by slow evaporation. The behaviour of bulk phonons propagating in investigated crystals was studied for different polarisations of incident and scattered beam. The components of the tensors of elasto‐optical and elastic properties of the crystals studied were determined. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Development of the microhardness of an al-2.0 at.% zn-1.3 at.% mg alloy with additions of technical impurities during a two-step ageing treatment (ii)

The alloy was preaged in the temperature range between T_pre = 50 °C and 160 °C for various times and afterwards postaged at R.T. The course of the microhardness and the related structure changes (TEM and SAXS investigations) were investigated. The main results are the following: – Preageing the samples times less than the incubation time no essential structure changes take place at least not in the sense that the further decomposition happening during the postageing at R.T. is remarkably influenced. – The samples preaged at T_a 110 °C for times somewhat longer than the incubation time show a distinct minimum after the end of the postageing period of 42 d at R.T. Most probably at T_a 110 °C in the starting period of the decomposition nuclei of the α-phase or of spherical hexagonal zones were formed. These nuclei lessen considerably the amount of G.P. zones able to be nucleated during the R.T. postageing.     

Isothermal Ageing in the T Range from 0°C to 180°C of a Technical Al-2.0 at.% Zn-1.3 at.% Mg Alloy after Direct Quench

The variations of the structure and the mechanical properties had been investigated by means of TEM investigations and determination of the microhardness number in the T_a range between 0 °C and 180 °C. The main results are the following: – At the starting period of the isothermal ageing an incubation time, t_inc, occurs. t_inc plotted versus T_a yields two intersecting C curves at about 100 °C. – At T_a = 70 °C the only decomposition product are G.P. zones. – At 70 °C <T_a <100 °C the G.P. zones formed in the first period of decomposition can be transformed into η-phase or spherical hexagonal (S.H.) zones. – Above about 110 °C the precipitates present in the matrix are quasi-homogeneously formed at nucleation sites delivered by clusters of impurity atoms.     

Dissolution of Si in Al–Si Melts. Effect of Sb on the Crystallization Process and As-cast Structure of Al–Si Alloys

The effect of Sb on the dissolution of Si single crystals (face {111}) in Al–Si melts is studied. The dissolution isotherms at three temperatures and fixed undersaturation are obtained. The experimental data are treated according to the Bliznakov's ratio. Some conclusions are made with reference to the crystal growth process. A probable mechanism of the influence of Sb on the structure of Al–Si alloys is given.     

Structural Changes during Post-Ageing of an Al-Zn (15 at.%) Alloy between 125 °C and 215 °C

The structural changes going on in an Al Zn (15 at.%) alloy postaged between 125 °C and 215 °C after pre-ageing between 20 °C and 90 °C (till start r_s = (20…︁ 55) Å are obtained) were followed by means of XSAS-investigations. The main results are the following:

• 1 An increase of r_s does not favour the growth of the precipitates during the reversion treatment.
• 2 At later reversion times the happening is obviously independent of the pre-history of the samples.
• 3 The integral intensity is decreasing with increase of T_rev, but the mean distance (d)-size (r_G) relation between the precipitates is not changed remarkably up to T_rev ≈ 200 °C in the early reversion stage.

Conclusions from this behaviour are drawn.     

Magnetic response of Cu (25 wt.%)-316 grade stainless steel processed by ball milling

Powder blend comprising Cu (25 wt.%) and 316-stainless steel (75 wt.%) has been subjected to ball milling upto 70 h followed by isothermal annealing at the temperature range of 350–750 °C for 1 h to investigate the evolution of microstructure and magnetic properties. The ball milling of the powder blends after 10 h has resulted in partitioning of the austenite stabilizing elements such as Ni from 316-stainless steel to elemental Cu leading to the transformation of the Bravais lattice of the Fe-rich phase from fcc (γ) to bcc (α). During further ball milling of the powder mixture upto 20 h, the α-Fe has dissolved completely in Cu, leading to the formation of partial amorphous phase after 70 h of milling. The amorphous phase of the alloy has been found to stable after annealing at 350 °C and super paramagnetic in nature. Annealing of the alloy at higher temperatures has resulted in precipitation of nanocrystalline bcc-Fe in the Cu evolving ferromagnetic properties. Annealing at 750 °C has resulted in collapse of the hysteresis loop due to the diminished exchange interaction as the result of grain coarsening of the α-Fe and Cu.     

The interaction of oxygen with the iron (111) surface,mainly studied by AES

The cleaned surface was oxidized by means of pure oxygen at a pressure of 3 × 10⁻⁶ – 3 × 10⁻³ pa and at temperatures of 300 K, 370 K, and 620 K. The Auger peaks of high energy (for oxygen at 510 eV and for iron at 703 eV) were used for the quantitative determination of the oxygen coverage on the iron surface. The experimental results on the change of oxygen coverage as a function as a function of the dose are interpreted by means of a theoretical model and provided definite informations about the bonding state of the adsorbate. In the lower energy part of the Auger spectrum there appear peak shape modifications during oxidation. These observations allow us to suppose that in the process of oxidation there are in action two kinds of chemical bonding of oxygen with iron differing by density of states in the valence band.     

Saturation Behaviour of In – Ga – As Melts and Growth of In.53Ga.47As Lattice-matched to (001) InP Substrates

This paper presents the saturation behaviour of In – Ga – As melts with monocrystalline GaAs. The coulometric As-analysis confirmed that the source-seed-technique produces In – Ga – As melts of defined compositions. The growth results of the step cooling technique applying both the source-seed technique and the single phase melts are compared.