On the reversion behaviour of an Al-15 at.% Zn alloy between 155°C and 175°C

The structural changes going on in an Al-15 at.% Zn alloy postaged in the vicinity of the kink of the C-curve (161°C) in the T.T.T.-diagram after pre-ageing at R. T. or 90°C (till start radii r_S between 1.0 and 4.0 nm were obtained) were followed by means of SAXS-investigations. The main results are the following:

• 1 No indication could be found that the reversion behaviour is remarkably changed by crossing the temperature of the kink of the C-curve in the T.T.T.-diagram, i.e. the reversion as well as the growth mechanism of the zones does not differ considerably below and above 161°C.
• 2 The decrease of the integral intensity Q₀ during the reversion process is mainly due to both the loss of zinc content of the zones and its rise in the matrix and not essentially by the diminution of the volume fraction of the precipitates.
• 3 Zones of a start radius of about 4 nm are much more stable against the decrease of the zine content during the reversion treatment that those of 2 nm.

     

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.     

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.     

Structure Changes in an Al-4.5 at.% Zn-2.0 at.% Mg Alloy during Continuous Heating in Dependence on the Starting Conditions

Die in einer AlZn(4.5 at.-%) Mg(2.0 at.-%)-Legierung beim kontinuierlichen Aufheizen vonstatten gehenden Strukturumwandlungen wurden mit Hilfe von DSC und RKWS in Abhängigkeit vom Startradius, r_S = (0.7 … 2.7) nm, der metastabilen Phasen (GP-Zonen und/oder η'-Phase), die bei T_pre = R.T. oder 100°C gewachsen waren, untersucht. Die Stabilität der bei T_pre gebildeten metastabilen Phasen nimmt mit deren Größe zu, während eine Veränderung von T_pre keinen merklichen Einfluß zeigt. Startet man mit Partikeln von r_S > 1.6 nm, so erscheinen die Minima, die durch die Bildung von η'-Phase aus den aufgelösten GP-Zonen (1. Minimum) bzw. aus überkritischen Zonen entstanden sind (2. Minimum), mit Zunahme von r_S immer undeutlicher.     

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.     

Study of calcium phosphate precipitation at 37 °C

The nature of the calcium phosphate formed when equal volumes of 20 mM CaCl₂ and 15 mM or 12 mM K₂HPO₄ with pH 7.4 are rapidly mixed at 37 °C has been investigated by following the pH of the suspension as a function of time, by calorimetric and light scattering methods, chemical analysis and TEM. It is found that in the early stages of this process, there appears a spherular amorphous form of calcium phosphate, called ACP1, which transforms into a floccular amorphous form, called ACP2. We suggest that this transformation is solution-mediated. ACP2 has not previously been clearly described as a separate phase and has not before been shown to be amorphous.     

Mechanical Behaviour of LiF Single Crystals in Compression and Tension at Temperatures from −196°C to 800°C

The work hardening characteristics of LiF single crystals in compression and tension has been investigated as a function of temperature. It was shown that in temperature range from 200 °C to 600 °C four stages of hardening were observed and the main part of plastic flow proceeds in stage IV. The parabolic hardening was not observed. The flow stresses and hardening rates in all stages in compression are higher than those in tension. As a temperature increase this difference decreases. The hardening rates, stresses which correspond to the beginning of the linear hardening stages and the duration of those stages decrease as a temperature increase. At temperatures above 600 °C the yield drop occured. Taking into account a good agreement between the temperature dependences of the critical resolved shear stress and τIII the assumption was made that τIII corresponds to resolved shear stress in a secondary glide system.     

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.     

Kinetics of crystallization of potassium chloride from aqueous solutions at 30 °C

The aim of this work is to use a diffusion layer model for the determination of individual rate constants of reaction and diffusion step of KCl at 30 °C. Crystal growth rate was measured by travelling microscope technique. From the measured values of the mean linear growth rate the resulting individual rate constants and the effectiveness factors (GARSIDE) are evaluated as a function of the driving force and of the flow velocity of solution. The significance of diffusional and surface reaction resistances against mass transfer is discussed in relation to the experimental conditions used.     

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.     

Growth mechanisms in excimer laser photolytic deposition of gallium nitride at 500°C

The mechanisms of controlling laser-induced chemical vapour deposition of GaN at substrate temperatures between 350 and 650°C have been investigated. Ultraviolet (193 nm) photolytic decomposition of trimethylgallium (TMGa) and ammonia (NH₃) precursors was examined in this range. Laser-induced fluorescence studies support the view that the dissociated intermediate fragments GaCH₃ and NH are the reacting species in GaN film formation, irrespective of substrate temperature. It was found that two crystal phases coexist in films grown at substrate temperatures below 500°C, wurtzite crystal structure with (0002) orientation forms at substrate temperatures above 500°C. The growth rate increases with both NH₃/TMGa ratio, and TMGa flow rate, while the temperature dependence shows a thermal activation energy of 0.2 eV which is smaller by a factor of five than that of films prepared by conventional thermal CVD. The large NH₃/TMGa ratios needed to achieve stoichiometry are interpreted in terms of the two-photon dissociation cross section of NH₃.     

Mechanical behaviour of magnesium oxide single crystals at temperatures from 196 to 2400 °C

The stress — strain characteristics of MgO single crystals in compression are investigated in the temperature range from –196 °C to 2400 °C on samples of the same shape and size. It is shown that the stress — strain curves may be subdivided into three basic types, each of them corresponding to the definite temperature range. The work hardening rate at stage I is temperature independent and at stage II lineary decreases with temperature increase in overall temperature range where work hardening is observed. The high temperature maximum in the temperature dependence on the proportional limit is found which is not due to the effects of impurities. The effect of geometrical form of specimens on stress-strain parameters and the local true strain distribution in compressed specimens have been studied.     

The Effect of Additions of Mn and Fe on the Structure of an Al-2 at.% Zn-1 at.% Mg Alloy

The structure of an Al-1.9 at.% Zn-1.3-at.% Mg mother alloy with additions of Mn and Fe, respectively, were investigated by TEM and microprobe analysis for two different heat treatments, namely direct quench from 490 °C to 160 °C and a two-step ageing treatment (preageing at RT for 7d before storing at 160 °C), to explain the differences in the course of the hardness obtained during the ageing at 160 °C. The differences of the structure observed are explained in terms of the tendency of the additives to form intermetallic phases, particularly in the grain boundaries.     

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.     

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.     

The Double Salts Cd(C6H5COO)2 · CdCl2 and Cd(C6H5COO)2 · CdBr2

The systems Cd(C₆H₅COO)₂ CdCl₂ H₂O and Cd(C₆H₅COO) ₂ CdBr₂ H₂O have been studied at 25 °C. Formation of the double salts Cd(C₆H₅COO)₂ · CdCl₂ and Cd(C₆H₅COO)₂ · CdBr₂(1:1:0) has been established and their chemical individuality has been confirmed by chemical, X-ray, and DT-analyses. Proceeding from data on the structure of Cd(C₆H₅COO)₂ · 2 H₂O and the composition of the 1:1:0 double salts as well as using Pauling's rules for the most probable spacial disposition of the ions, the crystal structure type of the double salts obtained are predicted.     

Information on the Operating Mode of Decomposition during the Cooling of an Al-10 at.% Zn Alloy to Various Tq

Information about the operating mode of decomposition during cooling to various T_q were gained by SAXS method. After reaching the T_q wanted the samples were quenched into water of RT and the SAXS intensity curves were measured at RT. Strong evidence was found that below about 175 °C the decomposition starts in this alloy with the nucleation and growth of Guinier-Preston zones. As proved by other authors, 175 °C is the upper limit temperature T_rhm for the onset of the rapid homogeneous nucleation of precipitates.     

烧结温度对(Ba0.85 Ca0.15)(Ti0.9 Hf0.1)O3陶瓷压电性能的影响

采用传统固相反应法在系列烧结温度下制备了(Ba0.85 Ca0.15)(Ti0.9 Hf0.1)O3无铅压电陶瓷,并对陶瓷样品的晶格结构、表面形貌、铁电性能、压电性能和常温介电性能进行了测试与分析.结果表明:样品均具有纯的钙钛矿结构,随着烧结温度的升高,样品的晶体结构由立方相转变为四方相;压电系数和机电耦合系数随之增大,当烧结温度为1340℃时,样品的晶粒尺寸较为均匀,约为2～7μm,并且样品压电系数最大为250 pC·N-1,此时,2Pr=13.33μC·cm-2,2Ec=5.91 kV·cm-1,i=0.05 mA,ε=1803,tanδ≈0.017,Kp=0.32,Qm=116.     

The properties of hydrogen forms of zeolite T obtained by the treatment with hydrochloric acid at 22°C

Hydrogen forms of zeolite T obtained by the treatment of synthetic zeolite of erionite-offretite type with hydrochloric acid at 22°C were tested with the methods of chemical analysis, sorption and X-ray diffraction. The influence of the variations in the number of treatments with acid, the concentration of acid and the calcination at 500°C on the properties of the samples was observed. It was found that the modification runs in two steps. The existence of the threshold of stability of the zeolite T structure under the treatment with 0.1 N HCl was indicated at the degree of decationization of 45–48% and the degree of dealumination of 25–28%. It was also found that about 30–33% of alkali cations in zeolite T are at the more accessible positions.