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.     

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.     

On the influence of the pre-ageing temperature on the reversion behaviour of an Al-4.5 at.% Zn-2.0 at.% Mg alloy

The reversion behaviour of an Al-Zn(4.5)-Mg(2.0) alloy was investigated by SAXS in dependence of the pre-ageing temperature, T_pre, (ranging between 60°C and 100°C) and the reversion temperature, T_rev, (120°C till 200°C) starting with precipitates having a radius of (1.2 ± 0.1) nm and (1.5 ± 0.1) nm, respectively. During the reversion treatment applied up to T_rev = 160°C three stages could be distinctly distinguished, namely the dissolution of unstable zones, the growth of the stable zones on the expense of the dissolved one, and at last the coarsening of the precipitates by the OSTWALD-ripening process, where the structure changes become independent of the pre-history (start radius). The change of T_pre from 60°C to 100°C does not influence the structure changes going on at T_rev, that means between 60°C and 100°C the same type of G.P. zones grows.     

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.     

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 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.

     

On the influence of impurity additions and quenching temperature on the course of the microhardness (HV) during a two-step ageing treatment of an Al-2.0 at.%Zn-1.3 at.%Mg Alloy

A pure as well as the analogous technical alloy were directly quenched from T_q = 400 °C or 490 °C to T_a, preaged at T_a (ranging between 50 °C and 160 °C) various times (t_a), and subsequently postaged at R.T. up to t_post = 42 d. — Ageing the samples below the upper limit temperature of the homogeneous formation of G.P. zones (T_hn) no essential difference in the course of the HV-number attained after one- as well as two-step ageing in dependence on the addition of impurity atoms and T_q could be found. — Contrary to this behaviour after pre-ageing at T_a > T_hn and postageing at R.T. a variation of both T_q and the content of impurity atoms shows a considerable effect. The reason is the difference in the density of nucleation sites for heterogeneous formation of precipitates of the η′- and/or η-phase in dependence of T_q and impurity content.     

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.     

Investigations on the growth kinetics of GP zones and a′R-precipitates in Al-Zn alloys

X-ray small-angle scattering (XSAS) and resistivity (R) measurements were particularly done with an Al-Zn (15 at·,%) alloy in rather wide ranges of both the ageing time t_a and ageing temperature T_a, in order to obtain information on the dependence of the growth exponent m of the l = β₀t^m growth law and the activation energy E_act on t_a and T_a. The XSAS-measurements yielded that within the range of the GUINIER radius r_G between 1 nm and 2 nm the growth is essentially retarded (m < 0.1) and for r_G > 2 nm m depends on T_a ranging from 0.15 to 0.23 with a maximum at 175°C. Reasons for these effects are discussed. The differences between the m-values obtained by means of XSAS-and TEM-measurements are explained by the distinctions of the two methods applied. The E_act taken from XSAS- and R-measurements show a remarkable increase with t_a. At the beginning of the decomposition E_act = (0.49 ± 0.05) eV holds well explainable by the migration of quenched-in VZn pairs, but at the end E_act = (1.05 ± 0.07) eV was found. This value was also obtained from TEM-investigations (growth of the length). It fits well the E_act of ZnV pairs in thermal equilibrium at T_a.     

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.     

Phase Equilibrium in the System HgSeO4 – H2SeO4 – H2O at 100°C

The solubility of the system HgSeO₄ – H₂SeO₄ – H₂O was studied at 100 °C. The fields of crystallization of HgSeO₄ H₂O and HgSeO₄ were established. The compounds obtamed were identified by means of chemical, X-ray and crystal-optical analysis. The mechanism of the thermal dissociation of the compounds obtained was studied.     

The Description of the Isothermal Decomposition Processes in Al?Sc Alloys by means of Electric Resistivity Measurements and TEM Investigations

The decomposition behaviour of two AlSc alloys (c_Sc = 0.18; 0.36 at.%) after direct quench to room temperature and subsequent ageing at temperatures in the range 200 °C ≦ T_a ≦ 550 °C was investigated by means of isothermal resistivity measurements and TEM. In the temperature range 325 °C ≦ T_a ≦ 400 °C both alloys show independent of the Sc content an “inversion” of the decomposition kinetics, obviously caused by the transition of the coherent into the incoherent Al₃Sc phase.     

Re-determination of the temperature of the coherent spinodal curve in an-Al-15 at.% Zn alloy by transmission electron microscopic and hardness investigations

The character of the coarsening of the concentration modulations is studied in an Al-15 at. % Zn alloy applying TEM investigations (TEM micrographs and diffraction patterns) and hardness measurements in the temperature range from 40 °C to 130 °C. From the TEM investigations the temperature of the coherent spinodal curve is estimated to be (93 ± 7)°C. This value agrees fairly well with the result obtained by means of hardness measurements (kink in the TTT-curve occurring at about (84 ± 3) °C). The TEM observations of structure revealed in the T_a-range below the coherent spinodal a periodic structure relevant to spinodal decomposition. The periodicity of modulations is confirmed by electron diffraction patterns where a clear effect of sidebands near the matrix spots is observed.     

On the position of the upper limit temperature of homogeneous nucleation of precipitates in low concentrated Al?Zn alloys

The upper limit temperature T_hn of the onset of homogeneous nucleation of Guinier-Preston (GP) zones was determined for three Al Zn alloys with xZn = 4.5, 6.0 and 8.0 at.% by SAXS investigations (cooling from the range of homogeneity to various temperatures T_a and ageing at T_a). The results are T_hn = (95 ± 3) °C for xZn = 4.5 at.%, (118 ± 3) °C (6.0 at.%) and (154 ± 3) °C (8.0 at.%), respectively. The obtained results fit well the data known for the alloys with higher contents of Zn. It is stressed that one has to distinguish between T_hn, determined by isothermal measurements after a direct quench or cooling to the respective T_a, and the upper limit temperature T_rhn of the onset of the rapid homogeneous nucleation of GP zones (continuous cooling).     

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.     

The Microhardness and the Toughness Anisotropy of Paratellurite Single Crystals in the Tr — 650 °C Temperature Range

The temperature hardness (H_v) and toughness (K_1C) dependence of paratellurite (100), (010), (001) planes is investigated in the T_r −650 °C range. The correlation between the anisotropy degree of Young modulus and H_v is not found. The presence of 11 kind H_v and K_1C anisotropy is shown.     

The influence of the Mg content and the quenching conditions on the decomposition kinetics of Al?Zn?Mg alloys

By means of resistivity measurements, TEM and microhardness measurements the decomposition kinetics of A1-3 at.% Zn-1.5 at.% Mg and A1-4.5 at.% Zn-x at.% Mg (x = 0.2; 0.5; 1.0; 1.5; 2.25; 3.0) were investigated. For T_a < T_{a, crit} < T_h (T_h: temperature of the rapid homogenous nucleation) in A1-3 at.% Zn-1.5 at.% Mg and A1-4.5 at.% Zn-x at.% Mg (x = 1.0; 1.5; 2.25; 3.0) an inversion of the decomposition kinetics for T_q > T_inv: (T_inv: inversion temperature) was established. This result can be explained with reference to the binding of excess vacancies in dislocation loops. Proceeding from the discrepancy between the experimentally estimated concentrations of vacancies bound in dislocation loops and the concentrations of free vacancies at T_q, calculated by means of the Lomer equation, a model is proposed, which explains the high vacancy concentration of the loops. In terms of this model the values of the migration energy, obtained for A1-3 at.% Zn — 1.5 at.% Mg in the T_a-range 23 °C ≦ T_a ≦ 100 °C, and the shift of the inversion temperatures T_inv towards higher values with increasing T_a are interpreted. The same applies to the influence of the Mg content on the decomposition kinetics in A1-4.5 at.% Zn-x at.% Mg alloys.     

The effect of manganese additions on the structure changes proceeding in rapidly solidified Al-Cu alloys

The effect of Mn additions (0… 2 wt%) on the decomposition of rapidly solidified Al-4.0 wt% Cu alloys (cooling rate 10³ to 10⁴ K/s; LQ treatment) were studied during ageing between RT and 450 °C by hardness, X-ray methods and electron microscopy. The results were compared with alloys homogenized in the region of the solid solution (SQ treatment). (i) The LQ treatment results in a quite better homogeneous distribution of the alloyed elements than the SQ one, that is less particles of intermetallic phases are present in the ascast state. (ii) At T < 250 °C Mn additions affect the decomposition kinetics by trapping of vacancies (retardation) and the diminution of the solubility of Cu atoms (acceleration). The first effect dominates in the stage of G.P. zone formation, the second one during precipitation of intermediate phases. (iii) At T ≧ 300 °C the intermetallic compound Cu₂Mn₃Al₂₀ forms associated with a significant increase of the hardness.     

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.     

Structure of monoclinic acridine orange hydrochloride monohydrate,C17H19N3 · HCl · H2O

M_r = 319.82, monoclinic P2₁/a, Z = 4, a = 14.545(5), b = 15.562(6), c = 8.538(4) Å, β = 120.66(3)°, V = 1662(1) ų, D_m = 1.27 Mgm⁻³, D_x = 1.278 Mgm⁻³, λ(MoKα) = 0.71069 Å, μ(MoKα) = 0.24 mm⁻¹, T = 297 K. Final R = 0.049 for 1485 observed reflections. All hydrogen atoms were located and refined isotropically. Bond lengths (σ = 0.004 Å) and angles (σ = 0.3°) are normal. The substance is a derivative of acridine. The structure consists of chloride anions, bisdimethylaminoacridinium cations and water molecules. In the solid state the molecules are approximately planar. Pairs of cations form dimers with parallel planes connected by an I inversion centre and plane distances of 3.474 Å. They overlap with partial areas of all three six membered rings.