Precipitation Of Equilibrium Phase Research Articles

Microstructural stability, phase transformation and mechanical properties of a fully-lamellar microstructure of a Mo-modified high-Nb γ-TiAl alloy

Microstructural instability modes, phase transformation and mechanical properties evolution of a fine-grained fully-lamellar Ti–44Al–6Nb–1Mo−0.3(B, Y) (at.%) alloy were investigated systematically. During isothermal exposure at application temperatures, dramatic discontinuous precipitation of γ and βo (ωo) grains proceeded along boundaries of prior lamellar colonies, i.e. via a transformation of L(α2/γ) → γ + βo (ωo). Meanwhile, the internal lamellar structures degraded by α2-lath dissolution and simultaneous γ-lath coarsening. Furthermore, thermal loading was found to contribute to the above decomposition processes. However, almost no βo and βo (ωo) grains formed in the interiors of lamellar structures. The precipitation of equilibrium phases γ, β and ωo abided by normal crystallographic orientation relations with their respective parent phases. The present microstructural decomposition was considered to be essentially induced by thermodynamic instability of α2 laths and high interfacial energy of nanoscaled lamellar structures. Finally, it was demonstrated that this microstructural degradation caused both high-temperature strength and room-temperature ductility to deteriorate.

Grain refinement during high temperature tensile testing of prior cold worked and peak aged Cu–Ti alloys: Evidence of superplasticity

The high temperature tensile properties of solution treated, cold worked and peak aged Cu–1.5 wt.%Ti and Cu–4.5 wt.%Ti alloys have been investigated in the temperature range from room temperature (RT) to 550 °C. Yield strength (YS) and tensile strength (TS) of Cu–1.5Ti alloy were found to be independent of test temperature up to 350 °C and decreased thereafter, up to 550 °C. In the case of Cu–4.5Ti alloy, behaviour similar to Cu–1.5Ti alloy was observed. The decrease in strengths beyond 350 °C is attributed to coarsening of β′ precipitate particles at 450 °C and precipitation of equilibrium phase β at 550 °C. TEM analysis of the alloys confirms the presence of metastable and coherent Cu 4Ti β′ precipitate when tested at 450 °C and equilibrium precipitate Cu 3Ti, β at the test temperature of 550 °C. It is interesting to note that in the cold worked and peak aged Cu–Ti alloys tested at elevated temperatures, ductility (% elongation) was found to increase with the test temperature up to 550 °C which is attributed to dynamic recrystallisation. The large elongation (160%) observed in Cu–4.5Ti alloy at the test temperature of 550 °C is an indication of superplasticity. We are reporting the discovery of superplasticity in Cu–4.5Ti alloy for the first time. Ductile mode of fracture was observed in these alloys tested at elevated temperatures, i.e. at 350 and 450 °C. High temperature tensile properties of Cu–Ti alloys are found to be superior to those of Cu–Be alloys (C17200 and C17510).

Elevated temperature tensile behaviour of a Cu–4.5Ti alloy

The high temperature tensile properties of a Cu–4.5Ti alloy have been investigated in the temperature range from room temperature (RT) to 550 °C in solution treated (ST) and peak-aged (PA) conditions. Yield strength (YS) and tensile strength (TS) in solution treated condition were found to be independent of test temperature up to 350 °C followed by a marginal increase at 450 °C and decreased, thereafter up to 550 °C. In situ age hardening during testing at 450 °C resulting in the precipitation of ordered, metastable and coherent Cu 4Ti, β′ phase is attributed to the increase in strength. Coarsening of β′ precipitate particles at 450 °C and precipitation of equilibrium phase β at 550 °C decrease the strengths. In the peak-aged condition, the YS and TS were found to be constant up to 350 °C, beyond which they decreased with increasing test temperature. The decrease in strengths beyond 350 °C in the peak-aged alloy is attributed to the coarsening of β′ precipitate particles, as well as precipitation of equilibrium phase β at 550 °C. High temperature tensile properties of Cu–4.5Ti are superior to those of Cu–Be alloy (C17200). Transmission electron microscope (TEM) analysis of the alloy confirms the presence of metastable and coherent Cu 4Ti, β′ precipitate when tested at 450 °C and both Cu 4Ti, β′ and equilibrium precipitate Cu 3Ti, β at the test temperature of 550 °C in solution treated as well as peak-aged conditions. Ductile mode of fracture was observed in both solution treated and peak-aged conditions of the alloy tested at RT as well as at elevated temperatures, i.e. 350, 450 and 550 °C.

Characterisation of a 6082 Aluminum Alloy after Thixoforming

The heat treatment response and the microstructure of a 6082 aluminum alloy produced by thixoforming were investigated. The initial microstructure was characterised by globular dendrites separated by a low volume fraction of (i.e. of eutectic). The heat-treatment response was investigated by testing the alloy in the form of thixoformed bars. The analysis of the hardness variation with ageing time and the electrical conductivity measurements suggested that the hardening of the alloy reflected the presence of coherent zones or semi-coherent precipitates at low temperature (160 °C) and the precipitation of equilibrium phases at high temperature (210 °C). These preliminary studies were partially repeated on samples obtained by sectioning a large component of more complex shape. The small fraction of liquid available at the thixoforming temperature was found to result in the formation of voids and hot-cracks in some locations of the component. The heat-treatment response was quite similar to that observed by testing thixoformed bars, even if the microstructure, and consequently the initial value of the hardness, was substantially less homogeneous in the component.

Reverse transformations in CuAlNiMnTi alloy at elevated temperatures

The reverse transformation sequences in an as-quenched Cu11.88Al5.06Ni1.65Mn0.96Ti (wt%) shape memory alloy during heating or isochronical annealing up to 650°C have been studied by means of differential scanning calorimetry, high temperature X-ray diffraction as well as transmission electron microscopy and optical observations. The results show that at least seven stages of structural evolution, depending upon heating rate, can be distinguished: the reverse martensitic transformation and the martensite stabilization; atomic reordering in parent phase; two phase separation in D0 3 and the reversion; atomic disordering in matrix; bainitic transformation; D0 3 = B2 transition; and the precipitation of equilibrium phases. The different dependence of the evolution stages on heating rate is analyzed and discussed.

Ａｌ‐Ｚｎ‐Ｍｇ合金溶接金属部の時効性に及ぼす冷却速度の影響

Aging processes of Al-6%Zn-2% Mg alloy weld metals obtained under the heat input of 10, 000, 5, 400 and 3, 600J/cm were examined by means of hardness measurements. X-ray small angle scattering methods and transmission electron microscopy. Aging characteristics of the weld metals strongly depended on weld heat input; the initial aging rate was least in the medium heat input. The age hardenability decreased as weld heat input increased. These results were discussed in terms of effects of cooling rate during and after solidification on aging characteristics of the alloy rapidly solidified. Introduction of dislocations during solidification retarded the initial aging rate of the weld metal. Precipitation of equilibrium phases during cooling after solidification and solute segregations decreased the age hardenability of it.

Precipitation of equilibrium phases in vapour-quenched Al-Ni. Al-Cu and Al-Fe alloys

In situ annealing experiments in the transmission electron microscope have been used to investigate the thermal stability and precipitation behaviour of non-equilibrium phases in vapour-quenched Al-Ni, Al-Cu and Al-Fe alloys prepared by co-sputtering. The non-equilibrium phases exhibit surprisingly high thermal stability and the general nature of precipitation is different from that in liquid-quenched or conventional alloys. These phenomena are caused by the very small as-quenched grain size.

Precipitation of equilibrium phases in vapour-quenched Al-Ni. Al-Cu and Al-Fe alloys

Precipitation of equilibrium phases in vapour-quenched Al-Ni. Al-Cu and Al-Fe alloys

Pb-0.038 wt%As合金の時効析出過程

Ageing Characteristics in a supersaturated Pb-0.038 wt%As alloy have been investigated resistometrically with particular emphasis on the quenching method, to make clear the anomalous enhancement effect of As in Pb-Sb alloys.Three stage are found in the isochronal resistivity curves. The rate of resistivity decrease in the first stage (∼−50°C) cannot be explained by the diffusion of As atoms under the equilibrium vacancy concentration. The activation energy for migration of As atoms in this stage is found to be 0.51±0.10 eV. This is near the activation energy for migration of a vacancy in lead. It has been shown that this stage is accelerated and rate controlled by the diffusion of quenched-in excess vacancies. The second stage having activation energy 1.21±0.04 eV is the precipitation of intermediate phase. The third is the precipitation of equilibrium phase, rhombohedral arsenic, which gives the activation energy 1.18±0.07 eV. A faint resistivity increase between the second and the third in the isochronal curves is due to the reversion reaction of intermediate precipitates.Anomalous enhancement effect in Pb-Sb alloys with small As addition may be due to the large binding energy between an As atom and a vacancy, and small As precipitates formed homogeneously with large rate in the early stage of ageing act as nuclei of Sb precipitates. The inevitability of affinity between As and Sb atoms is also discussed.

Changes in the electrical conductivity and the tangent of the dielectric loss angle during decomposition of oversaturated solid solutions of the NaCl/AgCl system into equilibrium phases

1. We examined the effect of the decomposition of solid solutions of alkali halides and silver halides upon the electric conductivity and dielectric loss angle using the example of the NaCl/AgCl system. 2. The decomposition of solid solutions of alkali halides and silver halides passes through several complicated stages. In the first decomposition stage, the electric conductivity passes through one maximum, and the dielectric loss angle through two maxima. In the second stage of the decomposition process, both the electric conductivity and the dielectric losses decrease. 3. In addition to decreasing values of both the electric conductivity and the dielectric losses, the decomposition of solid solutions of the NaCl/AgCl system changes the activation energy for charge-carrier migration. 4. Measurements of the electric conductivity and of the dielectric losses permit one to follow the precipitation of equilibrium phases in oversaturated solid solutions. This method of observing the decomposition of solid solutions is convenient because it makes it possible to continuously observe the decomposition and to examine the initial stages of the decomposition process; these are important points for formulating an atomistic theory of the decomposition of oversaturated solid solutions.