Age-hardening associated with ordering and spinodal decomposition in a AgCu-40 at % Au pseudobinary alloy

The age-hardening mechanism in an AgCu-40 at% Au alloy was studied by means of electrical resistivity measurement, hardness tests, X-ray diffraction and electron microscopy. Two stages of hardening were found by isothermal ageing below 648 K, which was higher than the critical temperature of ordering, T _c=620 K, in the present alloy. The first stage of hardening took place by formation of a modulated structure resulting from spinodal decomposition. Further hardening was brought about by ordering, yielding metastable AuCu I and/or AuCu II ordered platelets grown from the copper-rich portion of the modulated structure. Transitional ordering which gave rise to a marked hardening of the second stage was found, even though the temperature of below 648 K was higher than the T _c of the present alloy. Drastic softening was also found on disappearance of the transitional ordered phases. Although the modulated structure was observed by ageing at 773 K, there was no age-hardening.     

Age-hardening behaviour and microstructure of a silver alloy with high Cu content for dental application

Age-hardening behaviour and the related microstructural changes of a silver alloy with relatively high Cu content were elucidated by means of hardness test, X-ray diffraction (XRD), scanning electron microscopic (SEM) observations and electron probe microanalysis (EPMA). The microstructure of the solution-treated specimen was composed of the Ag-rich matrix, the Cu-rich particle-like structures containing Pd, and the lamellar structure of both phases. By the age-hardening heat-treatment, the Cu element began to precipitate from the Ag-rich matrix by the solubility limit, and the very fine Cu-rich precipitates became coarsened by further aging. The silver alloy with relatively high Cu content showed apparent age-hardenability. The hardness of the solution-treated specimen began to increase and reached a maximum value with increasing aging time, and then the hardness decreased gradually after maintaining the maximum value for short periods of time. The early stage of precipitation of the Cu-rich phase from the Ag-rich matrix seemed to have caused the increase in hardness. The decrease in hardness was attributed to the coarsening of the Cu-rich precipitates in the later stage of the age-hardening process.     

Age-hardening behavior of a low-gold dental alloy

The age-hardening behaviors of a low gold dental alloy were studied by means of differential scanning calorimetry, hardness testing, X-ray diffraction, optical microscopy and transmission election microscopy. Two distinct hardening behaviors were found at two different aging temperatures. Age-hardening at 290°C was attributed to the formation of the metastable AuCuI′ ordered phase, and the gradual softening in the overaging stage resulted from the slow growth of this phase. The rapid increase in hardness in the early stage at 495°C was due to the precipitation of the metastable AuCuI′ or/and AuCuII′ ordered phases, and the rapid decrease in hardness in the overaging stage was a consequence of the growth of these phases and the loss of the coherency strain at the interface between the spindal-like AuCuI platelets and the matrix.     

Age-hardening reactions and microstructures of a dental gold alloy with palladium and platinum

The age-hardening reactions and microstructures of a dental casting gold alloy with some palladium and platinum were investigated by means of hardness tests, X-ray diffractometry, electrical resistivity measurements, and scanning and transmission electron microscopy. Ageing reactions during isothermal annealing were completed by two stages. The first stage corresponded to the formation of a metastable CuAu I phase within grains, and the second stage to a cellular reaction at grain boundaries. The former contributed to the hardening and the latter to the softening. An activation energy of 128 kJ mol^–1 for hardening was obtained. Two types of cell growth were observed and could be distinguished in terms of the homologous temperature.     

Age-hardening by miscibility limit of Au–Pt and Ag–Cu systems in an Au–Ag–Cu–Pt alloy

The age-hardening by miscibility limit of Au–Pt and Ag–Cu systems in an Au–Ag–Cu–Pt alloy was examined by characterizing the hardening behavior, phase transformations and changes in microstructure, and elemental distribution during aging. The hardness increased by the transformation of the parent α₀ phase into the α₁ and metastable AuCu I′ phases, but not by the further transformation of the metastable AuCu I′ phase into the stable AuCu I phase due to the simultaneously initiated lamellar-forming grain boundary reaction. The replacing ratio of matrix by lamellar structure was not directly proportional to the AuCu I phase formation. The relatively high Pt content caused the severe exclusion of Au from the Cu-rich layer of the lamellar structure due to the overlapped miscibility limit of both Au–Pt and Ag–Cu systems.     

Aging behavior of a CuCr2Fe2NiMn high-entropy alloy

The effect of aging treatment on the hardness and microstructure of a CuCr₂Fe₂NiMn high-entropy alloy is investigated. The results show that the alloy exhibits a good high-temperature age hardening phenomenon and temper resistance. The aged alloy can obtain a peak hardness of 450 HV at 800 °C. Softening anneal occurs at 1100 °C. Age hardening is mainly attributed to the precipitation hardening of the ρ phase and a more homogeneous microstructure, whereas the softening of the aged alloy may be related to the decomposition of the ρ phase and Cu-rich FCC1 and to the coarsening of the FCC2 phase in the interdendrite regions.     

Orientation relations between carbon nanotubes grown by chemical vapour deposition and residual iron-containing catalysts

Orientation relationships between the growth direction of carbon nanotubes and encapsulated residual iron-containing particles have been determined using transmission electron microscopy. The nanotubes that are prepared by Fe-catalysed chemical vapour deposition on sol–gel Fe(NO₃)₃-tetraethyl orthosilicate substrates are the helical multiwall type. Nanoscale particles of both the low-temperature α-Fe (ferrite) and high-temperature γ-Fe (austenite) were found in the cavity of the carbon nanotubes with , and parallel to the tube growth direction, respectively. Cementite Fe₃C, the most abundant Fe-containing phase in present samples was also found to be entrapped in nanotubes with or parallel to the tube axis. The metastable retention of γ-Fe particles at room temperature is ascribed to the strain energy induced at the particle-nanotube interface due to volume expansion upon the γ- → α-Fe phase transformation. The decomposition of initially high aspect-ratio, rod-shape particles into a string of ovulation, while encapsulated in carbon nanotubes is accounted for by the Rayleigh instability. Ovulation leading to reduced particle size has also contributed to increase the surface energy term that counterbalances the total free energy change of phase transformation from γ- to α-Fe and further aids to the metastable retention of γ-Fe.     

Aging behavior of Cu–Ti–Al alloy observed by transmission electron microscopy

Aging behavior of Cu–3 at.%Ti–4 at.%Al alloy at 723 K has been examined from mechanical, electrical, and microstructural points of view. Compared with binary Cu–3 at.%Ti alloy, the electrical conductivity improved six times to about 6%IACS (international annealed copper standard); whereas the peak hardness decreased from 280 to 180 Hv. The major strengthening phase is the tetragonal α-Cu₄Ti, which forms not via spinodal decomposition but based on the nucleation and growth mechanism. The precipitates grow in the c direction of the tetragonal phase, which lies along one of the axes of the matrix fcc Cu phase. This growth mode minimizes the strain energy arising from the lattice mismatch of about 2% between the matrix and precipitate; and results in a square rod shape, which reaches about 50 nm in length after 100 h anneal. Another precipitating phase is AlCu₂Ti (D0₃, Strukturbericht notation), with the major habit plane close to {110} of the fcc Cu matrix. The orientation relationship was not definitely determined, but it was found that the angle between the 100 and 110 poles of the matrix and precipitates, respectively, is about 5°, while the angle between the two 001 axes being about 7°. It was suggested that the formation of this ternary phase reduced the solute Ti concentration, leading to the decrease in the resistivity.     

Age hardening and softening related to ordering and first and second grain boundary reaction in dental low carat gold alloy

Abstract

A commercial dental Au–Ag–Cu–Pd alloy with small amounts of Ir and Zn was examined to elucidate the age hardening and softening mechanisms by characterising the phase transformation and microstructural changes via a first and second grain boundary reaction by means of a hardness test, X-ray diffraction study, field emission scanning electron microscopy and energy dispersive spectrometry. The hardness increased to the maximum value without an incubation period, and the overaging process showed a softening course in two steps. The increase in hardness was caused primarily by lattice distortion along the c axis due to the tetragonality of the metastable AuCu I′ and stable AuCu I phases, and secondarily by the coherence or semicoherence strain in the interfaces of the metastable AuCu I′ phase with the parent α₀, product α₁ and stable AuCu I phases. The fine lamellar forming first grain boundary reaction did not result in hardening, because it was not related directly to the phase separation process. The softening was in proportion to the quantity of lamellar structures by the first and second grain boundary reaction, and the softening effect of the fine lamellar forming first grain boundary reaction was larger than that of the coarse lamellar forming second grain boundary reaction.     

Age-hardening and the associated phase transformation in an Au-55.2 at% Cu-17.4 at% Ag ternary alloy

To develop a low gold content dental alloy, age-hardening characteristics in an Au-55.2 at% Cu-17.4 at% Ag alloy were studied by means of hardness, electron microscopy and X-ray diffraction examination. Three distinct age-hardening behaviours depending on temperature were found in the alloy, i.e. (i) a dual mechanism of spinodal decomposition and Cu₃Au ordering below 673 K, (ii) a single mechanism of spinodal decomposition at 693 K, and (iii) a single mechanism of nucleation and growth of silver-rich precipitate at 773 K. A marked over-ageing was observed by lengthy ageing over the whole range of temperature. The long-period superstructure of Cu₃Au(II) was found only in the grain boundary product at temperatures between 623 and 633 K.     

Quasicrystal–crystal structural transformation in Al–5 wt.% Mn alloy

The atomic structure of Al–5 wt.%Mn (Al–5Mn) alloy, prepared by rapid solidification, and pre-annealed at 623 and 773 K for 5 and 1 h, respectively, were characterized by X-ray powder diffraction (XRD) and extended X-ray absorption fine structure (XAFS) techniques. The sample in as-quenched stage was found crystalline, consisting of metastable α-Al (Al–Mn solid solution) and icosahedral quasicrystalline I-Al₆Mn phases. Five hours annealing at 623 K proved thermal stability of both the phases. Pre-annealing at 773 K/1 h on the other hand leads to α-Al phase decomposition and structural transformation of metastable I-Al₆Mn to stable orthorhombic Al₆Mn phase. The EXAFS results indicate that Mn atoms are located preferably on the outer shell of icosahedrons. During the I-Al₆Mn→o-Al₆Mn transformation the total Al atoms coordinating one Mn were found to be constant (∼10). Based on the results, only distance/symmetry changes in atomic arrangement around Mn atoms were suggested.     

Strain hardening of structural metals with metastable structure under complex loading

We investigated distinctive features of strain hardening under complex loading for quenched and low-tempered steels of ferritic-pearlitic (40KhN), martensitic (30Kh3NSMV), and maraging (Kh16N5D3) types and an Al-4Cu age-hardening alloy whose matrix phase is characterized by the metastable structure corresponding to supersaturated interstitial and substitutional solid solutions susceptible to disintegration in the process of plastic deformation. We analyzed structural indicators of the depletion of the matrix phase of supersaturating alloying elements forming carbides and intermetallic compounds and the mechanisms of hardening and softening influence of preliminary deformation (within reasonable ranges of its variation) on the value of the yield limit when repeated loading changes its sign. It is shown that, for the investigated type of metastable materials, in the analyzed range of preliminary deformations governed by alloying, the application of repeated biaxial tension leads to a uniform extension of the limiting yield curves. As the degree of supersaturation decreases (in the process of thermal treatment) and we observe the formation of equilibrium solid solutions of the matrix phase, the character of strain hardening evidently changes from predominantly isotropic to kinematic. Timoshenko Institute of Mechanics, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 1, pp. 85 – 94, January – February, 1998.     

Precipitation behavior of Ag–Pd–In dental alloys

Age-hardening characteristics and precipitation behavior of Ag–25%Pd–3%In–1%Zn–0.5%Ir alloy were investigated in detail by means of hardness testing, X-ray diffraction, electron microscopy and resistivity measurement. The solution treating could be accomplished at 980 °C and the aging in the temperature range from 950 to 850 °C occurred by continuous precipitation. The aging in the temperature range from 850 to 450 °C occurred first, forming GP-zones with a hardness increase and then in overaging stage by forming discontinuous precipitation, which consisted of lamellae of solute (Pd, In, Zn) depleted Ag-rich phase and (Pd,Ag)₃(In,Zn) intermetallic phase. The hardness increased very fast to its peak in 10 min during aging at temperatures between 450 and 550 °C.     

The modulated structure formed by isothermal ageing in ZrO2-5.2 mol % Y2O3 alloy

The modulated structure produced by isothermal ageing of ZrO₂-5.2 mol % Y₂O₃ alloy was examined mainly by electron microscopy. It was found that the modulated structure was formed at ageing temperatures between 1400 and 1600° C, but not at 1700° C. The structure is developed by spinodal decomposition, which produces compositional fluctuation in the elastically soft 111 direction in cubic zirconia. The hardness increase caused by the development of modulated structure during ageing is larger than the hardening by precipitation of tetragonal phase in the cubic matrix.Graduate Student, Tohoku Univerisy, Sendai, Japan.     

Spinodal decomposition in fine grained materials

We have used a phase field model to study spinodal decomposition in polycrystalline materials in which the grain size is of the same order of magnitude as the characteristic decomposition wavelength (Xsu). In the spirit of phase field models, each grain (i) in our model has an order parameter (η _i) associated with it;η _i has a value of unity inside the ith grain, decreases smoothly through the grain boundary region to zero outside the grain. For a symmetric alloy of composition,c = 0–5, our results show that microstructural evolution depends largely on the difference in the grain boundary energies, y_gb, of A-rich (a) and B-rich (β) phases. If Y _gb ^α is lower, we find that the decomposition process is initiated with an a layer being formed at the grain boundary. If the grain size is sufficiently small (about the same as λ_sd), the interior of the grain is filled with the β phase. If the grain size is large (say, about 10λ_SD or greater), the early stage microstructure exhibits an A-rich grain boundary layer followed by a B-rich layer; the grain interior exhibits a spinodally decomposed microstructure, evolving slowly. Further, grain growth is suppressed completely during the decomposition process.     

Effect of thermomechanical treatment on the phase transformation in Cu-44Ni-5Cr alloy

Cu-44Ni-5Cr alloy has been subjected to thermomechanical treatment which consisted of plastic deformation of as-quenched material by 50, 65 and 80% reduction in thickness followed by ageing in the interval of 500 to 650 °C for various durations of time. Progress in age-hardening was studied by means of hardness measurement and X-ray diffraction studies. The wavelength of composition modulation and strain amplitude were measured. It was found that age-hardening was a result of interaction between spinodal decomposition and recovery processes. Prior deformation was found to enhance the kinetics of both spinodal decomposition and coarsening. It was concluded that this resulted from increased vacancy concentration and increased coherency strain in the cold-worked material.     

Morphological changes and hardness evolution in Cu–30Ni–5Cr and Cu–45Ni–15Cr spinodal alloys

Abstract

The development of increased strength in Cu–Ni–Cr alloys, compared with binary Cu–Ni alloys, is dependent upon heat treatment. These alloys have compositions which permit them to be solution treated at elevated temperature and then aged at a lower temperature, in a two phase field, to produce hardening. Decomposition into two phases may occur by nucleation and growth or by a spinodal reaction, depending on alloy composition and heat treatment temperature. As part of a more extensive study of ternary Cu–Ni–Cr alloys, the decomposition of Cu–30Ni–5Cr and Cu–45Ni–15Cr (wt-%) has been studied in the spinodal range. The evolution of microstructure has been determined together with the coarsening kinetics for the modulated spinodal decomposition products. Specimens rapid quenched from 1050°C, were aged in the temperature range 300–800°C. The progress of spinodal decomposition was followed via hardness measurements, X-ray diffraction, and scanning and transmission electron microscopy. Modulation wavelengths were measured from both X-ray diffraction patterns and electron micrographs. It was found that during the early stages of aging the modulation wavelength remained constant while the hardness increased continuously. After a certain period of aging, the hardness remained constant at its peak value, while the modulation wavelength increased continuously. The results are consistent with current theories of spinodal decomposition and hardening.

MST/1733     

Effects of annealing treatment on microstructure and hardness of bulk AlCrFeNiMo0.2 eutectic high-entropy alloy

Bulk AlCrFeNiMo_0.2 (in molar ratio) alloy ingot was prepared by vacuum induction melting and casting methods. The effects of annealing temperature variations (550–1050 °C) on the crystal structure, microstructure, and hardness were investigated. The hardness of the as-cast alloy was HV467, and the alloy exhibited a typical eutectic cell microstructure consisting of FeCrMo-rich solid solution (BCC structure) and NiAl-rich intermetallic compound (B2 structure). After annealing at 750–950 °C, a Mo-rich σ phase precipitated from the FeCrMo-rich solid solution, and apparent annealing hardening appeared. The hardness increased from HV479 to HV542. The hardening of this alloy was attributed to the transformation of the BCC phase to the σ phase. The σ phase changed back to the BCC phase at 1050 °C and the hardness decreased to HV478 which was nearly equal to that of the as-cast alloy. Overall, the AlCrFeNiMo_0.2 alloy exhibited excellent annealing softening resistance.     

Half-Metallicity and High-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Ferromagnetism in Si-containing Half-Heusler Alloys

A possibility of half-metallic ferromagnetism for the homogeneous phase in the half-Heusler alloy-based DMS CoTi_1−x Fe _x Sb is predicted. A phase diagram of the spinodal and binodal decomposition is constructed through the evaluation of the mixing free energies. By applying the Monte Carlo simulation method to the Ising model with realistic chemical pair interactions between Fe magnetic impurities, we simulate the spinodal decomposition and nanoscale separation in this alloy.     

The phase field investigation of B2 (FeAl) phase antisite defect during homogenous transformation of Fe–24Al alloy

The B2 (FeAl) antisite defects and microstructure evolution during homogenous transformation of Fe–Al alloys are investigated by phase field kinetic model considering long-range elastic interaction energy. The results show that congruent ordering transformation dominates the aging initial stage, and two phase structures of B2 ordered phase and disorder phase are formed. The antisite atom occupation probability which indicates antisite defects decreases during congruent ordering transformation of aging initial stage. With increasing aging time, spinodal decomposition occurs within B2 structure ordered phases, and spinodal decomposition and ordering coexistence microstructures are formed at this stage. At the aging of final stage, the increase of antisite atom occupation probability is attributed to the presence of spinodal decomposition. For Fe–Al alloys with different composition, we find that with increasing Al atom concentration, the Al_Fe antisite defects increase, and the Fe_Al antisite defects decrease. In contrast, with the increase of Fe atom contents, the numbers of Fe_Al antisite defects increase and that of the Al_Fe antisite defects decrease.