Analysis of phases in a Cu–Cr–Zr alloy

Phases of a Cu–0.31%Cr–0.21%Zr alloy were analyzed by scanning electronic microscope and energy dispersive X-ray spectroscopy (EDXS) and transmission electronic microscope (TEM). The EDXS results showed that there are three types of phases in the alloy, Cu-matrix, chromium-rich and zirconium-rich phases; coarse phases mainly consist of zirconium-rich phase. TEM result showed that fine chromium distributed in matrix and Cu₅₁Zr₁₄ phase was found in matrix.     

Nucleation and growth behaviors of primary phase in hypoeutectic Al–Cu alloy in high magnetic field

The nucleation and growth behaviors of primary Al phase in the hypoeutectic alloy of Al–20.8%Cu (mass fraction) in high static magnetic fields were investigated by differential thermal analysis (DTA). The DTA curves indicate that the nucleation temperature of primary Al phase decreases as the magnetic induction increases. The average growth rates of primary crystals increase with the increase of magnetic induction. The dendrite structures show that primary Al phase dendrites change from disorderly without the magnetic field to regularly with the field. The effect of magnetic field with the magnetic induction order of 10 T on driving force for the nucleation of Al crystals is negligible. The reduction of nucleation temperature of primary Al phase is mainly caused by the increase of the interfacial free energy between the melt and the nucleus. The change in dendrite morphology can be attributed to the suppression of melt flows in the magnetic field and magnetic anisotropy of Al crystals.     

Microstructure of phases in a Cu–Zr alloy

The morphology and crystallography of phases in the Cu-0.12% Zr alloy were investigated by scanning electron microscope(SEM), transmission electron microscope(TEM), and high-resolution transmission electron microscope(HRTEM). The results show that the as-cast microstructure of Cu–Zr alloy is mainly Cu matrix and eutectic structure which consist of Cu and Cu5Zr phases with a fine lamellar structure. The disk-shaped and plateliked Cu5Zr phases with fcc structure are found in the matrix, in which habit plane is parallel to {111}a plane of the matrix.Between the copper matrix and Cu5Zr phase,there exists an orientation relationship of [112]a|| [011]Cu5Zr;(111)a||(111)Cu5Zr. The space structure model of Cu5Zr phase can be established.     

Comparison of the corrosion resistance of an Al–Cu alloy and an Al–Cu–Li alloy

ABSTRACT

In this study, the corrosion mechanisms of the AA2024-T3 and the AA2098-T351 were investigated and compared using various electrochemical techniques in 0.005?mol?L^?1 NaCl solution. The severe type of corrosion in the AA2098-T351 was intragranular attack (IGA) although trenching and pitting related to the constituent particles were seen. On the other hand, the AA2024-T3 exhibited severe localised corrosion associated with micrometric constituent particles, and its propagation was via grain boundaries leading to intergranular corrosion (IGC). Electrochemical techniques showed that the corrosion reaction in both alloys was controlled by diffusion. The non-uniform current distribution in both alloys showed that EIS was not a proper technique for comparing the corrosion resistance of the alloys. However, local electrochemical techniques were useful for the evaluation of the corrosion resistance of the alloys.     

Phase decomposition of the γ phase in a Mn–30 at.% Cu alloy during aging

The phase decomposition process of γ phase in a Mn–30 at.% Cu alloy, when aged at 723 K from 2 to 50 h, is investigated with electrical resistivity and magnetic susceptibility measurement. In conjunction with the antiferro-magnetic transition of the Mn-rich regions during cooling to room temperature from the aging temperature, the temperature coefficient of electrical resistivity shows a continuous increase in a certain temperature range. The temperature where the coefficient has the maximum increasing rate is defined as the T_N temperature of the Mn-rich regions. It was found that the T_N temperature was 20–30 K higher than the concomitant f.c.c.–f.c.t. transformation temperature T_t, determined with the minima of Young’s modulus in the aged samples. The increment of temperature coefficient of electrical resistivity involved in the magnetic transition is used to estimate the changes of volume fraction for Mn-rich regions vs aging time. At the same time, the paramagnetic feature above the spin-freezing transition temperature for quenched Cu-rich alloys is summarized, and the Mn concentration in Cu-rich regions of aged samples is calculated. It should be noted that Mn and Cu-rich regions had already formed in the 2 h-aged Mn–30 at.% Cu sample, and longer aging further enriched Mn or Cu, while the volume fraction of Mn-rich regions decreased slightly with aging time. Electrical resistivity measurement sensitive to Mn-rich regions and the magnetic susceptibility measurement for Cu-rich regions have shown the compositional heterogeneity in decomposed phases. TEM observation confirms the interconnectivity of the two regions in the aged microstructure. All the results support the hypothesis that the decomposition of γ phase in Mn–Cu alloys proceeds in the spinodal manner.     

Comparative TEM investigation on the precipitation behaviors in Cu–15wt%Sn alloy

The decomposition and precipitation behaviors of a quenched Cu–15wt%Sn alloy as a function of aging temperature were investigated using transmission electron microscopy (TEM). Focused ion beam (FIB) was employed to assist TEM specimen preparation. At 300 °C, the decomposition of the supersaturated α′ phase occurred at grain boundaries, displaying a cellular morphology. The lamellae were found with ζ and α phases, rather than with the equilibrium ε and α phases. The ζ and α phases exhibit a well-defined orientation relationship (OR) as $ (1\bar{1}0)_{\alpha } //(0001)_{\zeta } ,\;[11\bar{2}]_{\alpha } //[\bar{1}2\bar{1}0]_{\zeta } $ . On the other hand, at 320 °C, only incipient lamellar structures of several micron meters were observed, which were composed of the δ and α phases. At the same time, abundant intragranular precipitation of the ε phase in the form of platelets was observed, and OR as $ (1\bar{1}1)_{\alpha } //(001)_{\varepsilon } , $ [110] _α //[100] _ε exists between ε phase and the α phase. These contrasting precipitation behaviors are discussed from the viewpoint of crystallographic coherency of these phases.     

Structure and phase formation in Cu–Mn alloy thin films deposited at room temperature

Transmission electron microscopy characterization of Cu–Mn alloy thin films deposited by DC magnetron sputtering is applied to reveal the formation of phases throughout the composition range. Pure Cu and Mn films exhibit face-centred cubic (fcc) Cu and α-Mn phases, respectively. At room temperature the low Mn content films have fcc structure (γ-phase). Mn can substitute Cu in the fcc Cu lattice up to ～35 at.% Mn. The lattice parameter of fcc Cu–Mn alloy films follows a linear relationship of a₀ = a_Cu + 0.322c (in Å), where a_Cu = 3.615 Å is the lattice parameter of Cu and c is the Mn atomic concentration. At high Mn content, above 50 at.% Mn, a homogeneous one-phase structure is observed, possessing the short-range order of α-Mn. The incorporation of Cu into Mn suggests that this structure changes from crystalline α-Mn to disordered structure as the Cu content increases. A narrow two-phase region exists between 35 and 45 at.% Mn. A grain size minimum of 2–3 nm was observed in the 35–65 at.% Mn region.     

Comparison of the Isothermal Oxidation Behavior of As-Cast Cu–17%Cr and Cu–17%Cr–5%Al Part I: Oxidation Kinetics

The isothermal oxidation kinetics of as-cast Cu–17%Cr and Cu–17%Cr–5%Al in air were studied between 773 and 1,173 K under atmospheric pressure. These observations reveal that Cu–17%Cr–5%Al oxidizes at significantly slower rates than Cu–17%Cr. The rate constants for the alloys were determined from generalized analyses of the data without an a priori assumption of the nature of the oxidation kinetics. Detailed analyses of the isothermal thermogravimetric weight change data revealed that Cu–17%Cr exhibited parabolic oxidation kinetics with an activation energy of 165.9 ± 9.5 kJ/mol. In contrast, the oxidation kinetics for the Cu–17%Cr–5%Al alloy exhibited a parabolic oxidation kinetics during the initial stages followed by a quartic relationship in the later stages of oxidation. Alternatively, the oxidation behavior of Cu–17%Cr–5%Al could be better represented by a logarithmic relationship. The parabolic rate constants and activation energy data for the two alloys are compared with literature data to gain insights on the nature of the oxidation mechanisms dominant in these alloys.     

In situ observation of Cu segregation and phase nucleation at a solid–liquid interface in an Al alloy

This paper presents a systematic study comparing experimental in situ transmission electron microscopy observation of microstructural and compositional evolution with complementary thermodynamic calculations, to better understand the redistribution of solute elements and the nucleation behavior of different phases in a commercial Al-alloy powder (AA390). The results show that Cu segregation to the solid Si–liquid Al interface, as well as the significant undercooling achieved in the liquid under non-equilibrium conditions because the Al phase cannot nucleate homogeneously, play a important roles in nucleating Al₂Cu at the interface prior to the Mg₂Si phase in the alloy. Although Cu segregation can occur at various locations along the interface, the Al₂Cu phase appears to preferentially nucleate at a high-index Si–liquid interface as opposed to a low-index one. The Cu concentration during segregation remains essentially constant with time, indicating that the observed segregation behavior is a thermodynamic and not a kinetic phenomenon. These in situ observations and complementary thermodynamic calculations substantially enhance our understanding of potential crystal nucleation and growth processes.     

Corrosion and corrosive-wear behaviors of a high strength and toughness Cu–15Ni–8Sn alloy in seawater

The Cu–15Ni–8Sn alloy with high strength and toughness is a common bearing alloy, while it is often subjected to premature failure in service due to corrosion and wear. Therefore, the corrosion and wear behaviors of an ultra-high-strength and toughness Cu–15Ni–8Sn (wt%) alloy fabricated by hot isostatic pressing were investigated in this study. The results indicated that intergranular corrosion and pits were observed when the studied alloy was immersed into seawater for 30 days. When the Cu–15Ni–8Sn alloy slid in seawater, there was a synergistic effect between corrosion and wear. With the increase of normal load, the synergistic effect weakened initially and then enhanced the interaction between corrosion and wear changed from negative to positive, and the main wear mechanisms transferred from abrasive wear into delamination.     

Phase transformation of Cu precipitates from bcc to fcc in Fe–3Si–2Cu alloy

Phase transition of Cu precipitates during aging of an Fe–3Si–2Cu alloy was studied by transmission electron microscopy. The precipitation of 3–5-nm-sized body-centered cubic (bcc) Cu in ferrite matrix was confirmed by high-angle annular dark-field scanning transmission electron microscopy imaging. The bcc Cu precipitates transformed to 9R Cu as they grew. Many 9R Cu precipitates were twinned, but untwinned 9R Cu particles were also observed. The 9R Cu transformed to twinned face-centered cubic (fcc) Cu by the glide of ±a/3 [1 0 0]_9R Shockley-type partial dislocations. Formation of the 3R structure previously reported could not be confirmed in this study. Finally, twins in fcc Cu precipitates disappeared to form stable fcc Cu particles. The importance of electron beam-orientation-dependent moiré fringes in the correct identification of Cu structure is discussed in detail.     

The phase diagram of the terbium–gold alloy system

The terbium-gold phase diagram has been investigated in the 0–100 at% Au field by differential thermal analysis (DTA), X-ray diffractometry (XRD), optical microscopy (LOM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Eight intermetallic phases were found, namely: Tb₂Au orthorhombic oP12–Co₂Si, peritectic decomposition at 1000°C, TbAu, L.T. form, orthorhombic oC8–CrB type and H.T. form, cubic cP2–CsCl type, congruent melting at 1590°C, Tb₃Au₄ trigonal hR42–Pu₃Pd₄ type, peritectic decomposition at 1340°C, Tb₇Au₁₀ tetragonal tI136–Gd₇Au₁₀ type, peritectic decomposition at 1210°C, TbAu₂ tetragonal tI6–MoSi₂ type, congruent melting at 1265°C, TbAu₃ orthorhombic oP8–TiCu₃ type, congruent melting at 1215°C, Tb₁₄Au₅₁ hexagonal hP65–Gd₁₄Ag₅₁ type, peritectic decomposition at 1175°C, and TbAu₆ tetragonal tP56–SmAu₆ type, peritectic decomposition at 855°C. Four eutectic reactions were found to occur at 880°C and 20.0 at% Au, at 1195°C and 62.5 at% Au, at 1160°C and 71.0 at% Au and finally at 805°C and 89.0 at% Au. A catatectic reaction occurs in the Tb-rich region. The experimental results are discussed and compared with the general behaviour of the other R–Au systems.     

Age-hardening and overaging mechanisms related to the metastable phase formation by the decomposition of Ag and Cu in a dental Au–Ag–Cu–Pd–Zn alloy

The age-hardening and overaging mechanisms related to the metastable phase formation by the decomposition of Ag and Cu in a dental casting gold alloy composed of 56Au–25Ag–11.8Cu–5Pd–1.7Zn–0.4Pt–0.1Ir (wt.%) were elucidated by characterizing the age-hardening behaviour, phase transformations, changes in microstructure and changes in element distribution. The fast and apparent increase in hardness at the initial stage of the aging process at 400°C was caused by the nucleation and growth of the metastable Ag–Au-rich phase and the Cu–Au-rich phase by the miscibility limit of Ag and Cu. The transformation of the metastable Ag–Au-rich phase into the stable Ag–Au-rich phase progressed concurrently with the ordering of the Cu–Au-rich phase into the AuCu I phase through the metastable state, which resulted in the subsequent increase in hardness. The further increase in hardness was restrained before complete decomposition of the parent α₀ phase due to the initiation of the lamellar-forming grain boundary reaction. The progress of the lamellar-forming grain boundary reaction was not directly connected with the phase transformation of the metastable phases into the final product phases. The heterogeneous expansion of the lamellar structure from the grain boundary caused greater softening than the subsequent further coarsening of the lamellar structure. The lamellar structure was composed of the Ag–Au-rich layer which was Cu-, Pd- and Zn-depleted and the AuCu I layer containing Pd and Zn.     

Liquid phase separation in immiscible Cu–Fe alloys

Cu–Fe binary alloys containing 20–50wt. % Fe were studied by the combination of the melt fluxing and cyclic superheating technique. The microstructural evolution of Cu–Fe alloys was investigated with scanning electron microscopy. When the undercooling was larger than the critical undercooling, the Fe-rich spheroids were embedded into a Cu-rich matrix and the metastable phase separation was observed in microstructures. The size of separated particles in the Cu-35wt.% Fe alloy was larger than that of other Cu–Fe alloys with different compositions and the size of separated droplets was related to the ΔT_S, which was equal to the undercooling (ΔT) minus the critical undercooling (ΔT_C). Moreover, a large undercooling tended to promote the coagulation of the separated droplets, so the size of the separated Fe-rich spheroids in the microstructure of the immiscible Cu–Fe alloys increased with the increase in the undercooling.     

Rapid solidification of undercooled Cu–Ge peritectic alloy

Bulk samples of Cu–14.4 wt% Ge peritectic alloy has been undercooled by up to 200 K (0.166 T_L) with a glass fluxing technique. The solidified microstructures are mainly characterized by α-Cu dendrites plus ζ phase which forms in the interdendritic areas within the whole undercooling regime. With the increase of undercooling, both the secondary arm spacing of primary α-Cu dendrite and the layer thickness of the peritectic ζ phase decrease. The primary trunk and secondary arm of α-Cu dendrites show round shape under small undercooling condition, whereas they keep a good dendritic shape within a large undercooling regime, indicating that the peritectic reaction proceeds for a relatively longer period of time in the former case. The volume fraction of peritectic ζ phase increases with undercooling, but that of α-Cu dendrite shows a decreasing tendency. Furthermore, drop tube experiments were also performed to reveal the competitive nucleation and growth mechanisms of primary α-Cu dendrite and peritectic phase ζ. Calculations based on the current dendritic growth model are made to analyze the crystal growth kinetics during the rapid solidification of undercooled Cu–Ge peritectic alloy.     

Electrochemical behavior of a lead-free Sn–Cu solder alloy in NaCl solution

Electrochemical impedance spectroscopy (EIS), potentiodynamic polarization techniques and an equivalent circuit analysis are used to evaluate the electrochemical corrosion behavior of Sn–Cu alloy samples in a naturally aerated 0.5 M NaCl solution at 25 °C. It has been found that a better electrochemical corrosion resistance is provided by a coarser cellular microstructure array. It has also been found that the corrosion current density (i_corr) is of about a quarter when compared with that of the finest microstructure examined. Such behavior is attributed to both localized strains between the Sn-rich phase and intermetallic (IMC) particles and the cathode/anode area ratios. The effect of copper alloying on i_corr is also discussed.     

Electron microscopic investigation of aging in the Cu–0.06% Zr alloy

The decomposition of supersaturated solid solution in the Cu–0.06 wt% Zr alloy has been investigated. Upon aging of the initially quenched alloy the homogeneous precipitation of particles is dominating. The decomposition begins from the precipitation of a metastable copper–zirconium phase, the particles of which have the shape of nanodimensional disks. An increase in the aging temperature results in the formation of coarser rodlike particles of the Cu₅Zr equilibrium phase. Aging of the deformed alloy is characterized by the predominance of the heterogeneous precipitation of particles at subboundaries and dislocations, and the decomposition begins at a lower temperature. The particle size is less by an order of magnitude than that in the quenched state. The precipitation of nanodimensional particles at dislocations retards the formation of recrystallization centers.     

Quench sensitivity of Al–Cu–Mg alloy thick plate

The quench sensitivity of Al-Cu-Mg alloy was investigated at different thicknesses of the thick plate.The quenching process was simulated via finite element analysis (FEA);time-temperature-property (TTP) curves and time-temperature-transformation (TTT) curves were obtained through hardness test and differential scanning calorimetry (DSC) test;and the microstructural observation was carried out by scanning electron microscopy (SEM)and transmission electron microscopy (TEM).Experimental results ex...     

Microstructure and properties of aging Cu–Cr–Zr alloy

The crystallography and morphology of precipitate particles in a Cu matrix were studied using an aged Cu–Cr–Zr alloy by transmission electron microscopy(TEM) and high-resolution transmission electron microscopy(HRTEM). The tensile strength and electrical conductivity of this alloy after various aging processes were tested. The results show that two kinds of crystallographic structure associated with chromium-rich phases, fcc and bcc structure, exist in the peak-aging of the alloy. The orientation relationship between bcc Cr precipitate and the matrix exhibits Nishiyama–Wasserman orientation relationship. Two kinds of Zr-rich phases(Cu4Zr and Cu5Zr)can be identified and the habit plane is parallel to {111}Cu plane during the aging. The increase in strength is ascribed to the precipitation of Cr- and Zr-rich phase.