Precipitation reaction in alpha-Cu-Al-Ag alloys

Abstract

The Ag-rich precipitates formation reaction in the Cu-9.22%at. Al-4.66%at. Ag, Cu-9.31%at. Al-5.83%at. Ag and Cu-9.37%at. Al-7.08%at. Ag alloys was studied using microhardness change measurements with temperature and time, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The results indicated that the Ag-rich phase formation is an interface controlled process with a zero-order mechanism, due to the amount of unsolved Ag that makes the precipitation reaction rate independent of Ag concentration.Keywords: Ag additions, Cu-based alloys, kinetics. INTRODUCTIONIn the growth of precipitates from supersaturated solid solutions, the rate of growth depends upon the rate at which atoms are brought to, or removed from, the interface by diffusion and the rate at which they cross the interface. During the early stage of growth the interface reaction must be the slower of these steps because of the limited area of interface and because the distance over which diffusion is necessary tends to zero. At large particle sizes the reverse is true because the diffusional flux gets progressively slower due to the continuous removal of solute from solution reducing the concentration gradient which is the driving force for diffusion, whilst the flux across the interface increases due to the increase in area [1].According to the Cu-Al equilibrium diagram [2,3], the α-Cu solid solution limit is between 17.97 % at. and 18.91 % at. Al below 300 °C. In this way, α-(Cu-Al) alloys in this field are not susceptible to age hardening. Silver additions to Cu-Al alloys increase its hardness [4], corrosion resistance [5], and modify the aging characteristics of the alloys [6]. Phases in Cu-Al-Ag alloys are structurally analog to those present in the binary systems, without ternary intermediary phases [7,8,9]. In Cu-Al-Ag alloys the Ag-rich phase formation follows a growth and nucleation mechanism influenced by Ag content [10].In this work, the Ag rich phase formation in the Cu-9.22%at. Al-4.66%at. Ag, Cu-9.31%at. Al-5.83% at. Ag and Cu-9.37% at. Al-7.08% at. Ag alloys was studied using differential scanning calorimetry (DSC), microhardness change measurements with temperature and time, scanning electron microscopy (SEM) and X-ray diffractometry (XRD). MATERIALS AND METHODSThe Cu-9.22 % at. Al-4.66 % at. Ag, Cu-9.31 % at. l-5.83%at. Ag and Cu-9.37 % at. Al-7.08 % at. Ag alloys were prepared in an induction furnace under argon atmosphere using 99.95% copper, 99.97% aluminum and 99.98% silver as starting materials. Results from chemical analysis indicated a final alloys composition very close to the nominal one, with Pb, Fe, and Mn as main impurities (concentration less than 100 ppm). Cylindrical samples with 2.0 cm diameter and 6.0 cm length were cut in disks of 2.0 mm thickness. The disks were cold rolled and flat squares samples of 1.0 mm thickness and about 5.0 mm length were used for hardness measurements and scanning electron microscopy. Previous results [6] indicated that the Ag ...

Effect of alloying constituents on thermal, microstructural, mechanical and shape memory properties of Cu-Al-Ag shape memory alloys

“…Silver 8% weight was chosen as the upper limit for silver composition based on existing literature stating significant grain size reduction and increase of brittleness due to precipitate formation at grain boundaries (GB). Silver addition below 2% weight did not enhance shape memory properties significantly and, therefore, it was not chosen for study (Silva et al, 2012a).…”

“…The strain recovery of an alloy will be diminished if residual martensite is present in the austenite. The precipitation of a phase should be avoided to obtain a good SME (Silva et al, 2012a).…”

A Microstructural Study of Cu-10Al-7Ag Shape Memory Alloy in As-Cast and Quenched Conditions

“…Cu-based shape memory alloys (SMAs) exhibit remarkable properties, such as super elasticity and shape memory effect, attributed to a diffusionless reversible martensitic transformation [1][2][3]. Due to their affordability and ease of processing, Cu SMAs are utilized in various fields, including medicine, electronics and aerospace [4][5][6][7].…”

“…Ag in Cu-Al alloys affects the phase transformations, can promote the formation of pearlitic phases and can change the stability of this pearlitic phase [22]. The precipitation of Ag in Cu-Al-Ag alloys exhibits a zero-order reaction, indicating constant Ag diffusion rates and an interface-controlled formation of Ag-rich phases [2]. Furthermore, the addition of Ag to Cu-based SMAs significantly improves the martensitic transformation, ductility and operating temperature range compared to conventional Cu-Ni-Ti and Cu-Al-Ni SMAs [13,23].…”

Influence of Manganese Content on Martensitic Transformation of Cu-Al-Mn-Ag Alloy

“…Of the elements that are not completely soluble in the β-phase, Fe shows the greatest microalloying effect on the properties of Cu-Al-Mn alloys and, depending on the content added, increases the temperature of the martensitic transformation. Studies have also shown that the addition of Ti, Co, Cr, and Si mainly contributes to the poorer mechanical properties of Cu-Al-Mn SMAs [28][29][30]. Microalloying with magnesium, on the other hand, does not significantly change the properties of the ternary alloy and does not affect the change in martensitic transformation temperatures [18].…”