Microstructure and mechanical properties of twostep Cu-alloyed ADI treated by different second step austempering temperatures and times

Austempering ductile iron (ADI) is an attractive material due to its excellent comprehensive mechanical properties. However, the deficit in elongation and toughness always threatens its security application. Two-step austempering process is an effective way to improve elongation and toughness simultaneously. In the present work, the influence of the amount, morphology and distribution of ferrite and austenite on mechanical properties of ADI under different second-step austempering parameters has been analyzed. Results show that the amount of austenite and its carbon content decrease with increasing of second-step temperature. Carbide begins to precipitate as second-step austempering temperature reaches 380 °C. These factors together influence the mechanical properties of two-step Cu-alloyed ADI. Impact energy and fracture toughness are strongly affected by second-step austempering temperature, and are dramatically decreased with increase of second-step austempering temperature. Elongation remains constant when the second-step temperature is below 360 °C, and then it is rapidly decreased with further increase of second-step temperature. Strength is slightly influenced by second-step temperature. Ferrite morphology is not influenced by second-step austempering duration, while blocky retained austenite size is slightly decreased with the increasing of second-step austempering time. The amount of retained austenite is decreased while the carbon content of retained austenite is increased with the extending of second-step austempering time. The substructure of austenite is transformed from dislocation to twin when second-step austempering time exceeds 60 min. Strength and elongation are improved slightly with extending of second-step time. Impact energy and fracture toughness initially decrease with the extending of second-step time, and then remain constant when the time is longer than 60 min. This is a result of austenite content decreasing and carbon content of austenite increasing. The second-step austempering time mainly influences austenite content and its carbon content, which is a result of carbon diffusion behavior variation.

Corrosion Behaviours of Copper Alloy in Solutions Containing Na_2SO_4 and NaCl with Different Concentrations

Potentiodynamic polarisation, potential-time measurements, X-ray diffraction (XRD) and infrared spectroscopy (IR) have been used to investigate the effect of different concentrations of Na2SO4 in the absence and presence of NaCI, on the corrosion of Cu-alloy. The electrochemical measurements showed that the increase of Na2SO4 concentration led to increase the corrosion current density of Cu alloy and vice versa. The presence of NaCI shifted the potential to more cathodic potential, which had a great influence on the protectiveness of the Cu oxide layer formed on the surface in presence of Na2SO4. The spectrometric measurements indicated the constituents of the film formed on the alloy surface were mainly Cu2O, in addition to the oxides, NiO and Fe2O3, which were traced by XRD analysis.

Effect of Cu content on microstructures and mechanical properties of ADI treated by twostep austempering process

The effect of Cu content on the microstructures and mechanical properties (yield strength, ultimate tensile strength, impact energy, fracture toughness) of austempering ductile iron (ADI) treated by two-step austempering process were investigated. High Cu content in nodular cast irons leads to a significant volume fraction of retained austenite in the iron after austempering treatment, but the carbon content of austenite decreases with the increasing of Cu content. Moreover, austenitic stability reaches its maximum when the Cu content is 1.4% and then drops rapidly with further increase of Cu. The ultimate tensile strength and yield strength of the ADI firstly increases and then decreases with increasing the Cu content. The elongation keeps constant at 6.5% as the Cu content increases from 0.2% to 1.4%, and then increases rapidly to 10.0% with further increase Cu content to 2.0%. Impact toughness is enhanced with Cu increasing at first, and reaches a maximum 122.9 J at 1.4% Cu, then decreases with the further increase of Cu. The fracture toughness of ADI shows a constant increase with the increase of Cu content. The influencing mechanism of Cu on austempered ductile iron (ADI) can be classified into two aspects. On the one hand, Cu dissolves into the matrix and functions as solid solution strengthening. On the other hand, Cu reduces solubility of C in austenite and contributes more stable retained austenite.

Predicting the variation of stacking fault energy for binary Cu alloys by first-principles calculations

The variation of stacking fault energy(SFE)in a number of binary Cu alloys is predicted through considering the Suzuki segregation by the full potential linearly augmented plane wave(FPLAPW)method.The calculated results show that some solute atoms(Mg,Al,Si,Zn,Ga,Ge,Cd,Sn,and Pb),which prefer to form the Suzuki segregation,may decrease the value of SFE;while the others(Ti,Mn,Fe,Ni,Zr,Ag,and Au),which do not cause the Suzuki segregation may not decrease the SFE.Furthermore,it is interesting to find that the former alloying elements are located on the right of Cu group while the latter on the left of Cu group in the periodic table of elements.The intrinsic reasons for the new findings can be traced down to the valences electronic structure of solute and Cu atoms,i.e.,the similarity of valence electronic structure between solute and Cu atoms increases the value of SFE,while the difference decreases the value of SFE.

Microstructure evolution and mechanical property of Cu-15Ni-8Sn-0.2Nb alloy during aging treatment

Aging treatment of Cu-based alloys is essential to enhance their strength that is desirable for their extensive engineering applications in electrical industry,whereas the underlying mechanism of strengthening is essential for massive manufacturing of these alloys.Here,the microstructure evolution of a supersaturated solid solution Cu-15Ni-8Sn-0.2Nb alloy aged at 400℃for different time was characterized at atomic scale using state-of-the-art transmission electron microscopy(TEM)and the corresponding mechanical property was also measured.The results reveal that the modulated structure,DO_(22)/L1_(2)ordering,and discontinuous precipitation(DP)appeared in the advances of aging time.At the early stage of aging treatment,component modulation waves and satellite spots appeared from spinodal decomposition and the modulation wavelength was identified in the range of 1-7 nm.Subsequently the modulated structures formed-poor-rich solute regions,of which DO22ordering was present in the Ni-poor region while L1_(2)ordering appeared in the Ni-rich region.The sequence of ordering precipitates was further verified by density functional theory(DFT)simulations.Furthermore,orientation relationships and interfacial structures between DO_(22),L1_(2)phases and the parent matrix were determined.The measured hardness of alloy reached a maximum value of 335 HV after aging for 120 min due to the coherence between the two ordering phases and matrix.These results illustrated the importance of aging on structural evolution and mechanical property of Cu-15Ni-8Sn alloy at various heat treatment stages,which could potentially help in manufacturing promising alloys for their extensive engineering applications.