Effect of transient current bonding on interfacial reaction in Ag-coated graphene Sn-Ag-Cu composite solder joints

To address the problem of floating and aggregation of Ag-GNSs in the molten pool during the traditional reflow soldering process,Cu/SAC/Ag-GNSs/Cu sandwich joints were prepared under an applied current density(1.0×10^(4) A/cm^(2))for a few hundred milliseconds to produce Ag-coated graphene-reinforced Sn-Ag-Cu(SAC/AgGNSs)solder joints.The experimental results showed that Ag-GNSs were homogenously dispersed in the solder joints,providing more Cu6 Sn5 grain nucleation sites,which refined these grains and reduced the thickness difference at the anode and cathode.In addition,the Cu6 Sn5 morphology changed from rod-like to plate-shaped because of the uniform distribution of Ag-GNSs and constitutional supercooling.The significantly increased shear strength of the transient current bonding and the change in the fracture mechanism were due to the uniformly distributed Ag-GNSs and the microstructural changes.

Microstructure evolution of 55Ni−23Cr−13Co nickel-based superalloy during high-temperature cyclic deformation

The cyclic deformation behavior and microstructure evolution of the 55Ni−23Cr−13Co nickel-based superalloy were studied at 750℃ under the strain amplitudes from 0.35%to 0.6%.Coffin−Manson−Basquin and Smith−Watson−Topper relationships were employed,which satisfactorily predicted the fatigue life of the alloy under various strain amplitudes.The superalloy showed an initial cyclic hardening as a result of the interaction between the dislocations and the precipitates,and following cyclic softening behavior mainly due to the shearing of theγ′phase by dislocations and dislocations recovery under all strain amplitudes.Microstructure analyses showed that the M_(23)C_(6) carbides exhibited a continuous-chain distribution at lower strain amplitudes,while they showed a discontinuous distribution at higher strain amplitudes.As the strain amplitude increased,the size of theγ′phase decreased as the consequence of repeated shearing by dislocations.Fracture mechanisms were analyzed.Under higher strain amplitudes,cavities preferred to form around grain boundaries.

Microstructure and grain boundary engineering of a novel Fe-Cr-Ni alloy weldment made with self-developed composition-matched weld filler metal

The microstructure,texture,and yield strength of an advanced heat-resistant alloy weldment made with composition-matched weld filler were investigated.Scanning electron microscopy,energy dispersive spectroscopy,and electron backscatter diffraction were used to characterize the microstructural and textural changes.Various grain boundary engineering(GBE)processes were performed on the weldment.The yield strengths of the weldment at 973 K were obtained before and after GBE processing,and were mostly consistent with the theoretically predicted values.The coincident-site lattices,misorientation,and recrystallization of the weld metal after GBE were analyzed,and the results indicate that the increase in dislocation density and the improvement in special grain boundaries in the weld metal are the main reasons for the yield strength elevation of the weldment after GBE.The variation in elongation after high-temperature tests has the same tendency as that in the impact toughness with different GBE parameters,which is related to the coarsening behavior of carbides.

Creep Behavior and Life Assessment of a Novel Heat-Resistant Austenite Steel and Its Weldment

In the present study, creep activation energy for rupture was obtained as 221-348 kJ/mol for 22Cr15Ni3.5 CuNbN due to the precipitation-hardening mechanism. The extrapolation strength of creep rupture time of 10~5 h at 923 K for22 Cr15 Ni3.5 CuNbN is more valid(83.71 MPa) predicted by the Manson-Haferd method, which is superior to other commercial heat-resistant steels. The tensile creep tests ranging from 180 to 240 MPa at 923 K were conducted to investigate creep deformation behavior of welded joint between a novel heat-resistant austenite steel 22Cr15Ni3.5 CuNbN and ERNiCrCoMo-1 weld metal. Apparent stress exponent value of 6.54 was obtained, which indicated that the ratecontrolled creep occurred in weldment during creep. A damage tolerance factor of 6.4 in the weldment illustrates that the microstructural degradation is the dominant creep damaging mechanism in the alloy. Meanwhile, the welded joints perform two types of deformation behavior with the variation in applied stress, which resulted from the different parts that govern the creep processing. Also, the morphology evolution of the fracture surfaces confirms the effects of stress level and stress state.