Papers by Author: Zhong Guang Wang

Abstract: A cast nickel based superalloy M963 was coated by high-velocity oxy-fuel (HVOF) spraying process. The effect of HVOF MCrAlY coating on thermo-mechanical fatigue (TMF) and isothermal fatigue (IF) in M963 was studied to understand fatigue life and failure mechanisms in coated and uncoated M963 alloy. Two types of TMF tests, i.e. in-phase (IP) and out-of-phase (OP), were performed in temperature range of 450~900°C, and IF tests were conducted at 900°C. It was found that the coating had a detrimental effect on fatigue life under OP TMF, while a beneficial effect of the coating existed under IP TMF and IF. Crack initiation time in the coated specimen was shorter than that in the uncoated specimen and the former’s crack density was higher than the latter’s one under OP TMF. The relationship of deformation and fracture response with fatigue life was discussed based on microscopic analysis.

Abstract: Fatigue tests of nanometer-thick Cu films as deposited and annealed in vacuum were conducted under constant load ranges at room temperature. Fatigue strengths of the Cu films, which is defined as the critical load range being able to cause crack initiation within 106 cycles, are determined. The experimental results show that fatigue strength increases with decreasing film thickness. Fatigue cracking behaviors were characterized by electron microscope. It is also found that fatigue cracking resistance is dependent on film thickness and increases with decreasing film thickness. Size effects on fatigue properties of the nanometer-thick Cu films are discussed.

Abstract: The eutectic Sn3.8Ag0.7Cu alloy is widely considered a leading Pb-free replacement for the eutectic Pb-Sn solder alloy in electronic packaging where creep deformation and rupture is a major concern. In this study, creep rupture behavior of Sn-Ag-Cu solder alloy was investigated under the isothermal condition. Creep tests were conducted under a range of stresses and temperatures. Creep lifetime data were analyzed by the combined time-temperature equations following the Sherby, Larsen-Miller, and Manson-Haferd approaches. From these analyses, a series of material parameters were obtained from the experimental data. The results showed that the Manson-Hanferd method provided a better correlation with the creep rupture data. The mechanisms of creep deformation and rupture at different time-temperature combinations are discussed.

Abstract: Low cycle fatigue behavior of Sn-3.8Ag-0.7Cu solder was investigated under fully reversed cyclic loading, with particular emphasis on microstructural effects. The LCF behavior of the solder with equiaxed microstructure was found to differ greatly from that of the solder with a dendrite microstructure. At a given total strain amplitude, the dendrite microstructure exhibited a much longer fatigue life than the equiaxed microstructure. Such a strong microstructural effect on fatigue life arose from the difference in cyclic deformation and fracture mechanisms between the two microstructures. A large number of microcracks along grain boundaries of the equiaxed structure solder developed with increasing cycling, while for the dendrite structure solder, cyclic deformation took place along the direction of the maximal shear stress during fatigue tests and microcracks initiated and propagated along shear deformation bands. Besides, the fatigue behavior of the dendritic microstructure was very sensitive to cyclic frequency whereas the fatigue behavior of the equiaxed microstructure showed less sensitivity to cyclic frequency.

Abstract: NiCrAlY coatings were prepared by the detonation spraying (DS) and high velocity oxygen fuel (HVOF) spraying processes, respectively. The DS NiCrAlY coating has higher oxidation rate than the HVOF NiCrAlY coating at 1100 oC due to its rougher surface and more mixed oxides. The residual compressive stress in thermally grown oxides (TGO) on the DS NiCrAlY coating is higher than that on the HVOF coating because the θ-Al2O3 to α-Al2O3 transformation causes the tensile stress in the TGO on the HVOF coating.

Abstract: Ti₃SiC₂ has been widely investigated for their intriguing physical and mechanical properties [1]. In the past decades, dense and pure polycrystalline Ti₃SiC₂ has been fabricated by some techniques [1,2]. Studies on deformation and fracture behavior at elevated temperature showed that Ti₃SiC₂ exhibited substantial plasticity with high strength [3]. It is pointed out that Ti₃SiC₂ can be possibly used for some contact applications, such as bearings, for its relative high compression strength, self-lubricating and machinability [1]. In the present paper, fatigue crack-growth rates and Hertzian contact damage of Ti₃SiC₂ were studied. Shielding ability of Ti₃SiC₂ was investigated specially in the small crack region. Damage evolution and strength degradation under Hertzian contact were studied to reveal damage behavior and mechanisms in Ti₃SiC₂.