Applied Mechanics and Materials Vol. 782

Abstract: In this paper, the shock phase transformation of β phase in Ti-5Al-5Mo-5V-3Cr-0.5Fe (Ti-5553) was investigated. Split Hopkinson Pressure Bar (SHPB) and light gas gun were employed to investigate the dynamic properties under high strain rates from 1000s^-1 to 3500s^-1. Microstructure characterization was carried out by optical microscopy (OM), scanning electronic microscopy (SEM) and transmission electron microscope (TEM). The experimental results demonstrate that the Ti-5553 alloy with β phase exhibit no obvious strain rate hardening effect with the high strain rate from 1000s^-1 to 3000s^-1. However, compared with the quasi-static compression test results (10^-3s^-1), this alloy shows an evident strain rate hardening effect, with the yield strength significantly improved. Second time loading indicates light gas gun dynamic tensile loading and then SHPB dynamic compression loading in Ti-5553 alloy with β phase. The results show that the shock-induced β to αʺ martensite phase transformation dramatically influences the postshock mechanical properties of these alloys. The yield strength of this alloy decreased after the shock wave effect of light gas gun, its ductility increasing. Higher shock pressures yielded an increased dislocation density and a gradual increase in the yield strength. Adiabatic shear band (ASB) exists in second time loading Ti-5553 alloy under 10³s^-1 strain rate. SHPB loaded the alloy: The results show that the Ti5553 alloy with β phase is adiabatic shear failure in high strain rate (3000s^-1).

Abstract: Ti-6Al-4V compact bulk was fabricated by Spark plasma sintering (SPS) with initial pressure of 1.7 MPa, holding pressure of 50 MPa, heating rate of 100 °C/min, and holding time of 5 min at different sintering temperature. The fracture morphology of the specimen sintered at different temperatures was observed to investigate the sintering mechanism. It can be concluded that there are four stages in the SPS process: activation and rearrangement of particles, connection of particles, growth of sintering neck and bulk deformation. The high-quality bulk compact can be obtained when the above mentioned four sintering stages proceed in turn and are all fully completed. The compact bulk has the best mechanical properties when the sintering temperature was 1050°C. The relative density of the bulk Ti-6Al-4V exceeds 99.5%. The tensile strength and the elongation of Ti-6Al-4V obtained by SPS process are 901 MPa and 13.9%, respectively.

Abstract: Spark plasma sintering (SPS) technique was employed to fabricate nanocrystalline aluminum (Al) with the organic-coated Al nanopowders as raw material. A low initial pressure of 1 MPa and a high holding pressure of 300 MPa were used in the investigation. The effect of sintering temperature on the microstructure and the physical properties of the nanocrystalline Al was investigated. The results demonstrated that both the grain size and the thermal diffusivity of the nanocrystalline Al increase with an increase in sintering temperature. However, the resistivity of the nanocrystalline Al deceases with increasing sintering temperature.

Abstract: TiB-Ti/Ti-6Al-4V composites were fabricated by spark plasma sintering (SPS) technique under a pressure of 50MPa, with sintering temperature of 1300 °C and heating rate of 100 °C /min. The effect of the TiB content in TiB-Ti composite layer on microstructures and mechanical properties of the TiB-Ti/Ti-6Al-4V composites were investigated. The results indicate that as an advanced welding method, SPS technique provided the excellent welding combination of TiB-Ti and Ti-6Al-4V. The relatively excellent mechanical properties of the joints, including the relative density of 98.6%, micro-hardness of 10.2GPa, fracture strength of 177MPa were achieved as TiB content in TiB-Ti composite layer reaches 50%.

Abstract: The bulk nanocrystalline (NC) aluminum (Al) 5083 was synthesized by spark plasma sintering (SPS) technique with low initial pressure of 1 MPa, high holding pressure of 300 MPa and holding time of 4 min at different sintering temperatures, using surface passivated nanopowders. The effect of sintering temperature on microstructure and mechanical properties of the bulk NC Al 5083 were investigated. Results indicate that the density, grain size, the hardness and the compressive strength of the bulk NC Al 5083 increase with an increase in sintering temperature. The mechanical properties of the material are greatly improved due to the fine grain size. The bulk NC Al 5083 sintered at 723 K has the highest micro-hardness of 2.37 GPa and the best compressive strength of 845 MPa.

Abstract: Quasi-static and dynamic compressive mechanical properties of Ni-Ti shape memory alloys have been investigated by using electronic universal testing machine and Split-Hopkinson Pressure Bar (SHPB), respectively. The experimental results show that the dynamic yield stress and the fracture stress are higher than those in the static. Martensitic phase transformation is induced by stress before the plastic deformation. The critical stress of SIM and the slope of stress plateau increase with the increase of strain rates.

Abstract: The high strength steel which was subjected with isothermal heat treatment at three different temperatures, namely 330°C, 350°Cand 380°C after different quenching temperature namely 880°C and 900°C,was investigated in this paper. The quasi-static and dynamic mechanical properties of new high strength steel was tested by universal material testing machine and Split Hopkinson Pressure Bar (SHPB). Experimental results have showed that the yield strength and tensile strength of the steel reach 1100MPa and 1400MPa respectively. Hardness, yield strength and toughness are found to decrease with the consequently increasing of isothermal temperature under the same quenching temperature. The compression properties of the steel under quenching temperature of 880°C are higher than that of 900°C with the same isothermal temperature. It can be found that the steel which is subjected with isothermal heat treatment show strain rate sensitivity under high velocity impact. When isothermal temperature is set 380°C, the steel exhibits the most obvious strain rate hardening effect.

Abstract: To explore the thermo-mechanical response of H96 brass alloy, the quasi-static (universal-testing machine) and dynamic (the split Hopkinson pressure bar apparatus) uniaxial compression experiments have been performed under the temperatures from 293 K to 873 K and the strain rates from 0.001 s^-1 to 6000 s^-1, and the strains over 60% are obtained. Results show that, H96 brass alloy has strong strain hardening behavior, and it becomes weaker with the increasing temperature. In addition, this alloy is sensitive to strain rates; and, it has temperature sensitivity, the dynamic strain aging occurs at the temperature of 473 K and a quasi-static strain rate of 0.001 s^-1. Based on the thermal activation dislocation mechanism, paralleled with the experimental results, a plastic flow constitutive model with the physical conception is developed. The model is suitable to predict the plastic flow stress at different temperatures and strain rates. According to comparing results, the model predictions are in good agreement with the experimental results.

Abstract: Ti-5553(Ti-5Al-5Mo-5V-3Cr-1Fe) is a new near-β Titanium alloy with some applications as structural components in aircrafts. In this paper, Ti-5553 alloy was heat treated at four different solid solution temperatures: 770°C, 790°C, 830°C and 850°C, followed by the same aging treatment. Quasi-static compressive and tensile experiments as well as Split Hopkinson Pressure Bar (SHPB) compression experiment were carried out to investigate the mechanical properties of Ti-5553 alloy. The microstructure evolutions were characterized using Optical Microscope (OM) and Scanning Electron Microscopy (SEM). The experimental results demonstrated that different microstructures of Ti-5553 alloy were obtained through four different heat treatment procedures. The main microstructures are aα+βb with different volume fractions. Ti-5553 alloy treated at 770°C, with secondary α precipitates embedded in β matrix, behaves the best combination of strength and toughness. In dynamic compression, strain rate hardening effect occurs exclusively in the samples with the Widmanstatten structure and not occurs in other samples.

Abstract: The evolution process of a high strength steel which subjected with three different heat treatment proceedings and gets different quasi-static tensile properties was investigated in this paper. To precisely control the plastic deformation of the cylinder and capture the development process of adiabatic shear bands, stopper ring was used in Split Hopkinson Pressure Bar (SHPB). Combining the stress-strain curves and microstructures after SHPB tests, the microstructure evolution from the nucleation of adiabatic shear bands to fracture of the cylindrical steel were observed. The experimental results have demonstrated that there are similar fracture procedures of the steel treated through different heat treatments. Shear bands form firstly, then micro-cracks develop from shear bands, and lead to macro-crack finally. However, the critical strains for nucleation of ASBs and the time spending on the fracture procedure of the steel treated at different heat treatments are different. Samples treated at 900°C/AC exhibit the best resistance to adiabatic shear sensitivity when compressed under high strain rates.