The Precipitated Particle Refinement in High-Cr ODS Steels by Microalloying Element Addition

Two oxide-dispersion-strengthened (ODS) steels with different compositions (14Cr-ODS and 14Cr-Zr-ODS) were investigated to reveal the influences of microalloying element addition on the microstructure and to clarify the refining mechanism of precipitated particles. TEM and HRTEM results indicated that precipitated particles in the Zr-containing ODS steel had finer sizes and dispersed more homogeneously within the grains. It was found that rhombohedral Y4Zr3O12 particles with complex lattice structures were formed and could pin the migration of the grain boundaries during heat treatment due to their high thermal stability. In addition, the Zr-containing ODS steel exhibited a finer and more uniform grain morphology. Tensile tests showed that microalloying element addition could significantly improve the comprehensive mechanical properties of 14Cr ODS steels at room temperature.

Effects of Adding Active Elements to Aluminum-Based Filler Alloys on the Bonding of 6061 Aluminum Alloy and Alumina

In this study, AA6061/AA6061 and AA6061/alumina were directly brazed with Al10.8Si10Cu, Al10Si10Cu4Ti and Al10Si10Cu4Ti0.1RE filler alloys at 530 °C for 10 min without the use of flux. The addition of titanium and rare-earth elements into Al10.8Si10Cu alloy effectively improved the bonding shear strengths of AA6061/AA6061 and AA6061/alumina joints. The highest joint shear strengths were 61.1 and 19.2 MPa, respectively. The Al10.8Si10Cu filler alloy without titanium and rare-earth elements could not wet on the alumina and caused failure of the AA6061/alumina joint. The shear strengths of the AA6061/AA6061 and AA6061/alumina joints both strongly depended on the active element addition. Due to the high chemical activity of the rare-earth elements, they formed AlLa between the Al10Si10Cu4Ti0.1RE filler alloy and alumina. The addition of rare-earth elements into Al10Si10Cu4Ti filler alloy resulted in significant enhancement of the average bond strength of AA6061/alumina joints, from 8.0 to 14.8 MPa.

Low Cycle Fatigue Properties of a Low Rare Earth-Containing Me20 Magnesium Alloy

A newly-developed rare earth-containing ME20 magnesium alloy was studied in this thesis. As a potential structural material for applications in automotive industry, low cycle fatigue properties are inevitably required. Strain-controlled low cycle fatigue tests were conducted on this wrought alloy with different specimen orientations. Microstructure, tensile behaviors and low cycle fatigue properties were studied. The effect of different specimen orientations on microstructures and mechanical properties were also discussed. Results show that rare earth element addition in this alloy helped weaken the texture and specimen orientations had little influence over microstructure and fatigue properties of this magnesium alloy.

A novel Mg-Sn-Zn-Al-Mn magnesium alloy with superior corrosion properties

The corrosion behaviors of the hot-pressed Mg-Sn-Zn-Al-Mn magnesium alloys with the addition of Al in different proportions have been investigated. Paraffin coating technique was applied to Mg powders before production. After debinding at 300 °C, the sintering process was applied at 610 °C under 50 MPa pressure for 70 min. All of the alloys were immersed in Hank’s solution for 10-days. The results indicated that the corrosion properties of the alloys were affected by the production method (hot pressing) and alloying element addition. After immersion, magnesium hydroxide (Mg(OH)2), hydroxyapatite (HA), and Mg-Al hydrotalcite structures were determined by the X-ray diffraction (XRD) analysis on the surfaces of Mg-Sn-Zn-Al-Mn alloys. The Mg-Al hydrotalcite protective layer was effective in preventing corrosion. Superior corrosion properties (weight loss: 1.2%, total volume of evolved H2 gas: 4 ml/cm2, corrosion rate: 0.39 mm/year) were obtained from TZAM5420 alloy (5 wt.%Sn, 4 wt.%Zn, 2 wt.%Al, 0.2 wt.%Mn).

Effect of Metallic or Non-Metallic Element Addition on Surface Topography and Mechanical Properties of CrN Coatings

Alteration of the phase composition of a coating and/or its surface topography can be achieved by changing the deposition technology and/or introducing additional elements into the coating. Investigation of the effect of the composition of CrN-based coatings (including AlCrN and CrON) on the microparticle height and volume, as well as the construction of correlations between the friction coefficient at the microscale and the geometry of microparticles, are the goals of this study. We use atomic force microscopy (AFM), which is the most effective method of investigation with nanometer resolution. By revealing the morphology, AFM allows one to determine the diameter of the particles, their heights and volumes and to identify different phases in the studied area by contrasted properties. The evaluation of the distribution of mechanical properties (modulus of elasticity E and microhardness H) on the surfaces of multiphase coatings with microparticles is carried out by using the nanoindentation method. It is found that the roughness decreases with an increase in the Al concentration in AlCrN. For the CrON coatings, the opposite effect is observed. Similar conclusions are valid for the size of the microparticles and their height for both types of coating.