Mechanical anisotropy and deep drawing behaviour of AZ31 and ZE10 magnesium alloy sheets

Current issues in magnesium sheet alloys: Where do we go from here?

Mg alloys are among the lightest alloys but they are usually weak. Here we report a new mechanism to make them ultrastrong while maintaining good ductility. Stacking faults with nanoscale spacing were introduced into a Mg–8.5Gd–2.3Y–1.8Ag–0.4Zr (wt%) alloy by conventional hot rolling, which produced a yield strength of ∼575 MPa, an ultimate strength of ∼600 MPa, and a uniform elongation of ∼5.2 %. Low stacking fault (SF) energy enabled the introduction of a high density of SFs, which impeded dislocation slip and promoted dislocation accumulation. These findings provide guidance for developing Mg alloys with superior mechanical properties.

Ultrastrong Mg Alloy via Nano-spaced Stacking Faults

The effects of non-metallic inclusions in nucleating acicular ferrite in steels during cooling from a weld or cooling from an austenitic temperature are reviewed. The influence of the acicular ferrite (AF) structure on mechanical properties of steels such as strength and toughness is briefly mentioned. The different factors affecting the formation of acicular ferrite, such as the soluble content of alloying elements in steel, cooling rate from austenitizing temperature, austenite grain size and inclusion characteristics in steel, are discussed. The mechanisms of acicular ferrite formation on non-metallic inclusions, such as reduction of interfacial energy, mismatch strain between the inclusion and ferrite/austenite, thermal strains at the inclusions and changes in matrix composition near the inclusions are also discussed. Finally, the effects of inclusion characteristics, such as size, number and composition are described and their effectiveness in nucleating acicular ferrite is discussed.

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On the Role of Non-metallic Inclusions in the Nucleation of Acicular Ferrite in Steels

Improvement of formability of Mg–Al–Zn alloy sheet at low temperatures using differential speed rolling

Microstructure and mechanical properties of Mg–Al–Zn alloy sheets severely deformed by asymmetrical rolling

Although it is well known that there is a significant contribution of precipitation strengthening to the strength of structural steels, it is difficult to experimentally separate the precipitation contribution from the overall strength due to the complex nature of steel microstructures. In this study, the contribution of precipitation strengthening to the matrix strength of ferrite in steel was experimentally investigated through nanoindentation tests. Nanoindentation hardness values of the ferrite matrix in precipitates-containing steels were greater than those in precipitate-free steels, which may be due to the Orowan strengthening mechanism based on the interaction between precipitates and dislocations. To better understand the precipitation strengthening contribution, interparticle spacing values of precipitates were calculated for each microalloyed steel and used for the analysis of hardness change. It was revealed that the trend of nanoindentation hardness change with variation in interparticle spacing corroborated the expectation based on classic Orowan theory.

Orowan strengthening effect on the nanoindentation hardness of the ferrite matrix in microalloyed steels

A model to predict the austenite grain size in a Ti-microalloyed steel weld heat affected zone (HAZ) was developed. Grain boundary mobility for the austenite grain growth was expressed as a function of aging temperature and alloying elements. By analyzing isothermal austenite grain growth behavior, the Zener coefficient of cubic TiN particle was measured. From quantification of the effect of grain boundary pinning by TiN particle and alloying elements on the grain boundary mobility, an isothermal grain growth model of Ti-microalloyed steel is presented. The predicted austenite grain sizes from the proposed model were in agreement with the experimental results. Finally, combining with the additivity rule, a general austenite grain growth model during the continuous welding thermal cycle was developed.

Prediction for the austenite grain size in the presence of growing particles in the weld HAZ of Ti-microalloyed steel

Coarsening kinetics of TiN particle in a low alloyed steel in weld HAZ: Considering critical particle size

Submerged arc welding was performed using metal-cored wires and fluxes with different compositions. The effects of wire/flux combination on the chemical composition, tensile strength, and impact toughness of the weld metal were investigated and interpreted in terms of element transfer between the slag and the weld metal, i.e., Δ quantity. Both carbon and manganese show negative Δ quantity in most combinations, indicating the transfer of the elements from the weld metal to the slag during welding. The amount of transfer, however, is different depending on the flux composition. More basic fluxes yield less negative Δ C and Δ Mn through the reduction of oxygen content in the weld metal and presumably higher Mn activity in the slag, respectively. The transfer of silicon, however, is influenced by Al2O3, TiO2 and ZrO2 contents in the flux. Δ Si becomes less negative and reaches a positive value of 0.044 as the oxides contents increase. This is because Al, Ti, and Zr could replace Si in the SiO2 network, leaving more Si free to transfer from the slag to the weld metal. Accordingly, the Pcm index of weld metals calculated from chemical compositions varies from 0.153 to 0.196 depending on the wire/flux combination, and it almost has a linear relationship with the tensile strength of the weld metal.

Jongbong Lee

Papers

Microstructure and mechanical properties of Mg–Al–Zn alloy sheets severely deformed by asymmetrical rolling

Orowan strengthening effect on the nanoindentation hardness of the ferrite matrix in microalloyed steels

Prediction for the austenite grain size in the presence of growing particles in the weld HAZ of Ti-microalloyed steel

Coarsening kinetics of TiN particle in a low alloyed steel in weld HAZ: Considering critical particle size

Effects of flux composition on the element transfer and mechanical properties of weld metal in submerged arc welding