Dimitrios Tsivoulas

Coherency Loss of Al3Sc Precipitates during Ageing of Dilute Al-Sc Alloys

Scandium additions are known to offer a number of benefits to aluminium alloy performance. Many of these benefits can be attributed to the precipitation of fine Al3Sc particles. These particles are fully coherent with the aluminium matrix when they are small, but can lose coherency as they grow or coarsen. In this work, the change in coherency has been studied in an Al- 0.12 wt%Sc alloy over the temperature range 300-425oC. Three coherency regimes were identified, consistent with previous observations. The time and temperature range over which coherency changes occur have been measured for a range of conditions and correlated with the precipitation kinetics and the predictions of a model for Al3Sc precipitation. The effect of the coherency change on the particle morphology has also been investigated.

Effects of Combined Zr and Mn Additions on Dispersoid Formation and Recrystallisation Behaviour of AA2198 Sheet

The present paper focuses on the influence of combined additions of Zr and Mn on the recrystallisation resistance of aluminium alloy 2198 sheet. Dual additions of these dispersoid forming elements have previously been reported to be beneficial for reducing recrystallisation during solution treatment, as they exhibit opposing microsegregation partitioning on solidification. Contrary to expectation, it was found that the addition of Mn, to a standard Zr-containing 2198 sheet material, reduced recrystallisation resistance. The reasons for this behaviour are explored by analysis of the morphology, size, chemistry, and distribution of the dispersoid families formed, as a function of the Mn and Zr level, traced back to the homogenisation stage.

Heterogeneous Nucleation of the T1 Phase on Dispersoids in Al-Cu-Li Alloys

The major strengthening phase of modern Al-Cu-Li alloys, T1, is so far known to nucleate on a number of sites in the microstructure. However, its preferential nucleation on Al20Cu2Mn3 dispersoids has never been reported with strong evidence up to now. The present work suggests that such heterogeneous nucleation is possible and performs a comparison with the precipitates distributed homogeneously in the matrix. This phenomenon is shown to promote a particle decohesive fracture mode.

Comparison of the Effect of Individual and Combined Zr and Mn Additions on the Fracture Behavior of Al-Cu-Li Alloy AA2198 Rolled Sheet

The effect of individual and combined addition of dispersoid-forming alloying elements Zr and Mn on the fracture behavior of the Al-Cu-Li alloy 2198 has been investigated by the Kahn tear test. Overall, the standard baseline 2198 alloy containing only Zr exhibited the best performance, while the alloy with the combined presence of Zr and Mn was slightly inferior. The lowest properties were seen for a Zr-free 2198-0.4Mn alloy variant. In the T351 temper fracture initiated at coarse constituent particles that formed large cavities and microvoid sheets linked the initial sites of void growth. In the Mn-containing alloys microvoids clearly nucleated at the coarser Al20Cu2Mn3 dispersoids within the microstructure, while this was not identifiable for the finer coherent Al3Zr dispersoids. However, this difference in the mechanism of cavity linkage had little effect on the overall toughness of the materials, which was more closely related to the effect of Mn and Zr on the level of recrystallization. Extended artificial aging promoted grain boundary decohesion due to the precipitation of high densities of T1 particles on GBs and favored a cleavage fracture mode. Particle decohesive fracture was also promoted by T1 precipitation on the Mn dispersoids.

Texture Formation in Flow Formed Ferritic Steel Tubes and the Influence of the Process Parameters

The crystallographic textures of flow formed parts are of great scientific interest as they result from a complex deformation mode that comprises strain components in the axial and hoop directions. In general they resemble those of the rolling type but with slight differences. The present paper analyses the effects of certain process parameters, such as roller contact angle, feed rate, and preform hardness, since these are crucial in defining the forces acting in each principal direction of the component. Additionally, the development of a texture gradient through the wall thickness is also discussed. Texture predictions from a crystal plasticity finite-element model were also employed to support the experimental data and interpret the deformation mechanisms. Finally, the diverse nature of the flow formed textures is verified by annealing treatments at 700°C, which yields the typical gamma-fibre encountered in rolled ferritic steels upon recrystallisation in conjunction with the strengthening of the (113)[1-10] component.

A high-strength silicide phase in a stainless steel alloy designed for wear-resistant applications

Hardfacing alloys provide strong, wear-resistant and corrosion-resistant coatings for extreme environments such as those within nuclear reactors. Here, we report an ultra-high-strength Fe–Cr–Ni silicide phase, named π-ferrosilicide, within a hardfacing Fe-based alloy. Electron diffraction tomography has allowed the determination of the atomic structure of this phase. Nanohardness testing indicates that the π-ferrosilicide phase is up to 2.5 times harder than the surrounding austenite and ferrite phases. The compressive strength of the π-ferrosilicide phase is exceptionally high and does not yield despite loading in excess of 1.6 GPa. Such a high-strength silicide phase could not only provide a new type of strong, wear-resistant and corrosion-resistant Fe-based coating, replacing more costly and hazardous Co-based alloys for nuclear applications, but also lead to the development of a new class of high-performance silicide-strengthened stainless steels, no longer reliant on carbon for strengthening.Wear- and corrosion-resistant hardfacing steels rely on carbon for strengthening. Here, the authors report an ultra-high strength silicide phase that could lead to a new class of silicide-strengthened stainless steels and alternative coatings for nuclear applications.