Volume 55, Issue 5

As-cast ZK40 alloys, modified with the addition of CaO, Gd, Nd and Y were investigated. Solution heat treatments were performed based on differential thermal analysis results. The unmodified ZK40 alloy exhibited microstructure with nearly no intermetallic compound but with precipitates formed during the solution treatment. The modified ZK40 alloys exhibited a semi-dissolved network of intermetallic compounds along the grain boundaries and zones of intermetallic compounds within the grains. Interestingly, no precipitates were observed immediately next to the grain boundaries. The energy dispersive X-ray spectroscopy line scans showed an enrichment of Zn and Zr in the regions where the precipitates are found, suggesting that they are Zn-Zr precipitates. The ageing behaviour was compared between the as-cast and the solution treated materials and it was found that apart from the ZK40-Gd, ZK40-Nd and ZK40-Y aged at 200 °C after solution treatment, there is no notable ageing response for the investigated alloys.

The microstructure and mechanical properties of dissimilar butt-joints of high-strength low-alloy steel to austenitic high Mn TRIP steels produced by manual TIG welding with AISI 309L filler metal were investigated. The heat input was varied by changing the welding current between 40 and 70 A. A fully austenitic microstructure with nearly equiaxed grains and without any iron/manganese carbide is observed in the high Mn TRIP base metal, whereas polygonal ferrite and pearlite colonies oriented along the rolling direction are found in the high-strength low-alloy steel base metal. Coarse and refined heat-affected zones (HAZ) were observed in the high-strength low-alloy steel, whereas no significant HAZ was detected at the TRIP side. Bainite and/or martensite grew in the coarse HAZ of the high-strength low-alloy steel steel joined with highest current. The weld seam was formed by austenitic dendrites with interdendritic ferrite and martensite. The best welding condition was achieved with an intermediate current of 50 A, where the hardness transition was smooth, fracture occurred in the HSLA base metal and the joint ductility was higher.

The interest for functionally graded materials (FGM) has grown up during the last 2 decades owing to the possibility of developing pieces and devices with gradual variation of properties along one or more dimensions. This allows for an optimum performance as a function of the intended use. On the other hand, two stage sintering is a relative new sintering technique, where the shaped pieces are heated up to a relative density of 75–92 % of theoretical density, followed by a fast cooling step to an inferior temperature, where the piece in maintained up to the end of the densification process. This results in the development of finer microstructures, responsible for better mechanical properties. In this work, we characterized alumina-ytria-stabilized zirconia composites, with different amounts of 3Y-ZrO2 by dilatometry. After fitting thermal behavior of the different composites during sintering and determining temperature of maximum densification rate, co-pressed FGMs were produced. Then the samples were sintered by pressureless traditional technique and two stage sintering, with a peak temperature of 1450 °C and holding temperature of 1350 °C. Microstructure was studied by SEM and mechanical properties (microhardness and indentation toughness) were determined in order to compare pressureless sintered FGMs at 1500 °C and two-stage sintered FGMs.

Duplex stainless steels have very good properties including good ductility, moderate toughness and good stress corrosion cracking resistance. These properties can be severely affected by the precipitation of sigma (σ) and chi (χ) phases, in particular ductility and toughness. The effect of cold deformation on the isothermal transformation kinetics of UNS S32205 steel was investigated. The steel was cold rolled up to 64 % reduction in thickness and annealed at 850 °C from 10 min up to 4320 min (3 days). Microstructural characterization used light optical and scanning electron microscopy, energy dispersive spectroscopy, and magnetic measurements. Formation of intermetallics is much faster in the cold-rolled samples. After 4320 min, samples show the same amounts of each phase for all rolling strains. Results have been validated by thermodynamic calculations.