What is laves phase in Stainless steel and Inconel 625?

Table of Contents

What is Laves Phases?

The Laves phase forms in the inter-dendritic region of a cast or weld structure during solidification. The existence of the Laves phase in the microstructure of Ni alloys has been associated with brittleness and hot cracking in Ni alloys. The inter-dendritic regions of the Laves phase create a favorable site for crack initiation and propagation, which affects the tensile and rupture properties, the ductility, the fracture toughness, and the fatigue life of the welded components. However, the formation and existence of the Laves phase in the microstructure is of concern mainly with more alloyed and age-hardenable alloys like 718.

The segregation of some elements, mainly Nb, Si and Mo in this last-to-solidify interdendritic eutectic melt, has a strong effect on the formation of Laves. The more the Nb segregates in the interdendritic region, thus depleting the Nb in the matrix, the higher is the volume fraction of laves phase.
Laves is a hexagonal closely packed phase and is generally accepted to be in the form of A2B, with Ni, Fe and Cr in the A position and Nb, Mo and Si in the B position. This unique chemical composition of the Laves phase, as mentioned above, distinguishes it from the other phases present in the Ni-Cr-Mo-Nb system (especially in terms of high Nb weight concentration). The formation of the Laves phase requires an Nb concentration ranging from 10–30%.
Consequently, the more the Nb segregates in the inter-dentritic region and thus depletes the Nb in the matrix, the higher is the volume fraction of laves phases. Laves phase depletes the matrix of significant amounts of useful strengthening alloying elements, making the matrix material softer.

How to remove laves Phases?

Once it has been formed during the solidification of the weld metal, the Laves phase is difficult to remove by homogenization treatments. In order to dissolve the Laves phase, very high homogenisation temperatures are needed. However, the required homogenization temperature depends on the extent or degree of segregation produced in the weld. Slower cooling and longer solidification time leads to more segregated phase structure than to rapid cooling and fast solidification.

Effect of Laves Phases in Inconel 718

Another effect of the Laves phase on alloy 718 is the precipitation of δ phase (delta) around the partially dissolved Laves particles during solution treatment. The δ phase requires about 6–8% Nb and precipitates in the range of 860–995 °C. The Delta phase cannot form during weld metal cooling because the time spent in the δ precipitation range is very brief. During postweld solution treatment in the δ precipitation range, as in the present case at 980 °C, δ
precipitation occurs as in the form of δ needles in these regions of moderate Nb concentration around the Laves particles [81]. A large amount of Laves phase, and consequently δ needles, after heat treatment has been found to be connected to poor fatigue performance of alloy 718 welds.

Recent Posts