Processing-microstructure-property correlation in thermo-mechanically processed Ti-6Al-4V alloys: A comparative study between conventional and novel approaches

Abstract

Ti-6Al-4V alloy offers an optimum combination of properties like strength, ductility, toughness, corrosion and fatigue resistance. Interestingly, thermomechanical processing (TMP) appears as an important technique to tailor the microstructure and therefore modifying the properties further. In the present study, two different TMP routes having minor differences in the processing conditions are used. While one of the routes is a conventional one to generate bimodal microstructure, the other one employs a novel approach with nominal variation in the parameters for the initial steps of TMP. Consequently, the as-received lamellar structure transformed into an equiaxed one, instead of a bimodal. To gain a deeper insight, the dominant mechanisms for microstructural evolution following each TMP steps are critically analyzed. Particular focus was to identify the variation in micromechanism with the alteration in the processing parameters, employed in the new approach. The corresponding effects in modifying the localized and globalized mechanical properties of the as-received and processed alloys are investigated extensively and the microstructure-property correlations are established. While the alloy in as-received condition revealed the least hardness, the equiaxed one obtained with the novel TMP approach showed the highest value, even improved from the bimodal microstructure. The role of individual constituents in affecting the hardness of the alloys are further evaluated. The research also emphasizes that the studied Ti-6Al-4V alloy in the form of different microstructures (lamellar, bimodal, equiaxed) do show indentation size effect leading to increased hardness with reduced load. The equiaxed structure is however least prone to the size effect, thereby revealing lesser variation in properties for small-scale applications. Furthermore, the equiaxed microstructure also exhibited a combination of higher global strength and ductility, as realized from tensile tests. The study is the first of its kind to highlight the scope for generating the equiaxed microstructure in Ti-6Al-4V alloy, through a novel processing route, leading to the best possible combination of mechanical properties.