Improving Mechanical Properties of cp Titanium by Heat Treatment Optimization

Abstract

The yield stress and the ductility are very important mechanical quantities for materials selection. The paper deals with the question: how far is it possible to increase the yield stress without a significant loss of ductility by optimizing the final heat treatment in the elaboration stage. Commercially pure titanium sheets are subjected to different thermo-mechanical treatments to produce seven metallurgical states. The textures and the microstructures of the samples are studied by Electron Back Scattering Diffraction measurements, the mechanical behavior by tensile testing along the previous rolling, and the transverse directions of the sheet. The obtained microstructures display different grain sizes and varying fractions of recrystallized grains, together with slightly dissimilar textures. The yield stress increases with the decreasing grain size and obeys the classical Hall-Petch law. The grain size reduction results in a small decrease of ductility for extension along the rolling direction when the recrystallized volume fraction is higher than 80%. For extension along the transverse direction, however, the homogeneous deformation strongly decreases as soon as the material contains a small fraction of non-recrystallized grains. A good compromise between high yield stress and ductility is identified in a metallurgical state close to the end of primary recrystallization. This material state insures a relatively small grain size with all grains being in a recrystallized state.

Combined Effects of Texture and Grain Size Distribution on the Tensile Behavior of α-Titanium

“…The experimental yield stress increases with decreasing mean grain size and follows rather well a Hall–Petch relationship ( Figure 7 ), i.e., a scaling , as it was already shown on a larger data set of cp Ti specimens that also included partially recrystallized microstructures [ 31 ]. The calculated yield stresses follow a pretty correct trend considering the unlikely ranking of measured yield stresses between samples RR 4.8 and RR 9.8 ( Figure 6 and Figure 7 ).…”

“…Because of the presence of the Hall–Petch type term in the expression of (Equation ( 10 ), the values of and had to be modified to retrieve CRSSs with about the same level as in [ 19 ]. The value of the resolved Hall–Petch type slope corresponds approximately to the macroscopic Hall–Petch slope determined in [ 31 ] divided by the Taylor factor. The value of was adjusted so that the product is about the former value of in [ 19 ] for a mean grain size of 10 µm.…”

“…The distribution of the slip activity is mainly influenced by the texture, the initial values of the CRSSs and the intergranular stresses that arise from strain incompatibilities. For tensile tests along the rolling direction, it is known that, as a result of the texture, a vast majority of the grains have large macroscopic Schmid factors for prismatic, pyramidal <a> and 1st order pyramidal <c+a> glide and low ones for basal slip [ 17 , 31 ]. On the contrary, along the transverse direction, there is no big difference of Schmid factors among the four main slip families, 1st order pyramidal <a> and <c+a> being nevertheless the most favorably oriented ones [ 17 , 31 ].…”

“…For tensile tests along the rolling direction, it is known that, as a result of the texture, a vast majority of the grains have large macroscopic Schmid factors for prismatic, pyramidal <a> and 1st order pyramidal <c+a> glide and low ones for basal slip [ 17 , 31 ]. On the contrary, along the transverse direction, there is no big difference of Schmid factors among the four main slip families, 1st order pyramidal <a> and <c+a> being nevertheless the most favorably oriented ones [ 17 , 31 ]. As a consequence, for deformation simulation along the last rolling direction, prismatic slip strongly dominates at the very beginning of deformation because of its low CRSS value.…”

“…The pole figures correspond to well known textures for this material [ 17 , 26 , 27 , 28 , 29 , 30 ] where basal planes are tilted from the normal toward the transverse direction. When the grains are small, which means that the annealing was stopped close to the end of the primary recrystallization, the texture resembles the deformation one whereas the grain growth texture is obtained after heat treatments made at higher temperatures (see [ 31 ] for further information). The data set of the grains (grain diameters in addition to volume fractions and orientations) are the microstructural information used further in the simulations.…”