Experimental evidence of elastic interaction between ω nanoparticles embedded in a metastable β titanium alloy

Abstract

Elastic interactions by means of correlation effects between precipitate particles often play an important role in determining the microstructure of systems in which a precipitation reaction takes place. Thus, the role of strain fields and the ensuring mechanochemical processes have historically been discussed in connection with the classic problem of precipitation and growth of misfitting metallic clusters in an anisotropic matrix [1]. However, although the case of coherent γ ′ precipitates has been intensively investigated in Ni based alloys [2–4], very little information is available in the literature concerning such mechanisms in isothermal ω phase growth kinetics in β titanium alloys. Such knowledge is of a great interest since metastable β Ti alloys can be primarily strengthened through precipitation of coherent secondary phases (ω and α), leading to extremely refined high strength microstructures [5]. The aim of this paper is to present preliminary results concerning the morphological aspect of the ω nanoparticles embedded in a BCC β matrix under the influence of coherent strain field interactions. Resistivity measurements and transmission electron microscopy (TEM) dark field imaging have been carried out and a short range elastic interaction between ω particles is proposed. The composition (by weight/%) of the titanium alloy provided by Timet was 87.2% Ti, 6.8% Mo, 4.5% Fe, 1.5% Al. In order to obtain the metastable β-Ti structure at room temperature, this alloy was heat treated (1.8 ks annealing at 1123 K) in a high vacuum induction furnace and then water quenched. The electrical resistance (R) measurements were made by a computer driven four point technique. R/Ri (Ri is the initial resistance) was plotted as a function of time and temperature. In this way, microstructural evolution and transformation kinetics can be investigated [6–8]. TEM observations were carried out on a Jeol 2000FX operating at 200 kV. From previous studies [8], ω phase has been shown to precipitate between 423 and 623 K in the Ti-6.8Mo4.5Fe-1.5Al alloy. The typical ω phase morphology is displayed by the TEM dark-field image in Fig. 1. On this micrograph, ω ellipsoidal nanoparticles are clearly observed to precipitate randomly through the β-Ti cubic matrix after a thermal treatment of 3.6 ks at 473 K. It is now reasonably well established that the ω phase can nucleate through the well-known (111)β collapsing mechanism first proposed by De Fontaine [9]. The classical orientation relationship between the two phases β and ω have been confirmed in the present TEM investigation and are consistent with the following epitaxial relationships: (111)β //(0001)ω and [110]β //[1120]ω. As a consequence, the lattice misfit between the metastable ω phase and the β matrix is expected to be strong and this lattice mismatch is accommodated by elastic displacements generating strain fields within both phases. Isothermal growth kinetics of ω phase were also investigated by resistivity measurements. Fig. 2 presents the change in resistivity of quenched Ti-6.8Mo-4.5Fe1.5Al as a function of time at three different isothermal heat treatments within the omega precipitation range (473, 508 and 523 K). The detailed growth mechanisms of ω phase have not been clarified yet [5], however, Ho et al. [7] assumed that resistivity changes could be correlated with an