A general model for the crystal structure of orthorhombic martensite in Ti alloys

Abstract

The present work develops a novel unified approach to describe the crystal structure of orthorhombic martensite (α′′) in Ti alloys independent of chemical composition. By employing a straightforward yet highly instructive solid sphere model for the basic tetrahedral structural unit the crystal structures involved in the β ↔ α′′/α′ martensitic transformation are categorized into several intermediate configurations. Importantly, a new metric is introduced, δ, which unambiguously characterizes the atomic positions inside the orthorhombic unit cell depending on unit-cell geometry. Furthermore, the exclusive use of relative quantities to describe unit-cell geometry and atom positions renders the approach developed herein independent of alloy content. In this way, shortcomings of commonly suggested structural metrics for α′′ are eliminated. Subsequently, the novel methodology is applied to analyse and compare the crystal structure of α′′ across a broad range of Ti alloys based on experimentally measured unit-cell parameters. From this analysis it emerges that a large fraction of structural configurations along the b.c.c.–Cmcm–h.c.p. transformation path is not observed in quenched alloys. The threshold between the not-observed and the remaining well observed configurations is identified with an ideal Cmcm crystal structure, relative to which the experimentally found α′′ is compressed along its c axis.

Kinetics of Martensite Decomposition and Microstructure Stability of Ti-6246 during Rapid Heating to Service Temperatures

“…Despite further developments in fast in situ XRD analyses to monitor heat treatment processes, merely a few small-scale studies were conducted on Ti-6246 in the past and thus little knowledge was gained in this regard. Moreover, the majority of the studies on martensitic Ti-phases are presently performed on highly β-stabilized alloys such as binary Ti-Nb or Ti-Mo [13][14][15][16][17]. When more complex alloying systems are investigated, the workhorse Ti-6Al-4V [18,19] regularly comes into play as well as further Ti-Al-V alloys with only a slight addition of Mo, Sn [20] and highly stabilized Ti-Nb-Sn [21] among others.…”

“…Bönisch et al [13][14][15] set their focus on the distinction of diffusive and diffusionless transformations in the decomposition path of α -martensite and were able to give specific data on the anisotropy of thermal expansion. Demakov et al [16] expanded these investigations and invented a novel model for the unique characterization of martensite crystal structure and facilitated the inter-alloy comparison. Comprehensive investigations for Ti-6246 focusing on the stability and the decomposition of martensite during heating to temperatures corresponding to possible service temperatures in the upper application regime (~550-650 • C) and elevated temperatures approaching the M S (~840-880 • C) are lacking.…”

“…Within the present study, these techniques were used for the data acquisition for Ti-6264. Regarding the analysis of martensite transformation, kinetics and crystal structure characterization, some studies served as guides [13,15,16]. Concerning data analysis and interpretation, the latter study by Demakov et al [16] expanded the standardized determination of the lattice parameter development with a new crystal-structure-based model.…”

“…Regarding the analysis of martensite transformation, kinetics and crystal structure characterization, some studies served as guides [13,15,16]. Concerning data analysis and interpretation, the latter study by Demakov et al [16] expanded the standardized determination of the lattice parameter development with a new crystal-structure-based model. This model allows the unique identification of the transformation status along the b.c.c.-Cmcm-h.c.p.…”

“…This model allows the unique identification of the transformation status along the b.c.c.-Cmcm-h.c.p. pathway by breaking down the whole crystal structure to an atom configuration of three atoms in layer A and one atom in layer B, which form a tetrahedron [16]. This procedure was firstly transferred from highly β-stabilized titanium alloys to Ti-6246 in order to give information about whether the martensite decomposition paths are comparable.…”