In order to elucidate the microstructural refinement mechanism and the effect of secondary phase on the microstructural evolution of beta-type titanium alloy, severe plastic deformation was conducted on samples of a precipitation-hardened Ti-29Nb-13Ta-4.6Zr (TNTZ). Specifically, TNTZ that was precipitation-hardened through an aging treatment (TNTZAT) was subjected to high-pressure torsion (HPT) processing (TNTZ(AHPT)). The microstructure of TNTZ(AHPT), which has been evaluated as a function of the torsional rotation number, N, exhibits ultrafine elongated beta grains. The needle-like alpha precipitates in TNTZ(AT), which exhibit a diameter of approximately 12 nm, are homogeneously distributed within the beta grains. The dislocation density and subgrain diameter, estimated by X-ray line profile analysis, saturate at approximately 4.2 x 10(16) m(-2) and 12.2 nm, respectively, at N >= 10. The beta grains contain nanostructured subgrains having non-uniform morphologies surrounded by blurred and wavy boundaries. A saturated hardness distribution at approximately 450 HV indicates that microstructural homogeneity has been achieved at N >= 10. The alpha precipitates enhance the beta grain refinement and microstructural homogeneity is achieved in TNTZ(AHPT) at N >= 10, whereas this occurs at later stages (N > 20) in TNTZ which is solution-treated and therefore does not contain any alpha precipitates. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.