Search

Abstract The effects of ω precipitates and orthorhombic (α″) martensitic transformation on stress-strain behavior were studied in metastable β Ti-10V-2Fe-3Al alloy single-crystal in submicron scales. We successfully distinguished the size effects induced by ω precipitates and α″ phase transformation through compressing the metastable β Ti-10V-2Fe-3Al along the orientation of <011> β , which is unfavorable for α″ phase transformation. Two individual exponential curves express well the relationship between the strengthening exponents, which were regressed using the flow stresses at different ranges of plastic strains of 0.2–13% and the pillar widths, and the corresponding plastic strain. It is worth noting that the strengthening exponent jumps from 0.256 to 0.293 at 8% plastic strain. Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars. The first (ω precipitate-related) size effect has been predicted with the proposed models, and the second (martensitic transformation-induced) size effect is interpreted with the modified Liu's equation. [ABSTRACT FROM AUTHOR]

The damping and transformation behaviors of Ti 50 (Pd 50− x Cr x ) shape memory alloys with x ranging from 4.0 to 5.0 are systematically investigated. The damping capacity (Q −1 ) at the martensitic transformation is found to be inversely proportional to the square root of frequency, i.e., Q −1 ∝ ω −0.5 . A relaxation peak or shoulder is observed slightly below the martensitic transformation damping peak for compositions within the compositional crossover region (4.5 ⩽ x ⩽ 4.8). Furthermore, the damping capacity at the martensitic transformation is smaller within the compositional crossover region (4.5 ⩽ x ⩽ 4.8), compared with that of compositions at both sides ( x = 4.0 and x = 5.0 ). These observations can be ascribed to the hysteretic motion of interfaces between different phases near the compositional crossover region. [ABSTRACT FROM AUTHOR]

[Display omitted] Large-scale applications of elastocaloric cooling demand bulk materials showing both large adiabatic temperature change (Δ T a d ) and low-fatigue characteristics at room temperature. Using cold-rolling and aging treatment, we synthesize a bulk Ti 49.2 Ni 40.8 Cu 10 polycrystalline alloy that has microstructural features of nanocrystallinity and epitaxially related Ti(Ni,Cu) 2 nanoprecipitates. It exhibits a large Δ T a d of 13.8 K and a coefficient of performance of 13 at room temperature. Moreover, the degradation of Δ T a d is only 0.3 K after 450 tensile cycles. We attribute the favorable properties to the enhanced reversibility of martensitic transformation during stress cycling, aided by the internal epitaxy-generated stress at the interface between the Ti(Ni,Cu) 2 nanoprecipitates and matrix, together with grain refinement. The results indicate that the alloy offers a good balance of multiple objectives, holding promise for solid-state refrigeration applications. [ABSTRACT FROM AUTHOR]

Shape memory alloys (SMAs) have attracted considerable attention for their high damping capacities. Here we investigate the damping behavior of Ti 50 (Pd 50− x D x ) SMAs (D=Fe, Co, Mn, V) by dynamic mechanical analysis. We find that these alloys show remarkably similar damping behavior. There exists a sharp damping peak associated with the B2–B19 martensitic transformation and a high damping plateau ( Q −1 ~0.02–0.05) over a wide ambient-temperature range (220–420 K) due to the hysteretic twin boundary motion. After doping hydrogen into the above alloys, a new relaxation-type damping peak appears in the martensite phase over 270–360 K. Such a peak is considered to originate from the interaction of hydrogen atoms with twin boundaries and the corresponding damping capacity ( Q −1 ~0.05–0.09) is enhanced by roughly twice that of the damping plateau for each alloy. Moreover, the relaxation peaks are at higher temperatures for the TiPd-based alloys (270–370 K) than for the TiNi-based alloys (190–260 K). We discuss the influence of hydrogen diffusion, mobility of twin boundaries and hydrogen–twin boundary interaction on the temperature range of the relaxation peak. Our results suggest that a martensite, with appropriate values for twinning shear and hydrogen doping level, provides a route towards developing high damping SMAs for applications in desired temperature ranges. [ABSTRACT FROM AUTHOR]

• Deformation kinking is found in Ti-V metastable β -Ti alloys for the first time. • Kink bands co-appear with slip bands in the deformed β -grains. • Deformation-induced ω -precipitates are produced in kink bands due to local strain. • Kink bands are suggested to follow a dislocation-based formation mechanism. [Display omitted] Unlike stress-induced martensitic transformation, deformation twinning as well as dislocation gliding, deformation kinking is an uncommon deformation mechanism in metastable β -titanium (Ti) alloys. In this study, the unique deformation mechanism was reported in Ti-V binary β -Ti alloys for the first time. It was found that a large number of kink bands were distributed in β -grains after cold forging at a strain rate of ~103 s−1. Microstructural characterization manifests that the appearance of kink bands is frequently accompanied by slip bands, whilst dense dislocations are accumulated around the kink bands on a microscopic scale. Such local plastic deformation is quite strong so that the pre-existing athermal ω -precipitates in the initial microstructure is completely destroyed within the kink bands and instead fresh deformation-induced single variant of ω -precipitates is produced. This deformation localization mainly originates from the impediment of dislocation activity caused by both pre-existing athermal ω -precipitates and the intrinsic slip retardation in BCC crystal structures. The deformation kinking is thus suggested to follow a dislocation-based formation mechanism, whilst the resulting Taylor axis of lattice rotation was deduced to be<011> β on basis of extensive crystallographic analysis. These findings enrich fundamental understanding on deformation kinking, and provide helpful information for utilizing the unique deformation mechanism to tune mechanical properties of β -Ti alloys. [ABSTRACT FROM AUTHOR]

The ω -phase transformation in metastable β -titanium (Ti) alloys has attracted great attention in the past decade due to its intrinsic complexity and modifying mechanical properties. Interfacial twin boundary (ITB) ω -phase was reported to appear along {332} β or {112} β twin boundaries in metastable β -Ti alloys. The formation of such ITB ω -phase was proposed to arise from the reverse α″ to β martensitic transformation and subsequent stress relaxation along the twin boundaries. In this study, a new formation mechanism is revealed in Ti-10wt.%Cr alloy containing ω -precipitates in the initial microstructure. It is experimentally found that the formation of ITB ω -phase is closely correlated with the favored pre-existing one ω -variant at the expense of the other three-siblings, i.e. , reorientation of ω -variants. The ω -reorientation mechanism is further rationalized by first-principles calculation in terms of the energy barrier of transformation pathway between ω -variants. These findings advance our fundamental understanding to the ω -phase transformation and further plastic deformation behavior of Ti alloys. Unlabelled Image • A new formation mechanism of interfacial twin boundary (ITB) ω -phase has been revealed in metastable β -Ti alloys. • The formation of ITB ω -phase is experimentally ascribed to the reorientation of ω -variants. • The ω -reorientation mechanism is further rationalized in terms of the encountered energy barrier by first-principles calculation. [ABSTRACT FROM AUTHOR]