Grain boundaries (GB) and phase boundaries (PB) are planar discontinuities in metal crystals that effectively regulate the strength and toughness of polycrystalline alloys, which are often two key properties that are mutually exclusive. As a result, GB engineering, such as adjusting the number or arrangement of GBs/PBs, is widely used to design super-strong, super-tough lightweight alloys, such as titanium (Ti) alloys. However, the fineness and type of microstructure that can be achieved with titanium alloys is limited, as their grains quickly coarse once exposed to thermal loads. Therefore, the high fluidity of these crystal interfaces with relatively high GB energy limits the further improvement of GB-related performance. High-strength duplex titanium alloys strengthened by densely dispersed α nanoprecipitation face a strength-toughness trade-off, as the strain (geometric) incompatibility of semi-coherent α/β PB in the form of dislocation stacking leads to severe fracture stress concentrations, reducing the ductility and toughness of the material. Strain localization typically occurs in titanium alloys as soon as the moving dislocations pass through high-energy α/β PBs (dislocation channels are formed), accompanied by strain softening behavior. Therefore, it is a great challenge to design the microstructure, especially PB, to improve the strength and toughness of duplex titanium alloys at the same time.
Based on the engineering of variability, scholars from Xi'an Jiaotong University constructed a layered and orderly coherent interface through densely dispersed nanoparticles in ductile titanium-chromium-zirconium-aluminum alloy with ultra-high specific strength and super fracture toughness to optimize strength-toughness. The study reveals that these ordered coherent interfaces are both obstacles and sources of dislocations, and that a sustainable self-hardening deformation mechanism is formed through layered nanodiamond-dislocation interactions, so as to achieve the ultra-high strength and toughness of titanium alloys. The nanocorundum in this study is thermally stable at high temperatures below 400 °C, and at temperatures above 400°C, the transition from toughness to brittleness caused by tempering occurs due to the decomposition of the layered ordered nanoemery and the spheroidization of the previously β lamellae. The design strategy of hierarchical ordered coherent interfaces enables our cost-effective nanodiamond titanium alloys to obtain an unprecedented combination of strength, ductility and toughness, providing a new approach for the microstructure design of high-strength and ductile structural materials with excellent fracture resistance. The work was published in Acta Materialia in a research article titled "Hierarchically ordered coherent interfaces-driven ultrahigh specific-strength and toughness in a nano-martensite titanium alloy".
Paper Links:
Hatps://doi.org/10.1016/j.aktamat.2023.119540
In order to obtain super-strong and super-tough titanium alloys, this study is based on the "D-Electron Theory", which adjusts the stability of the β matrix according to the content of the β stabilizer Cr to create a self-assembled ordered nanoemery with a layered coherent α′/β interface. This composition is designed to ensure that hard dislocation structures α′ nanosensitizers with a thickness of λ < 50 nm can be generated after a simple water quenching process (WQ), rather than soft α′′ phases. In addition to reducing the alloy cost, Al, Cr, and Zr elements are selected as alloying elements mainly to improve the strength and corrosion resistance of Ti alloys, and to have good thermal stability. After careful adjustment of the alloy composition to a chromium content of 1.8 to 3.8 wt.%, a low-cost, ductile α′ nanomartensitic Ti-2.8Cr-4.5Zr-5.2Al (wt.%) alloy with ultra-high specific strength and superior toughness was obtained.
Figure 1. Thermomechanical machining scheme of Ti-2.8Cr-4.5Zr-5.2Al alloy with schematic diagram showing microstructure evolution. Transmission electron microscopy (TEM) images of Ti-2.8Cr-4.5Zr-2.5Al alloys showing microstructural characteristics at different stages.
Figure 2. Microstructure characteristics of WQ Ti-2.8Cr-4.5Zr-5.2Al alloy. (a1) Dark-field transmission electron microscopy (DF-TEM) images and schematics showing a β-transus microstructure consisting of β and α′ sheets. The schematic diagram shows the microstructure of the hierarchical ordered α′-martensite structure. (a2) The corresponding selection electron (SAED) pattern shows three martensitic variants. (a3-a4) low-magnification HAADF-STEM and corresponding IFFT micrographs show no additional planes associated with dislocation dislocations at α′/β PB, as well as IFFT micrographs using (011)β and (0110)α' diffraction points. The illustration in (a3) is the corresponding FFT pattern. (a5) High-magnification HAADF-STEM micrographs showing the traditional "platform-ledge" interface structure, as shown by the white dotted line. APT characterization of elemental assignment and composition of (b1-b2) α′ and β nanosheet layers. (c) Statistical distribution of α′ and β sheet thicknesses in WQ samples. (d) The relationship between the yield strength and α′ thickness of Ti-Cr-Zr-Al alloys and other reported martensitic Ti alloys, including Ti-4Mo, Ti-5Al-3Mo-1.5V, TC4 (original β grains), Ti-V-(Al,Sn) series, Ti-V-Sn series, TC4 traditional processes (α+α′ and full α′), TC4 SLM (α′), TC4 EBM (α′) and TC4 SLM (β + α′).
Figure 3. Microstructure characteristics of 400AC Ti-2.8Cr-4.5Zr-5.2Al alloy. (a1) BF-TEM shows αp and α′dec phases. (a2) SAED graphs of the β-transus microstructure showing three martensitic variants. (a3) HR-TEM image showing β nanoparticles inside the α′ martensite. (a4) Statistical distribution of β layer thickness in a 400AC sample. APT analysis of elemental partition and composition of 400AC samples (b1-b2) revealed calcium-rich β nanoparticles in α′ dislocation martensite, as indicated by the black arrows.
Figure 4. Microstructure characteristics of 500AC Ti-2.8Cr-4.5Zr-5.2Al alloy. (a) The DF-TEM image shows a large number of secondary α, indicated by orange arrows. (b1-b2) BF-TEM and corresponding DF-TEM images showing newly formed αs and β nanoparticles in the decomposed α′ sheet. The corresponding chromium element EDS (inset) is also provided. (c1) The BF-TEM image shows the β-cross-sectional microstructure after tempering at 500°C. (c2) The corresponding DF-TEM image shows the previous β splitting β 2D plates and nanoscale β particles. (c3) The corresponding EDS map shows the distribution of Ti, Al, Cr, and Zr and the diluted Cr region indicated by the white arrows. (c4-c5) shows the EDS line analysis of the marked line in (c1). (c6) Statistical distribution of β particle diameters in a 500AC sample. (c7) The HR-TEM image shows a transition region with significant lattice strain. (c8) The HR-TEM images show the emergence of a new αs phase in the transition region and some mismatch dislocations at the interface between αs/β and BOR.
Figure 5. Microstructure characteristics of 700AC Ti-2.8Cr-4.5Zr-2.5Al alloy. (a) HAADF-STEM image showing β and αs nanosheets. (b) The EDS plot shows the distribution of Cr, Al, Ti, and Zr. (c) Plots of Cr, Al, Ti, and Zr concentration profiles obtained by TEM-EDS along the white line line scan in a. Statistical distribution of αs and β lamellae thicknesses in (d-e) 700AC samples.
Figure 6. Room temperature mechanical properties of Ti-2.8Cr-4.5Zr-5.2Al alloy in this study. (a) The engineering stress-strain curves of the titanium alloy in this study in different states. (b) Comparison of the yield strength to the total elongation of this titanium alloy and the comparison of the yield strength to the total elongation of other martensitic α' and tempered martensitic α′ titanium alloys reported to date. Martensitic α′ Ti alloys: including Ti-4Mo, Ti-5Al-3Mo-1.5V, Ti-V-(Al,Sn) series, Ti-V-Sn series, Ti-4.5Al-(2.0-2.5)Fe-0.25Si, TC4 (different β grain sizes) and TC4-SLM (α′) series, tempered martensitic α′ Ti alloys (including partially decomposed α′ phase or fully decomposed α′ phase): TC4-SEBM, TC4-MA, TC4-Conventional Process, TC4-STA, TC4-SLM (α+β), TC4-SLM (Flake α+β), TC4-EBM (α′+β tempered) and TC4-EBM and TC4-EDE, TC4-SLM (as HIP′ed). (c) Comparison of the specific yield strength and raw material cost of the titanium alloy in this study with other reported α′/β titanium alloys.
Figure 7. Fracture characteristics of titanium-2.8Cr-4.5Zr-5.2Al alloy. (a) The true stress-strain curve of the WQ sample shows the calculated work at break. (b) Comparison of the work at break and yield strength of the titanium alloy in this study, and the comparison of the work at break and yield strength of the degradable titanium alloy that has been reported. (c) J-integral fracture resistance curves of current Ti-Cr-Zr-Al alloys under different heat treatment conditions. The J-R curve is measured from a unilateral bending specimen. (d) The alloys studied in this study are compared with other typical α+β Ti alloys (including TC4 series: (TC4-ELI and TC4-F) and TC4-DT, TC4-xFe (x = 0.1, 0.3, 0.5, 0.7, 0.9), TC4 (Widmanstätten, equiaxed and bimodal structures) and Ti-Al series: (Ti-6Al-2Mo-2Cr, Ti-4.5Al-3V-2Mo-2Fe, Ti-4.5Al-3V-2Mo-2Fe, Ti-7Al-4Mo, Ti-6Al-6V-2Sn, Ti-6Al-2Zr-2Sn-2Mo-1.5Cr-2Nb, Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.25Si and Ti-5Al-2.5Fe), and transferable β Ti alloys, including Ti-10V-2Fe-3Al, Ti-5Al-4Zr-8Mo-7V, Ti-5Al-5Mo-5V-3Cr、Ti-15Mo-2.7Nb-3Al-0.2Si、Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe、（Ti-6Al-2Sn-3Mo-1Cr-2Zr-2Nb、Ti-5Al-5Mo-5V-1Cr-1Fe、 Ti-15V-3Cr-3Sn-3Al and Ti-3Al-5Mo-5V-2Cr) and (Ti-5Al-4Mo-4Cr-2Sn-2Zr, Ti-6Al-2Sn-4Zr-6Mo and Ti-2.5Al-12V-2Sn-6Zr)
Figure 8. Fracture characteristics of Ti-2.8Cr-4.5Zr-5.2Al alloy in different states. (a1) Scanning electron micrograph of the fractured surface of a WQ sample. The illustration in (a1) is a magnified image of a fractured surface with a large number of depressions. (a2) SEM image of the fracture surface showing voids at αp/β and α′/β PB. (a3) SEM image of the fracture surface showing crack propagation/deflection along αp/β and α′/β PB. (b1) SEM micrograph of the fracture surface of a 400AC sample. The illustration in (b1) is a magnified image of a fractured surface, showing a large number of depressions. (b2) SEM images of the fractured surface showing voids at αp/β and α′dec/β PB. (b3) SEM images showing crack propagation/deformation along αp/β and α′dec/β PB. (c1) SEM micrograph of the fracture surface of a 500AC sample. The inset in (C1) is a magnified image of the fractured surface, showing transcrystalline crack-like features. (c2) SEM images show that the macroscopic cleavage plane is actually composed of discontinuous microcavities (orange arrows) and very fine grooves (white dotted lines). (c3) The corresponding subsurfaces with voids at αp/β and α′dec/β PB show that the traces of plastic deformation (void propagation or crack deflection) are not discernible even at a fairly close proximity to the voids.
Figure 9. Deformed substructure of WQ and 500AC Ti-2.8Cr-4.5Zr-5.2Al alloys. (a1-a4) WQ sample with 3.2 % deformation. (a1-a2) Two-beam conditional analysis of dislocations in a WQ sample. Dislocations are emitted by a α′/β PB along a substrate (0002) slip system within a α′ lamella, indicated by a yellow dash. A further zoomed-in micrograph of the square frame area (a1) shows a slip transport event for the PB, highlighted by a pink arrow. (a3-a4) HR-TEM and corresponding IFFT images show several RIDs at α′/β PB, verifying the dislocation transmission activity, with the RID marked with the yellow symbol "⊥". The (b1-b3)WQ sample was deformed by 6.1%. (b1) The TEM images show high dislocation density and dislocation interactions (yellow arrows) inside the α′ lamellae. In addition to the (0002) slip system, many differential rows are nucleated from coherent PBs, as indicated by the orange arrows. b1) is the corresponding selective electron diffraction (SAED) plot. (b2-b3)HR-TEM and corresponding IFFT images show more RIDs on PB. (c1-c2) Two-beam conditional analysis of dislocations in a 500AC sample at breaking strain. Only the slip zone along the plane (0002) is activated.
In this study, an ultra-high specific strength and large ductility Ti-2.8Cr-4.5Zr-5.2Al alloy with high-density ordered coherent α′/β PBs was successfully designed by martensitic transformation. The high-density nanomartensitic thickness is only ∼22 nm at its thinnest point and is stable at 400 °C, which makes this cost-effective titanium alloy with ultra-high strength and toughness with excellent fracture toughness. The layered ordered coherent PB strategy designed through nanomartensitic engineering not only overcomes the shortcomings of the low density of micron-scale PB in traditional titanium alloys, but also provides sufficient dislocation sources and obstacles for the sustainability and self-hardening ability of the alloy, thus achieving the combination of ultra-high strength and toughness. In addition, this study anticipates that this design strategy will be applicable to other metabolizable alloys, such as conventional steels and emerging multi-component alloys, to achieve ultra-high strength, high ductility, and superior toughness. (Text: SSC)
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