ABSTRACT
For decades, grain boundary engineering has proven to be one of the most effective approaches for tailoring the mechanical properties of metallic materials, although there are limits to the fineness and types of microstructures achievable, due to the rapid increase in grain size once being exposed to thermal loads (low thermal stability of crystallographic boundaries). Here, we deploy a unique chemical boundary engineering (CBE) approach, augmenting the variety in available alloy design strategies, which enables us to create a material with an ultrafine hierarchically heterogeneous microstructure even after heating to high temperatures. When applied to plain steels with carbon content of only up to 0.2 weight %, this approach yields ultimate strength levels beyond 2.0 GPa in combination with good ductility (>20%). Although demonstrated here for plain carbon steels, the CBE design approach is, in principle, applicable also to other alloys.
ABSTRACT
The composition of nano-sized alloy carbides formed by interphase precipitation in V-Nb and V-Ti multiple microalloyed low-carbon steels is analyzed by using three-dimensional atom probe. Carbide-forming alloying elements including V, Nb, and Ti, are simultaneously precipitated from the early stage of isothermal treatment, whose atoms are uniformly distributed in the carbide particles, even after prolonged holding. Cluster analysis by the maximum separation method, with parameters optimized using different methods, is carried out to extract alloy carbides from matrix. The composition of alloy carbides evaluated by site fraction of substitutional carbide-forming alloying elements indicates that at the early stage of their formation, Nb and Ti are more strongly enriched than V.
ABSTRACT
The lath martensite structure contains hierarchical substructures, such as blocks, packets and prior austenite grains. Generally, high-angle grain boundaries in the lath martensite structure, i.e. block boundaries, are correlated to mechanical properties. On the other hand, low-angle grain boundaries play an important role in morphological development. However, it is difficult to understand their nature because of the difficulty associated with the characterization of the complex morphologies by two-dimensional techniques. This study aims to identify the morphologies of low-angle boundaries in ultra-low carbon lath martensite. A serial-sectioning method and electron backscatter diffraction analysis are utilized to reconstruct three-dimensional objects and analyse their grain boundaries. A packet comprizes two low-angle grain boundaries - sub-block and fine packet boundaries. Sub-blocks exhibit porous morphology, with two large sub-blocks predominantly occupying a block. Several fine packets with different habit planes from the surrounding regions are observed. Fine packets are present in blocks, which frequently share a close-packed direction with the neighbouring fine packets. In addition, fine packets are in contact with the sub-block boundaries.
ABSTRACT
In order to develop a novel alloy with a changeable Young's modulus for spinal fixation applications, we investigated the microstructures, Young's moduli, and tensile properties of metastable Ti-30Zr-(Cr, Mo) alloys subjected to solution treatment (ST) and cold rolling (CR). All the alloys comprise a ß phase and small athermal ω phase, and they exhibit low Young's moduli after ST. During CR, deformation-induced phase transformation occurs in all the alloys. The change in Young's modulus after CR is highly dependent on the type of deformation-induced phase. The increase in Young's modulus after CR is attributed to the deformation-induced ω phase on {3 3 2} mechanical twinning. Ti-30Zr-3Cr-3Mo (3Cr3Mo), which exhibits excellent tensile properties and a changeable Young's modulus, shows a smaller springback than Ti-29Nb-13Ta-4.6Zr, a ß-type titanium alloy expected to be useful in spinal fixation applications. Thus, 3Cr3Mo is a potential candidate for spinal fixation applications.
