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The different shapes of titanium carbide (TiC x ) grains formed at different growth stages of self-propagating hightemperature synthesis (SHS) were obtained in the quenched sample. The shape evolution of the TiC x grains and the growth mechanism are discussed. As the highly substoichiometric TiC x nucleates at the initial stage of the SHS, the (111) surfaces become stable and the TiC x grains nucleate as octahedra. With an increase of the TiC x stoichiometry, the free energy of the (111) surfaces increases. Hence, the area of the (111) surfaces on the TiC x grains decreases gradually while the (100) surfaces are exposed. The growth shape of the TiC x grains turns to truncated-octahedron. Moreover, when the combustion temperature during the SHS exceeds a certain value (about 1800 °C), the (100) surfaces of the TiC x grains turn round and these rounded (100) regions grow and coalesce with further increasing of the TiC x stoichiometry. The growth shape of the TiC x grains then turns to close-to-sphere.

With using the carbon nano-tube (CNT) of high chemical activity, nano-TiCx particles with different growth shapes were synthesized through the self-propagating high temperature in the 80 wt.% metal (Cu, Al, and Fe)-Ti-CNT systems. The growth shapes of the TiCx particles are mainly octahedron in the Cu- and Al-Ti-CNT systems, while mainly cube- and sphere-like in the Fe-Ti-CNT system.

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The morphologies of the transition metal carbide (TMC) (ZrC x , NbC x , and TaC x ), transition metal nitride (TMN) (TiN x ), and transition metal diboride (TMD) (NbB 2x and TaB 2x ) particles formed during the selfpropagating high-temperature synthesis (SHS) were investigated. The results indicate that the ceramics with wide stoichiometric ranges all show a stoichiometry-induced morphology evolution, i.e., octahedron → truncated-octahedron → spherelike → sphere, for TMCs and TMNs, and hexagonal prism → polyhedron → spherelike, for TMDs. For TMCs and TMNs, the increase in the stoichiometry leads to the increase in the growth rate in the ⟨111⟩ crystalline direction. Hence, their morphologies show an evolution process of gradual exposure of the {100} surfaces and shrinkage of the {111} surfaces. When the exposed {100} surfaces are roughed because of the extremely high combustion temperatures during the SHS and thus turn round, the growth shapes of the TMC and TMN crystals change to spherelike. On the other hand, when the TMCs and TMNs are stoichiometric or near stoichiometric, the critical transition temperature for thermodynamic roughening of the {100} surfaces could be very high. Then, the rounded {100} will restore to the flat surfaces, and the cubic and truncatedcubic TMCs and TMNs particles are formed. For TMDs, the morphology evolution could be caused by the decrease in the stability of the {0001} and {101̅ 0} surfaces at high stoichiometries. With the increase in the stoichiometry, these two surfaces are less-exposed gradually while the {11̅ 01} surfaces are exposed and expand. The growth shapes of TMDs change from regular hexagonal prism to polyhedron. With the rounding (roughening) transition of the {11̅ 01} surfaces at high temperatures, the TMDs particles become spherical.

Nanocrystalline (NC) metals are stronger and more radiation-tolerant than their coarse-grained (CG) counterparts, but they often suffer from poor thermal stability as nanograins coarsen significantly when heated to 0.3 to 0.5 of their melting temperature (Tm). Here, we report an NC austenitic stainless steel (NC-SS) containing 1 at% lanthanum with an average grain size of 45 nm and an ultrahigh yield strength of ~2.5 GPa that exhibits exceptional thermal stability up to 1000 °C (0.75 Tm). In-situ irradiation to 40 dpa at 450 °C and ex-situ irradiation to 108 dpa at 600 °C produce neither significant grain growth nor void swelling, in contrast to significant void swelling of CG-SS at similar doses. This thermal stability is due to segregation of elemental lanthanum and (La, O, Si)-rich nanoprecipitates at grain boundaries. Microstructure dependent cluster dynamics show grain boundary sinks effectively reduce steady-state vacancy concentrations to suppress void swelling upon irradiation.