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Powder metallurgy generally is a preferred method to consolidate and prepare some refractory metals considering their high melting temperatures. However, sintering such materials to high density while maintaining fine grain structure is challenging, and generally require ultrahigh temperature or need to be assisted by external pressure. Especially, the preparation of nanostructured oxide dispersion strengthened (ODS) refractory metals at low temperature is more difficult because grain coarsening and densification are simultaneously hindered by the common oxide addition. Thus, it is of great sense to explore a kind of oxide that can be used as both strengthening phase and sintering accelerator. In this work, after extensive theoretical calculation and experimental observation, it is found that the accelerated sintering at low temperature can be achieved in W-HfO 2 system. In the absence of external pressure or activator, this system can be rapidly densified without undesirable grain coarsening at a relative low temperature of only 1480 ˚C. Through kinetic analysis and density functional theory (DFT) calculation, it was confirmed that the intergranular HfO 2 particles provide a rapid diffusion transport pathway for W atoms in support of densification at temperature of interest. As a result, this alloy exhibits a homogeneous microstructure with ∼97% relative density and ∼200 nm grain size, which renders our alloy with an ultrahigh compressive strength of ∼3 GPa, much higher than the reported values. We believe that the accelerated sintering mechanism found in our W-HfO 2 system can be applicable to other refractory metals to prepare high-density nanocrystalline alloys with excellent properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Avoiding the formation of cracks to ensure a reliable printability and a good stability is crucial in the laser additive manufacturing of alloys. Contrary to previous studies that have generally tried to decrease the liquid film and solidification range, in this work, we innovatively utilized segregation engineering and abundant cell boundaries to introduce liquid backfilling as well as a network of segregation phases to alleviate thermal stress, consequently eliminating hot cracking. More specifically, zirconium was introduced into a nickel-based superalloy to form a continuous interdendritic liquid film during the laser additive manufacturing process. It was found that the continuous intermetallic Ni 11 Zr 9 segregation phase decorated the cell boundaries, and cracks were completely eliminated in the as-printed Haynes 230 alloys when their Zr content reached 1 wt.%. Moreover, this continuous Ni 11 Zr 9 network layer was able to act as a "skeleton" to significantly improve the yield strength of the as-printed samples. Following appropriate heat treatment, these Zr-modified Haynes 230 alloys exhibited an extraordinary combination of strength and plasticity, which were superior to those of the previously reported Haynes 230 alloy. These findings provide a new alloy design route for the laser additive manufacturing of crack-free alloys with excellent mechanical properties. [Display omitted] [ABSTRACT FROM AUTHOR]