# In-Situ Nanoparticles: A New Strengthening Method for Metallic Structural Material
###### Abstract  
Over the past several years, coherent interface strengthening was proposed and has since drawn much attention. Unfortunately, many fabrication techniques are restricted to very small size. Recently, a brand new method of in-situ nanoparticle strengthening was systematically investigated, which was proved to be an efficacious way to optimize microstructure and improve mechanical property by utilizing uniformly dispersed nanoparticles. In this review, we summarized recent related advances in investigated steels and Cu alloys, including details of preparation technique and characterization of in-situ nanoparticles. In-situ nanoparticles formed in the melt possess a coherent/semi-coherent relationship with the matrix, which has a similar effect of coherent interface strengthening. In this case, bulk metallic structural materials with dispersed nanoparticles in the matrix can be fabricated through conventional casting process. The effects of in-situ nanoparticles on grain refinement, inhibiting segregation, optimizing inclusions and strengthening are also deeply discussed, which is beneficial for obtaining comprehensive mechanical response. Consequently, it is expected that in-situ nanoparticle strengthening method will become a potential future direction in industrial mass production. casting; strengthening method; in-situ nanoparticles; coherent interface; grain refinement; mechanical properties +
Footnote †: journal: Applied Physics Letters  
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[PERSON], [PERSON], X.  
as described by Hall-Petch relationship, increase in strength is proportional to the reciprocal square root of the grain size [7; 8]. In addition, ultrafine crystalline materials (grain size: 100-1000 nm) exhibit ultra-high strength at room temperature and ultra-high plasticity at high temperature [9], which can be manufactured by severe plastic deformation (SPD), such as equal channel angular pressing (ECAP) [10; 11; 12; 13; 14] and high pressure torsion (HPT) [15; 16; 17]. Regrettably, samples processed by SPD are always limited in millimeter-sized scale and cylinder or cuboid shape. As for solidification processing, it was reported that adding nucleating agents to metal melt [18] and various casting techniques [19; 20; 21], such as centrifugal casting, die-casting, squeeze-casting and ultrasonic vibration casting, have been developed in order to modify the microstructures with finer grains (about 100 um). In that case, strengthening effect is not remarkable because grain size is still large. In particular, spray forming [22; 23], a novel rapid solidification technique which has quickly developed in recent decades, can refine grain size to 15-20 um but still has limitations in size and volume [24]. Therefore, fabrication of bulk metallic structural materials with very fine grains in industrial mass production is still a problem. Second phase strengthening is an important reinforcement approach for as-cast metallic ingots [25; 26], which are generally strengthened by dispersed micron/sub-micron particles, thus to dramatically reduce the plasticity especially for ceramic-type strengthening particles in high concentration [27; 28; 29; 30; 31; 32]. According to the Orowan-Ashby model, the refinement of the second phase is conducive to both strength and plasticity for materials [33; 34; 25; 35; 36]. When the particles are refined to nanometer scale, the second phase/matrix interface turns to be coherent or semi-coherent and therefore the interaction mode between dislocations and the second phases will change [37]. In other words, in the process of particles blocking dislocation motion, some dislocations enter the interior of particles along the coherent interface, which reduces the dislocation accumulation and meanwhile improves the macroscopic strength and plasticity of alloys [38]. As a result, overall mechanical performance of metals can be improved when refining second phase to the nanometer level [33; 38; 39; 40; 41; 42].