Laser powder b e d fusion of copper matrix iron particle reinforced nanocomposite with high strength and high conductivity

Liquid-liquid phase separation,and the resulted solute segregation,during conventional solidification have been a long-term challenge to produce copper(Cu)-iron(Fe)immiscible composites with high strength and high conductivity.The present work reports an effective solution to this issue through laser powder bed fusion(L-PBF)in-situ alloying of Cu-8 wt.%Fe.Microstructure observation showed that the fast cooling within micron-scale melt pools fully eliminated the Fe segregation and therefore the L-PBF fabricated nanocomposite achieved the homogeneous microstructure,which featured equiaxed fine grains around 1μm in size.Ageing of the nanocomposite at 600℃for 1 h enabled precipitation of two types of nanoparticles.One is coarser Fe nanoprecipitates with body-centered cubic(BCC)structure and diam-eter of 100-300 nm,mainly distributing along grain boundaries.The other is smaller Fe nanoprecipitates with face-centered cubic(FCC)structure and diameter of 10-35 nm,being observed within the grains and having coherent interfaces with the Cu matrix.As a result,the aged Cu-Fe nanocomposite achieved tensile strength of 462.9±6.6 MPa with 30.4%±1.7%elongation to failure and 74.5%IACS(International Annealed Copper Standard)electrical conductivity.The formation mechanisms of the nanoprecipitates and the strengthening mechanisms of the nanocomposite are discussed.

Phase/grain boundary assisted-3D static globularization mechanism of TC17 alloy based on the microstructure reconstruction and in-situ TEM observation

Lamellar globularization in the dual-phase titanium alloy is the key to improving plasticity and strength.However,the mechanism has not been fully elucidated so far.In this work,the role of phase/grain bound-ary in the static globularization of TC17 alloy was systematically studied by setting differentαphase con-tent before annealing through low-and high-temperature deformation.Isothermal compression causes the parallel distribution and fragmentation of 3Dαplates and few globularαparticles are formed at a strain rate of 1 s^(-1).Post-deformation annealing promotes the static globularization ofαphase while it is affected by initialαphase content.After 730°C deformation,the development ofα/αinterface by absorbing dislocations promotes the formation of globularαgrains based on the nucleation of sepa-ratedαparticles and pre-recoveryαsubgrain during subsequent annealing.Theα/α/βandα/β/βtriple junctions formed due to highαcontent with about 36%volume fraction are favorable for the further nucleation and growth of globularαgrains by reducing interface energy,forming a 3D irregularαplate.Then nucleation and growth of theβphase dominate the microstructure evolution during subsequent an-nealing,resulting in the local dissolution of the plate and formation ofαrods.After 850°C deformation,theαphase tends to nucleate at theβ/β/βtriple junctions and grow into a lamellar shape along the high energyβ/βgrain boundary due to lowαcontent with about 7%volume fraction.Theαnucleation that maintains the Burgers orientation relationship(BOR)with the surroundingβphase grows along the habit plane and thickens slowly,resulting in the formation of a precipitatedαplate with a flat surface and the suppression of static globularization.The comprehensive investigation of lamellar globularization provides guidance for optimizing the 3D microstructure and properties of dual-phase titanium alloy.

Effect of processing parameters on the densification of an additively manufactured 2024 Al alloy

Understanding the densification behaviours and formation mechanisms of defects are essential to fabricate high quality and high strength aluminium components using selective laser melting(SLM) technology. In this work, the effects of laser power and scanning speed on the densification, defects evolution and their formation mechanisms in a SLMed 2024 aluminium(Al) alloy were investigated in consideration of the corresponding laser energy input, melting mode transition and microstructural evolution. The results showed that optimizing the processing parameters effectively reduced the porosity level below1% by avoiding the lack of fusion and keyhole melting mode, and minimizing the gas pores. However,optimization of the processing parameters could not eliminate the columnar structure associated with the SLMed 2024 Al alloy, which contributed to the hot-tearing cracks in the SLMed parts. It was found that the dependence of porosity formation on SLM processing parameters was contrary to the crack density. Hence, to further improve the SLM-processability of the 2024 Al alloy it is necessary to develop SLM methods in order avoid the hot-cracking within the optimized processing parameter window associated with the minimum porosity formation.

Heterogeneous lamella design to tune the mechanical behaviour of a new cost-effective compositionally complicated alloy

A heterogeneous lamella(HL)design strategy was applied to manipulate mechanical properties of a new cost-effective Fe_(35)Ni_(35)Cr_(25)Mo_(5)compositionally complicated alloy(CCA).The HL structure was produced by single-step heat treatment(800℃for 1 h)after cold rolling.This HL structure consists of alternative lamellae regions of coarse-grained FCC matrix(5-20μm),and regions containing ultra-fine grains or subgrains(200-500 nm)together with nanoprecipitates(20-500 nm)and annealing twins.As compared with other cost-effective CCAs,the 800℃annealed sample with HL structure demonstrated a comparable tensile property,with yield strength over 1.0 GPa and total elongation of~13%.Formation of the annealing twins and nanoprecipitates decorated HL structure was a result of the concurrent partial recrystallization and precipitation ofσphase at the shear bands with a high density of lattice defects(e.g.high-density dislocation walls and deformation twins).The latter restricted the growth of recrystallized grains,leading to the formation of ultrafine subgrains within the HL structure.The high yield strength resulted from the multistage hetero-deformation induced(HDI)strengthening and precipitation strengthening associated with heterogeneous lamella structures containing nanoprecipitates.The ductility was originated from the coexistence of multiple deformation mechanisms,which started with dislocation slip and formation of stacking faults at the initial stage,followed by nano-twinning at the higher strain level.This HL design strategy,comprising composition and thermomechanical process designs,and the resultant microstructure tuning,open a broader window for the development of cost-effective CCAs with enhanced performance.