Microstructure,cracking behavior and control of Al-Fe-V-Si alloy produced by selective laser melting

Selective laser melting(SLM)technology based on atomized powder was used to fabricate Al-8.5Fe-1.3V-1.7Si(wt%)alloy parts.The microstructure and crack characterization of SLM samples fabricated at various conditions were presented.Results show that the cracks appear periodically along the building direction,initiate preferably at the outer edges of the as-built samples and propagated along the remelting border zone(RBZ)into deposited layers.Solid-phase cracking is proposed according to the fracture morphology.The thermal-induced residual stress during SLM combined with the precipitation of relatively large-sized Al_mFe phase in the RBZ results in the formation of cracks.Enhancing scanning speed and hatch distance enable to reduce the cracking sensitivity.The crack-free Al-8.5Fe-1.3V-1.7Si parts can be fabricated at optimized parameters of laser power of 320 W,scanning speed of 1000 mm·s^(-1)and hatch distance of0.10 mm along with proper laser pre-heating procedure.The samples built horizontally show good ultimate tensile properties of 454 MPa in average with the elongation of7.2%.

Selective Laser Melting of an Al–Fe–V–Si Alloy:Microstructural Evolution and Thermal Stability

Selective laser melting was used to produce an aluminum alloy Al-8.5Fe-1.3V-1.7Si（wt%）. The effects of heat treatment on microstructure evolution and phase stability during long-term thermal exposure of the deposits were investigated. Results show that the microquasi-crystalline phase, Al12（Fe,V）3Si and AlmF e metastable phases coexisted with α-Al in the as-produced alloy. Annealing at 400 ℃ resulted in decomposition of microquasi-crystalline phase and supersaturated α-Al into Al12（Fe, V）3Si phase in the fusion zone, accompanied by the decrease in alloy hardness. The activation energy of this decomposition process was 115 k J/mol. A more homogenous microstructure was obtained after annealing at 400 °C for 60 min,which was resistant to coarsening exposed at 425 °C up to 500 h. The Al12（Fe,V）3Si and AlmF e phases were coarsened at 475 and 525℃ with increasing the exposure time. Coarsening of Al12（Fe,V）3Si phase was attributed to a combination of volume diffusion and grain boundary diffusion mechanism of Fe. Heat treatment at 600℃ resulted in accelerated microstructure coarsening and formation of large-sized equilibrium phases, which signi？cantly degraded the room temperature microhardness.

Microstructural evolution of Ti–47Al–2Cr–2Nb–0.8B alloy prepared by semi-solid process

The refinement of large boride ribbons in the as-cast TiAl alloy is the essential issue for aerospace industry application,which is difficult to avoid by classical casting techniques.The present paper seeks to explore the possibility of the semi-solid process in manufacturing Ti-47Al-2Cr-2Nb-0.8 B(at%)alloy.An important result is that,except forming a nondendritic globular structure,the semi-solid process also plays a crucial role in refining large borides for the TiAl alloys with boron.In the current alloy,the long ribbon borides can be successfully converted to fine,equiaxed particles with an average size of 2-5μm.Furthermore,subsequent proper heat treatment is necessary to control and achieve a fine,nearly fully lamellar microstructure.The key microstructure-refinement mechanism is due to the fine borides obtained during semisolid process,which act as the effective nucleation centers for the formation of interdendritic fine y grains,providing the pinning effect on priorα-grain boundaries.These results prove that semi-solid process can be an ideal candidate for the microstructure refinement in TiAl alloys.

Microstructural characterization in 7 at% Al-containing NiTi-based alloys

The microstructural evolution of Ni–42Ti–7Al and Ni–41Ti–7Al alloys as a function of solution and aging heat treatment was investigated using transmission electron microscopy（TEM）, electron probe, and X-ray diffraction（XRD）. The results reveal that the volume fraction of Ti2 Ni phase as well as its composition does not change significantly after as-solution treated at 1200 °C and aged at 850 °C. At the early stage of the aging treatment at 850 °C for 1 h, the cuboidal β＇ precipitate keeps coherency with the matrix; further aging, β＇ precipitate coarsens, and the semicoherency between the β/β＇ two phases are observed.The shape of coarsened β＇ precipitates changes to the globule, and the interface dislocations are introduced accompanied by the occurrence of semicoherent precipitates. Under the same heat treatment, compared to the Ni–42Ti–7Al alloy, the lattice misfits of the Ni–41Ti–7Al alloy between the β and β＇ two phases are larger, so the β＇ precipitates in Ni–41Ti–7Al alloy are coarsened severely and easily lose coherency with the matrix. The thermal stability of Ni–41Ti–7Al alloy is much worse when aging at 850 °C.

Evolution of carbides on surface of carburized MSONiL bearing steel

The dissolution and precipitation behaviors of the carbides in carburized M50NiL steel were derived from different solution and tempering treatments.Totally four kinds of carbides of (V,Cr)-rich MC,(Mo,Fe)-rich M2C,Fe-rich M3C and (Fe,Cr)-rich M7C3 were obtained from the carburized M50NiL steel after different heat treatments.The key carbides of carbufized M50NiL steel were proved to be tough V-rich MC and Cr-rich M7C3.The highest hardness (634 HV)and the optimal surface structure with 1.0% volume fraction of uniformly distributed MC carbides were obtained after the carbufized M50NiL steel was solution-treated at 1150 ℃ and tempered at 500 ℃.The quantitative statistics show that 63%of the MC carbides were less than 200 nm under that heat treatment.The variety of carbides changed with solution and tempering conditions.When the solution temperature increased from 1050 to 1150 ℃,the undissolved carbides were proved to be Fe-rich M7C3,Mo-fich MC and (Mo,Fe)-fich M2C.Besides,the equivalent content of V-rich MC was found increased when the tempering temperature changed from 500 to 550 ℃.The combination of high-temperature solution and low-temperature tempering is recommendable heat treatment for the high hardness as well as the tiny and uniformly distributed carbides.

Halysis Høeg,1932—An ancestral tabulate coral from the Ordos Basin,North China

The problematic calcareous microfossil Halysis is abundant in the Middle Ordovician Darriwilian Stage of the western edge of the Ordos Basin,North China.The rich and well-preserved specimens of Halysis in this area facilitate detailed studies for its skeletal construction and tube microstructure.Halysis differs from calcified cyanobacteria and calcareous red and green algae in morphology,skeletal construction and microstructure,as well as reproduction mode.Halysis typically consists of multiple juxtaposed parallel tubes arranged in sheets(‘multiple-tube'type)or is just composed of one tube(‘single-tube'type).In‘multiple-tube'Halysis,tube fission by bifurcation results from the insertion of a microcrystalline wall at the center of a mother tube.This study demonstrates for the first time that the tube walls of Halysis have a laminofibrous(fibronormal)microstructure,composed of fibrous calcite perpendicular to wall surface,and recognizes the‘single-tube'type Halysis composed of one tube;in addition,for the first time,this study finds out that‘multiple-tube'Halysis develops buddings from the conjunction of two tubes and‘single-tube'Halysis shows wide-angle Y-shaped branchings.Based on these findings,this study further compares Halysis with tabulate corals.Halysis appears stratigraphically earlier than Catenipora and Aulopora,and has a smaller tube size.‘Multiple-tube'Halysis resembles Catenipora and‘single-tube'Halysis resembles Aulopora in skeletal construction and microstructure,and in their tube walls of laminofibrous microstructure composed of fibrous calcite perpendicular to the tube wall surface.Catenipora and Halysis are both characterized by the absence of septal spines.The similarities suggest that Halysis may be the ancestor of Catenipora-like and Aulopora-like tabulate corals.

Microstructural refinement and enhanced mechanical properties of suction-cast NiTi-Al alloy for structural use

A significant refinement of the microstructure of structural NiTi alloy was achieved through a combination of Al addition and fabrication by suction casting.This is attributed to the change from a layer-by-layer to volume solidification mode with Al content increasing,which causes a change in grain shape from columnar to equiaxed.Furthermore,when compared with conventional casting,suction casting optimizes the microstructure of the alloy by refining the grain size and ensuring a homogeneous distribution of Ti2 Ni and Ni_(2)TiAl phases.The high cooling rate of suction casting also delays the formation ofβ'-Ni_(2)TiAl phase.Finally,this fine microstructure and uniform distribution of intermetallic phases result in the ultimate compressive strength of up to 2463.7 MPa,representing an improvement of 370 MPa over conventionally cast NiTi-Al alloy.