Study on primary carbides precipitation in H13 tool steel regarding cooling rate during solidification

This study aims to investigate the primary carbides precipitation in H13 steel solidified at relatively high cooling rates,ranging from 300 to 6,000℃·min^-1,based on in situ observations with a high temperature confocal laser scanning microscope.In the cooling rate range investigated,the solidification microstructure becomes more refined as cooling rate increases and the relationship between the secondary dendrite arm spacing(SDAS),λ2,and cooling rate,.T,can be expressed asλ2=128.45.T-0.124.Regardless of cooling rates,two kinds of primary carbides,i.e.,the Mo-Cr-rich and V-rich carbides,are precipitated along the interdendritic region and most of them are the Mo-Cr-rich carbides.The morphology of Mo-Cr-rich carbide is not obviously influenced by the cooling rate,but that of V-rich carbide is obviously affected.The increasing cooling rate markedly refines the primary carbides and reduces their volume fractions,but their precipitations cannot be inhibited even when the cooling rate is increased to 6,000℃·min^-1.Besides,the segregation ratios(SRs)of the carbides forming elements are not obviously affected by the cooling rate.However,compared with the conventionally cast ingot,the SDAS and primary carbides in the steel solidified at the investigated cooling rates are much finer,morphologies of the carbides have changed significantly,and SRs of the carbides forming elements are markedly greater.The variation of primary carbide characteristics with cooling rate is mainly due to the change in SDAS.

Effect of Zr on solidification and microstructure of a Ni-based superalloy with high Al and Ti contents

The total content of Al and Ti in advanced Ni-based wrought superalloys is up to 7.5wt.%,which makes it easier to form harmful nonequilibrium eutectic(γ+γ′)andηphase.It has been reported that the addition of certain amount of Zr can modify precipitation of the nonequilibrium phases obviously,but the mechanism is still controversial.The effect of Zr ranging from<0.0006wt.%to 0.150wt.%on solidification behavior,segregation and microstructure of a Ni-based superalloy with high Al and Ti contents was investigated,eliminating the interferences of C and B.Results show that increase in Zr content significantly promotes the formation of eutectic(γ+γ′),ηand Zr-rich phase in the interdendritic region.Besides the Zr-rich phase,Zr dissolves slightly in the eutecticγ′and obviously in theηphase.An interesting phenomenon is discovered that the Zr addition significantly increases the area fraction of liquid pools and enlarges the forming range ofγdendrites,which suggests that Zr markedly retards the solidification.Zr affects the eutectic(γ+γ′)andηformation mainly due to the retard of solidification and dissolution of Zr in them.The retard of solidification obviously increases the residual liquid fraction and undercooling.Zr can serve as a forming element for the eutectic(γ+γ′)andηphase,and the obvious dissolution of Zr inηphase significantly decreases the critical concentration of Ti for its precipitation.

Microstructure and hot ductility behavior of Ni-based superalloy U720Li with boron addition

High-alloyed superalloy U720 Li is difficult to be processed through the conventional cast and wrought route.Boron is an important strengthening element for superalloys,but the role of boron in the hot ductility of U720 Li alloy is not clear.This study is focused on the effect of boron addition,in the range of<0.0005 wt%-0.0240 wt%,on the microstructure and hot ductility behavior of superalloy U720 Li.It is found that boron addition significantly increases the content of eutectic(γ+γ’)and greatly affects the degree of continuity of the grain boundary eutectic(γ+γ’)precipitation.Boron addition dramatically worsens the 1100℃ tensile ductility of conventional cast U720 Li alloy and causes a change in tensile fracture mode from transgranular to intergranular.The eutectic(γ+γ’)represents the most effective site for crack initiation.Boron deteriorates the hot ductility mainly by promoting the continuous precipitation of eutectic(γ+γ’)along the grain boundary.

Transformation Mechanism of (γ+γ') and the Effect of Cooling Rate on the Final Solidification of U720Li Alloy

The transformation mechanism of （γ ＋γ′） was studied by analyzing the microstructure and elemental distri- bution of the U720Li samples heated at 1250℃ and cooled at the rates in the range of 1-100℃/s. Although the （γ ＋γ′） is deemed to be formed by a eutectic reaction and has been called eutectic （γ ＋γ′）, it was found in the present study that the （γ ＋γ′） precipitation begins with a peritectic reaction of （L ＋ γ）γ′, and develops by the eutectic reaction of L （γ ＋γ′）. The energy for the γ′ nucleation is low because the interfacial energy for the γ /γ′ interface is about one-tenth of the solid/liquid interface, and hence, the nucleation rate is high and the fine structure of （γ ＋ γ′） is formed at the initial precipitation stage. The γ and γ′ in （γ ＋ γ′） tend to grow into a lamellar structure because it is difficult for them to nucleate directly from the residual liquids, and hence, the γ′ precipitates naturally tend to grow divergently direction of the regions rich in AI and Ti, forming a fan-like structure of the （γ ＋γ′）. As a result, the γ′ precipitates will coarsen finally because the space between them is enlarged. The solidification of the final residual liquids is a diffusion dependent process. When cooled at a higher rate, a higher degree of super cooling is reached and finally the solidification is finished by the pseudo- eutectic reaction of L → （γ ＋ boride） and L→ （γ ＋γ′）, which can absorb Zr and B. When cooled at a rate low enough, most of the residual liquids are consumed by the （γ ＋γ′） growth due to the sufficient diffusion, and the boride and Zr- bearing phase are precipitated at a quasi-equilibrium state. Under this condition, Ti is depleted at the （γ ＋γ′） growth front. However, the η-Ni3Ti phase is formed there occasionally due to the boride precipitation, because the compositions of the two phases are complementary.

The Minor Precipitation at the Final Stage of U720Li Solidification

The minor precipitations caused by B and Zr which are the normal constituents of U720 Li alloy have been studied by analyzing the solidification process and the composition evolution. The present study aims to supply the elementary information about the existing form of B and Zr in the as-cast microstructure, which is helpful for the subsequent processing, such as homogenization treatment. The M_3B_2 and Ni_5Zr phases were observed in the U720 Li alloy in as-cast state, which were usually accompanying with each other together with g-Ni_3 Ti phase at the edge of eutectic（γ＋γ＇）. Combining the DTA analysis and heating and quenching tests, the solidification sequence was determined to be the following： c matrix, eutectic（γ＋γ＇）, g-Ni_3Ti, M_3B_2 and Ni_5Zr. The in situ composition analysis by EDS and EPMA revealed that the precipitation and microstructure were governed by the composition evolution in the liquids. The solidification of c matrix increased the Ti concentration in the residual liquids and resulted in the eutectic（γ＋γ＇） formation; the（γ＋γ＇） formation increased the Ti/Al radio in the liquids and the g-Ni_3Ti was formed in front of the eutectic（γ＋γ＇）; the g-Ni_3Ti precipitation consumed up Al and Ti and increased the concentration of B, Mo and Cr, and M_3B_2 boride is formed;the previous precipitation of the phases consumed up most of the elements other than Ni and Zr, and Ni_5Zr is formed finally. The melting points are in the ranges of 1130–1140 °C for Ni_5Zr phase, 1180–1190 °C for M_3B_2 boride and1190–1200 °C for g-Ni_3Ti phase.