Ding Hongsheng

Effects of ultrasonic vibration on the microstructure and mechanical properties of high alloying TiAl

To modify the microstructure and enhance performances, the ultrasonic vibration is applied in the mould casting of TiAl alloy. The effects and mechanism of ultrasonic vibration on the solidifying microstructure and mechanical properties are investigated and the model for predicting lamellar colony size is established. After ultrasonic vibration, the coarse microstructure is well modified and lamellar colony is refined from 534 μm to 56 μm. Most of precipitated phases are dissolved into the lamellar colony leading to a homogenous element distribution. The phase ratio of α2-Ti3Al and γ-TiAl is increased, and the chemical composition is promoted to more close to equilibrium level by weakening the influence of β-alloying elements. The microhardness and yield strength are gradually improved by 23.72% and 181.88% due to the fine grain strengthening, while the compressive strength is enhanced by 24.47% through solution strengthening. The critical ultrasonic intensity (Ib) for TiAl alloy is estimated at 220 W cm−2 and the model for average lamellar colony size is established as . The ultrasonic refinement efficiency exponentially increases as the ultrasonic vibration time with a theoretic limit maximum value of Elim = 88% and the dominating refinement mechanism by ultrasonic vibration is the cavitation-enhanced nucleation rather than cavitation-induced dendrite fragmentation.

Coarsening mode and microstructure evolution of Al-In hypermonotectic alloy during rapidly cooling process

Abstract A numerical model reflecting the real physical processes well is developed to predict the coarsening mode of the second phase droplets and the microstructural evolution under the common action of nucleation, diffusional growth, collision coagulation during rapid cooling Al–In hypermonotectic alloys.

EFFECT OF PROCESSING PARAMETERS ON SOLID-LIQUID INTERFACE OF Nb-Si BASE ALLOY FABRICATED BY ELECTROMAGNETIC COLD CRUCIBLE DIRECTIONAL SOLIDIFICATION

Nb-Si base alloys have attracted considerable attentions as the potential high temperature structural materials working in the service temperature range of 1200~1400 ℃ because of their high melting points(1750 ℃),moderate densities(6.6~7.2 g/cm3) and excellent high temperature strength.However,the mismatching between room temperature fracture toughness and high temperature strength has limited their practical applications.Directional solidification(DS) and alloying have been proved to be the effective methods to overcome this critical issue.The DS processes used to prepare Nb-Si base alloys included Czochralski directional solidification in a copper crucible,electron beam directional solidification,optical floating zone melting,integrally directional solidification and electromagnetic cold crucible directional solidification(ECCDS).The previous studies focused on the effect of process parameters on microstructure and mechanical properties in the steady-state growth region(SSGR).However,the microstructure in the SSGR was controlled by the solid-liquid interface,and the solid-liquid interface was controlled by process parameters.Therefore,the study about the effect of process parameters on solidliquid interface was very important.In this work,the master alloy with the nominal composition of Nb-22Ti-16Si-3Cr-3Al-2Hf(atomic fraction,%) was prepared by vaccum non-consumable arc-melting first,and then induction skull melting.The DS experiments were performed in the ECCDS device equipped with a square water cooled copper crucible(internal dimension:26 mm×26 mm×120 mm) and a Ga-In alloy pool.There were three processing parameters in ECCDS including heating power of power supply,withdrawal rate and holding time.The DS ingots were prepared according to the orthogonal test(L9(33)).Instability degree was defined as the ratio of the height of solid-liquid interface to the width of the DS ingot.The results showed that there were three macroscopic morphologies of solid-liquid interfaces;the increase of holding time,decrease of withdrawal rate and elevation of heating power were conducive to keeping the solid-liquid interface macroscopic morphology planar.With the increase of withdrawal rate,primary dendrite arm spacing(d1) and secondary dendrite arm spacing(d2) decreased gradually;with the increase of heating power,d1 and d2increased gradually;with the increase of holding time,d1 and d2increased first and then decreased.The higher withdrawal rate,lower heating power and less holding time were beneficial to refining the d1 and d2.

MICROSTRUCTURE EVOLUTION OF Ti44Al6Nb ALLOY DIRECTIONALLY SOLIDIFIED WITH COLD CRUCIBLE

Ti44A16Nb(atomic fraction,%) ingots with industry size were directionally solidified without contamination with electromagnetic cold crucible.Effects of pulling rates and powers on the surface quality,the S/L interface,grain morphology and phase selection were studied.The results show that the surface quality are improved by decreasing pulling rate or increasing power,they influence the surface quality by changing the superheat degree of the melt or the volume of the mushy zone.The S/L interface becomes concave and the grain width becomes big with increasing of pulling rate,the columnar grains grow discontinuously when the pulling rate is increased.The grain width decreases with the increase of the power.Columnar grain to equiaxed grain transition(CET) is easy to occurred when the pulling rate is lower or the power is higher.At the beginning of solidification,the initial phase is α phase when the pulling rate is 0.5 mm/min,whereas,it is β phase when the pulling rates are others.