The Effect of Growth Rate on the Microstructure and Tensile Behaviour of Directionally Solidified Ti-44Al-9Nb-1Cr-0.2W-0.2Y Alloys

Zhu-Hang Jiang,Ning Li,He-Xin Zhang,Cheng-Zhi Zhao

doi:10.3390/met8070535

Abstract

Ti-44Al-9Nb-1Cr-0.2W-0.2Y alloys were directionally solidified (DS) at different growth rates varying from 10 to 20 μm/s using a modified liquid metal cooling (LMC) method. The results show that an increase in the growth rate leads to both a decrease in the size of the columnar grains in the directional solidification stable growth zone and a deterioration of the preferred orientation of the α2(Ti3Al)/γ(TiAl) lamellar structure in the columnar grains. The growth direction of the primary dendrite in the quenching zone gradually deflected along the axial direction as the growth rate increased. At the same time, the morphology changed from dendrite to a cystiform dendritic structure, with considerable B2 phase segregation in the dendritic core. Correspondingly, the tensile properties of the alloy decreased at 800 °C with a gradual increase in the cleavage fracture area. These findings show that the low growth rate is beneficial for the preferred orientation and the mechanical properties of the alloy. The content of the B2 phase and the change in the lamellar orientation are the main limiting factors for the tensile properties of the materials at high temperatures.

Highlights

TiAl base alloys containing a high Nb content have attracted extensive attention for potential applications in aero engines due to their low density, high specific modulus and creep resistance, as well as their excellent high-temperature properties [1,2,3,4,5]
During the directional solidification process, an increase in the growth rate can refine the size of the columnar grain and grain boundaries, which causes the columnar crystal to be more inclined to axial growth but will reduce the preferred orientation of the columnar lamellar layer and the uniformity of its size and composition
The main factor affecting the tensile properties of the directionally solidified (DS) Ti-44Al-9Nb-1Cr-0.2W-0.2Y alloy is the angle between the lamellar orientation and the tensile direction

Summary

Introduction

TiAl base alloys containing a high Nb content have attracted extensive attention for potential applications in aero engines due to their low density, high specific modulus and creep resistance, as well as their excellent high-temperature properties [1,2,3,4,5]. Amongst the various microstructures of TiAl alloys, the most typical is the fully laminated dual-phased structure that consists of γ-TiAl and α2 -Ti3 Al. Fully lamellar TiAl alloys with uniform composition and fine grains have good rupture toughness and strength both at room and at elevated temperatures. The poor room-temperature ductility, high-temperature resistance and fracture toughness still restrict the applications of these alloys in aerial materials and turbine engines [6,7]. A directional solidification technique can improve the room-temperature ductility of TiAl by obtaining a lamellar orientation that is parallel to the direction of the macroscopic stress or inclined at an angle of less than 45◦ [8,9]

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