Effect of various combinations of Ti and Zr interlayers on the tensile properties of laser welded joints of molybdenum

Highlights

• Zr is present as Mo₂Zr playing the role of second - phase strengthening in FZ of Mo joints.

• Ti appears in the atomic state in FZ of Mo joints with a solid-solution strengthening effect.

• The strength, hardness, and ductility of Mo joints can be controlled by different addition strategies of Ti and Zr.

• Zr is superior to Ti in improvement of the tensile strength of joints.

• Ti is superior to Zr in improvement of the ductility of FZ in Mo joints.

Abstract

Molybdenum (Mo) is of great potential in the nuclear energy field, however, the embrittlement and the lower tensile strength limit the application of the fusion welded joints. Titanium (Ti) and zirconium (Zr) were synchronously added into the fusion zone (FZ) to improve the tensile strength and certain ductility of laser beam welding joints. By utilizing high-resolution scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and nanoindentation, the microstructures and mechanical properties of FZs in the joints under different alloying strategies with Ti and Zr were analyzed. Zr is mainly present in Mo as Mo₂Zr, playing the second-phase strengthening effect; Ti is mainly present in Mo in an atomic state, playing a role of the solid - solution strengthening. The addition of Ti and Zr with different mass proportions can control the relative contents of Mo₂Zr phase and solid - solution atoms in the FZ, control the distribution of zigzag and flat grain boundaries and influence the relative distributions of low - angle grain boundaries (LAGBs) and high - angle grain boundaries (HAGBs). In the end, combined addition of Ti and Zr is possible to improve the tensile strength while decrease the loss of ductility of the joint.

Introduction

The demands for the next generation novel nuclear fuel (accident-tolerant fuel (ATF)) and high - efficient heat exchangers for nuclear power make the focus to molybdenum (Mo) and Mo alloy. The study of El-Genk et al. [1] shows that Mo and Mo alloy are the ideal materials for manufacturing the above components due to their excellent high - temperature performance and high thermal conductivity; however, the poor weldability is the greatest obstacle to the engineering application of Mo and Mo alloy. Fukuhisa et al. [2] [3] studied systematically the mechanical properties, the number of pores and the crack tendency of the joint after electron beam welding and GTA welding of pure molybdenum and TZM alloys and found cracks and pores are likely to occur in the fusion zone (FZ) and tensile strength and ductility of the welding joints were very low. How to improve the strength and eliminate the weld cracks has become a priority problem.

The key to increasing the strength and eliminate the cracks is to reduce the brittleness of the grain boundary of Mo joints. According to the formation causes, the brittleness of the grain boundary of Mo can be divided into the intrinsic brittleness and extrinsic brittleness. Brosse et al. [4] studied the segregation and strengthening or embrittling effects of 29 metallic dopants at the ∑5(310) tilt and ∑5(100) twist grain boundaries using density functional theory (DFT) calculations. His results showed that the intrinsic brittleness is induced by the configuration of unsaturated electrons in the outermost layer of Mo atoms while the study of Kumar et al. [5] shows that extrinsic brittleness is caused by impurity elements oxygen (O) and nitrogen (N) distributed at the grain boundary. The intrinsic brittleness at the grain boundary can be decreased by adding some solid-solution elements (such as carbon (C), boron (B), rhenium (Re), titanium (Ti) and niobium (Nb)) into Mo. Mo and solid - solution atoms can share some electrons in the second outermost layer or the outermost layer to reduce the unsaturation of electrons in these layers, thus further decreasing the intrinsic brittleness of the grain boundary [6]. The extrinsic brittleness of the grain boundary is lowered by reducing the contents of free O and N at the grain boundary. For example, Noda et al. [7] obtained some samples with different O content by controlling the heating time in an oxygen atmosphere and measured the O content at the grain boundary surface by AES. His results showed that reducing the O content at the grain boundary could significantly improve the strength and ductile - brittle transition temperature; Zhang et al. [8,9] found Ti, Zr added in the fusion zone of laser welding joint of Mo could react with O and the content of O was decreased by adding active elements. The grain boundary is also strengthened by adding the second phases or second phase forming elements. For example, Liu et al. [10] found the La₂O₃ nano - particles could improve the strength of grain boundaries significantly. Among the above methods, the method to select Ti and zirconium (Zr) as alloying elements to strengthen Mo is the commonest and most mature.

The addition of both Ti and Zr elements can effectively improve the mechanical properties of Mo. The strengthening mechanisms of the two elements are basically consistent: (a) solid-solution strengthening; (b) reducing the O content at the grain boundary; and (c) second - phase strengthening. According to the equilibrium phase diagrams of Mo – Ti [11] and Mo – Zr [12], it can be found that Ti and Zr are reacted with Mo to separately form the solid solutions Mo (Ti) and Mo (Zr). The difference lies in that the atomic radius of Ti is lower than that of Zr. Therefore, if the content of solid - solution atoms are the same, Zr could cause the greater lattice distortion and thereby have a better strengthening effect, correspondingly; moreover, Lee et al. [13] comparing the experimental results of bending and compression of MoTi alloy containing 0.1 wt% Ti, 0.3 Wt% Ti and 1.5 wt% Ti respectively found that Ti could decrease the ductile - brittle transition temperature (DBTT) and further enhance the ductility of Mo. However, Mo₂Zr could be generated by the eutectic reaction when the Zr content is higher than the maximum solid solution limit (10 wt%) in Mo. A small number of Mo₂Zr phases can play the role of the second-phase strengthening when they are dispersedly precipitated at the grain boundary [14]; however, Zhuang et al. [15] found when the content of Zr was high, the Mo₂Zr one laves phase would precipitate in network at the grain boundary of Mo. According to the study of Zhang et al. [16], the network Mo₂Zr could decrease the strength and increase the brittleness of the grain boundary relative to the dispersedly distributed. Despite this, the mechanical performance of joints containing Mo₂Zr is still superior to that of the pure Mo joint. Ti and Zr added into the FZ are apt to undergo the oxidation reaction with O and N to generate the oxides or nitrides. As a result, the contents of O and N decreasing, thus weakening the effect of MoO₂ and excessive Mo₂N on the strength of the grain boundary. Zhang et al. [9] found the Zr added in the FZ of joints could reacted with O to form ZrO₂ then decreasing the content of O. And Ti added into the FZ of joints could reacted with O to form TiO₂ then decreasing the content of O [8].

Through the above analysis, it is found that both Ti and Zr show their own obvious advantages and disadvantages in improving the mechanical properties of Mo. The addition of Ti into Mo can realize the solid-solution strengthening effect to reduce the O and N contents at the grain boundary, decrease the DBTT and further increase the ductility of Mo; however, its strengthening effect is limited according to the mechanism of the solid-solution strengthening. Zr can be reacted with Mo to form Mo₂Zr phases. A small number of Mo₂Zr can obviously enhance the joint strength. However, the addition of Zr can also greatly rise the DBTT, thus strengthening the brittleness of Mo. Thus, when alloying the Mo joint during laser welding, Ti and Zr elements are added into the welded seam simultaneously, expecting to reduce the brittleness of the joint while improving the joint strength.