Microstructural and Corrosion Properties of Ti-to-Zr Dissimilar Alloy Joints Brazed with a Zr-Ti-Cu-Ni Amorphous Filler Alloy

Si-Young Lee,Hyun-Jun Lee,Sung Soo Park,Jung Gu Lee,Jong-Hee Baek

doi:10.3390/met11020192

Abstract

Titanium and Zircaloy-4 dissimilar alloys were brazed with a zirconium-titanium-copper-nickel amorphous filler alloy, and the resulting joint structures as well as their corrosion properties were examined. The microstructure of the brazed joints was investigated according to brazing holding time at 850 °C, and the corrosion property was analyzed by potentiodynamic polarization. During brazing, joints were produced by diffusion-induced isothermal solidification of the molten filler alloy. At a relatively brief brazing holding time of 5 min, a large segregation zone consisting of an active α-phase and a nobler intermetallic phase was generated in the joint center, which suffered from micro-galvanic corrosion. The presence of alloyed titanium deteriorated the nobility of the α-zirconium phase near the joint and induced galvanic coupling with cathodic base metals, resulting in massive localized corrosion. This localized corrosion caused the pitting behavior at the applied potential of −51.1~187.5 mV during anodic polarization. With a brazing holding time of 20 min, the concentration of the alloying elements was homogenized to eliminate the electrochemical potential difference and minimize the galvanic corrosion susceptibility of the joint region. This homogeneous joint resulted in a highly passive corrosion behavior comparable to that of the titanium base metal.

Highlights

Titanium (Ti) is the material of choice for applications involving corrosive environments, such as power generation and chemical processing, due to its high specific strength and outstanding resistance to corrosion [1,2]
Four distinctive regions were produced in the joint brazed for 5 min, as shown in Figure 1a, in which ZA indicates the acicular zone near the Zircaloy-4 base metal, C the central solution zone, S the segregation zone, and TA the acicular zone near the Ti base metal
A Zr-Ti-Cu-Ni amorphous alloy was proposed as a low-temperature brazing filler alloy for Ti-to-Zircaloy-4 dissimilar alloy joining, and its brazing characteristics were examined with respect to the produced joint structures and their corrosion properties

Summary

Introduction

Titanium (Ti) is the material of choice for applications involving corrosive environments, such as power generation and chemical processing, due to its high specific strength and outstanding resistance to corrosion [1,2]. To braze Ti and Zr, Zr-rich amorphous alloys have been widely investigated as promising fillers that contain Ti, with copper (Cu), nickel (Ni), iron (Fe), and/or aluminum (Al) added as a melting-point depressant [9,10]. These alloys are considered the best choices for high-temperature and corrosive environments, compared with other types of brazing filler, such as Al- and silver-based alloys [10,11]. Based on their chemical compatibility with Ti and Zr, investigations have been conducted on the brazing of Ti alloys for aerospace and chemical plant applications [10,12], and more recently, the brazing of Zr alloys in nuclear applications [13,14]

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