REFRACTORY METALS: NIOBIUM, MOLYBDENUM, TANTALUM, AND TUNGSTEN

ReviewersE. LiguoriScarrott Metallurgical

D. KautzLos Alamos National Laboratory

ContentsIntroduction 552

Fundamentals 552

Niobium andIts Alloys 557

Molybdenumand Its Alloys 559

Tantalum andIts Alloys 561

Tungsten 562

Joining of RefractoryMetals to OtherMaterials andDissimilar Metals 564

SuggestedReading List 565

CHAPTER 31

Photograph courtesy of Tosoh SMD, Inc.

AWS BRAZING HANDBOOK 551

INTRODUCTION

552 CHAPTER 31—REFRACTORY METALS: NIOBIUM, MOLYBDENUM, TANTALUM, AND TUNGSTEN AWS BRAZING HANDBOOK

The development of refractory alloys was influ-enced by the electric light and electronic tube indus-tries through their use of tungsten and molybdenum.Today, the greatest impetus for the development ofthe refractory metals niobium, molybdenum, tanta-lum, tungsten, and their alloys has come from theaerospace industry’s need for airframes, rocketmotors, and engine nozzles.

FUNDAMENTALS

Mechanical strength is of primary importance tohigh-temperature structural component applicationsmade of refractory metals. To understand the ele-ments that comprise the refractory metals and theirunique high-temperature properties, it is possible toclassify these metals according to their position inthe periodic table. It should be noted that there areother metals having high melting points. Figure 31.1indicates the region of the periodic table containingmetals having melting points approaching 3600°F(2000°C) or higher.

Only a dozen metals have melting points above3350°F (1850°C). Table 31.1 lists these metals in theorder of decreasing melting point. This group can befurther divided into two groups based on crystal

structure. Six metals—tungsten (W), tantalum (Ta),molybdenum (Mo), niobium (Nb), vanadium (V),and chromium (Cr) (i.e., the metals in Groups V andVI)—have body-centered cubic (BCC) crystal struc-ture. The remaining six elements—rhodium (Rd),osmium (Os), ruthenium (Ru), iridium (Ir), hafnium(Hf), and rhodium (Rh)—have either face-centeredcubic (FCC) or hexagonal-close-packed (HCP)structures.

The BCC metals are structurally superior for high-temperature applications as compared to other met-als with FCC or HCP structures. Since refractorymetals are chosen for structural applications forwhich strength and good mechanical properties arerequired at high temperatures, materials with HCPor FCC structures are not given further considerationin this chapter. In addition, the commercial applica-tions of these metals are limited due to their avail-ability and exceptionally high cost for engineeringfunctions.

If the two lower-temperature metals, vanadiumand chromium (the melting points of these metals arebelow 3600°F [2000°C]), are deleted, the remainingmetals to be considered are niobium, molybdenum,tantalum, and tungsten. Although these four metalshave much more in common than presented in therefractory metal classification and organizationscheme, there are differences that have a direct bear-ing on the manner in which these metals are used

REFRACTORY METALS: NIOBIUM, MOLYBDENUM, TANTALUM, AND TUNGSTEN

CHAPTER 31