Determination of the Effect of Hf Additions on Phase Stability in Nb-Silicide Based In-Situ Composites

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40a/o Si S30p,,Nb(Hf)5SI3

Nb3S P, ' Nb

/'f(b),i 04'/"f2Si

"'

or • . ,

1

51

filNb,HfdSi),quares aassw(in Nb

b

10

40

30

20

70 80

60

50

Hf

go

Figure 1(a). Schematic diagram showing the projection of the liquidus surface.

The projection shows the peritectic ridges, Pld P2 P3 s the eutectic a valleys, el, e2c e, and the invariant reactions, Up, U2 U3. dThe actual compositions investigated are also shown (infilled squares). N5+H2+H5 . %, .M= a(Hf,Nb,Si)

40

PZM= P(Nb'Hf'Si)

NS

Pp

/r

..

/ t•_/7NS•"204 N5 =Nb(f~si3/'O.-'-'.'.-.-'.•/ •n/•., '

/ ,,P/ ,," •.--" Nb 10

30

40

__H3__Hf3Si2 H2"J H3

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•.



so

60

H2 =Hf(Nb)2SI

Hf2Si

H2+H5 /I

,,"OM ++H2

4"-i

20

H2+H3+H5 N,

N5+H5 /1

, ,o 70

","•H2+oxM+IpM

H5 = Hf(Nb)5Si3

\\, o.-.Hf 80

90

am

Figure ml(b). Isothermal section of the Nb-Hf-Si phase diagram at 1500mC. Bulk compositions are shown as solid bullets. EMPA measurements of phase compositions are shown as open circles.

The aim of the present paper is to describe liquidus surface and the isothermal section at

1500hC of the metal-rich end of the Nb-Hf-Si phase diagram. The alloy compositions that were inves-

tigated are also shown in Figure 1. EXPERIMENTAL

The samples were prepared using cold crucible directional solidification [2, 3] after triple melting the starting charges from high purity elements (>99.99%). The directional solidification procedure has been described in more detail previously [3]. Table 1 shows the range of compositions that was

investigated. Samples for heat treatment were wrapped in niobium foil and heat treated at 1500'C for 100 hours. All of the samples were examined using scanning electron microscopy (back scatter electron (BSE) imaging) and energy dispersive spectrometry (EDS). Electron beam microprobe analysis (EMPA) [10], x-ray diffraction (XRD), and automated electron back scattering pattern analysis (EBSP) [8], were also performed on selected samples. RESULTS AND DISCUSSION In this section liquid-solid phase equilibria will be described first, and solid-solid phase equilibria second. Table 1 summarizes the phases that were identified. KK6.8.2

Table 1. Compositions and constituent phases of the range of alloys that were investigated. Phases After 1500WC H.T.

Composition

Phases in As-DS Condition

Regime I

Nb-15%Hf-16%Si

Primary P3(Nb,Hf,Si) dendrites with

P(Nb,Hf,Si), Nb(Hf),Si 3,

Regime 2

Nb-10%Hf-16%Si Nb-5%Hf-19%Si

Nb(Hf) 3 Si-P(Nb,HfSi) eutectic Primary Nb(Hf) 3 Si with Nb(Hf) 3Si-

Hf(Nb) 2Si, and Hf(Nb)5 Si3 P(Nb,Hf,Si) and Nb(Hf)5 Si 3

Regime 3

Nb-10%Hf-18.5%Si Nb-10%Hf-25%Si

P(Nb,Hf,Si) eutectic Primary Nb(Hf) 5 Si 3, peritectic Nb(Hf)3 Si,

P(Nb,HfSi), Nb(Hf)5 Si 3, and

Regime 4

Nb-30%Hf-25%Si

Nb(Hf) 3 Si-1(Nb,Hf,Si) eutectic Primary Hf(Nb) 5Si 3, peritectic Hf(Nb)2 Si,

Hf(Nb),Si 3 Hf(Nb) 2Si and P(Nb,Hf,Si)

Regime 5

Nb-60%Hf-25%Si

Hf(Nb) 2 Si-l3(Nb,Hf,Si) eutectic Primary Hf(Nb) 2 Si with Hf(Nb) 2Si-

Hf(Nb) 2Si and [3(Nb,Hf,Si)

Regime 5

Nb-40%Hf-30%Si Nb-80%Hf-14%Si

13(Nb,Hf,Si) eutectic Primary Hf(Nb) 2Si, Hf(

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