PSI - Issue 23

Václav Paidar / Procedia Structural Integrity 23 (2019) 402–406 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

404

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Table 1.Compositions in TiZrVHfNbMoTa(A7) and TiZrVHfNbMoTaW(A8) alloys. N denotes nominal compositions in atomic per cents, D compositions in coarse dendritic regions and ID in interdendritic regions. There are the relative changes in the two BCC regions with respect to the nominal compositions in the columns between D and ID. T m K is melting temperature of pure elements in Kelvin.

T m K

N

D

ID

N

D

ID

A7

A8

Ti Zr

1945 2128 2163 2504 2742 2896 3293 3695

16,4 16,4 13,6 15,6 15,1 15,4 7,5

12

-0,18 -0,95 -0,29 -0,67 0,38 0,53 0,87

15 24 15 16 13 11

12,7 13,3 11,5 13,9 10,9

7,9

-0,40 13 -1,50 24 -0,38 13 -0,73 16 0,18 12 0,50 9,5 0,89 6,8 1,68 4,9

8,5

4

V

11 11 19 19 21

8,6 5,8

Hf

Nb Mo

14 15 21 23

11

Ta W

7,6

15,9 10,8

3. Binary alloys of zirconium and of hafnium Let us examine first the data available for binary alloys with zirconium. All the data are summarized in Table 2. We take the difference between the melting temperatures of the BCC transition metal and zirconium as the first parameter characteristic for element separation. In the case of binary systems with an intermetallic phase separating the beta solid solution zones, the temperature interval between the melting temperature of pure element and the upper temperature of intermetallic phase is also considered and is denoted as  T im . The temperature and composition intervals are marked in the schematic phase diagram in Fig. 1. The composition width of the two-phase region above the intermetallic phase occurrence is denoted as  c . The height of composition gap,  T up , is just the difference between the upper temperature of the two beta phase mixture and T c for the case when the beta phase extends across the entire binary phase diagram. If there are two separate beta phase regions, there are also two temperatures corresponding to the limit concentrations of beta phase regions. Then the temperature interval,  T up , is taken between the medium temperature of the two beta zones (the points H and B in Fig. 1) and T c . Hence three values of this difference are listed in Tables 2 and 3, those for the points H and B with the medium value between them. Let us define two types of solid solution concentration gaps: First, the beta phase extends across the entire binary phase diagram and there is a concentration difference,  c s , at the critical temperature, T c , and second, the gap is composed of two parts corresponding to the beta zones of hexagonal and BCC transition metal elements,  cH and  cB , respectively. Then the entire composition gap is  c s =  cH +  cB.

Finally, the efficiency of element separation is defined as the sum

 =  T im .  c +  T up .  c s .

(1)

An analogous approach was applied for hafnium and the data are summarized in Table 3. Contrary to the Zr-Ta phase diagram with a pronounced composition gap in the beta zone, there is only very mild two phase composition region at the bottom of the Hf-Ta beta zone that has practically no effect on the element separation.