Scripta METALLURGICA Vol. 23, pp. 81-86, 1989 Pergamon Press plc Printed in the U.S.A. All rights reserved

ON THE EFFECT OF BISMUTH IN A CAST TITANIUM-SILICON EUTECTIC ALLOY

R.L.Saha, T.K.Nandy and R.D.K.Misra Defence Metallurgical Research Laboratory

PO: Kanchanbagh, Hyderabad - 500 258 (INDIA)

(Received August 25, 1988) (Revised October ii, 1988)

It is now well-known that casting of titanium and its alloys to requisite shapes has some inherent problems, notably, in the selection of mould material, severity of metal-mould reactions, relatively low castability and high energy consumption of the casting process. The difficulties can, however, be reduced to a significant extent if low melting eutectic alloys can be developed with appreciable mechanical properties. Binary eutectics of titanium are generally brittle in nature and are formed with metals only at higher percentage additions (~30-40wt%) of alloying elements. In view of the limited success (1,2) and the aforementioned problems, much of the work has been directed towards wrought alloys. Si and Be are two elements which form binary eutectics with titanium even with small additions (~Swt%) and possess the additional advantage of being low density elements. The Ti-Si system, on the other hand, has been investigated in some detail. The Ti-Si alloy system exhibits a eutectic reaction(3):

(TiSi) L > Ti + Ti5Si 3

at a temperature of 1603K and at 8.5 wt%Si. The few studies(4,5) that have been carried out on Ti-Si alloys indicate that the tensile strength of a Ti-Si alloy increases with an increase in Si-content and attains a maxima of ~ 750 MPa at 2wt%Si. Further addition of Si to the extent of ~ 3wt.% led to a decrease in tensile strength to ~ 540MPa from the maximum value. These alloys also exhibited very low ductility (~ 0.07%) when Si addition exceeded more than

2wt%.

More recently, a directionally solidified Ti-8.5 wt.%Si alloy has yielded a tensile strength as high as 1000MPa but with almost zero ductility(6). In addition, the alloy exhibited elastic modulus (~ 1,90,000 MPa) approximately twice that of pure titanium (~1,10,000 MPa).

The aforementioned observations prompted us to freshly examine the Ti-Si system. Our preliminary studies have shown that the tensile properties up to about 3wt% Si were similar to those reported in the literature. However, a significant improvement in tensile strength was noted for alloys, with a higher percentage of silicon (Table I). The near eutectic composition (Ti-6.5wt% Si) cast alloy which exhibited tensile strength of ~ 830 MPa and low ductility of

0.8% was thought to be an alloy of industrial importance provided its ductility could be enhanced.

One of the commonly used methods to improve ductility and toughness of eutectic alloys is that of modification of the microstructure by minor additions of a third element. A good illustration in this regard is the modification of the AI-Si eutectic alloy by minor additions of Na or Sr(7,8). Considering the important role played by Na or Sr in modifying the microstructure of an AI-Si eutectic alloy, initial attempts were aimed to improve the ductility of the Ti-6.5Si alloy by minor additions of low surface tension elements, namely, Na, St, Se, Te and Bi. It was, however, noted that only the Bi displayed a definite and beneficial effect on the ductility of the alloy. Thus, Bi was selected for

81 0036-9748/89 $3.00 + .00

Copyright (c) 1989 Pergamon Press plc

82 EFFECT OF Bi IN A Ti-Si ALLOY Vol. 23, No. 1

studying the behaviour of a Ti-6.5wt%Si alloy. The following paragraphs describe the microstructural changes that were induced upon the addition of a small % of Bi in the Ti-6.5wt%Si alloy. An attempt is also made here to examine whether there exists a correlation between the microstructural features and the mechanical properties (particularly ductility) of the alloy.

Ti-Si alloys in the form of pancakes were made by melting commercial purity Ti and Si in a water cooled copper crucible by non-consumable arc melting under an argon atmosphere. Repeated melting was carried out to ensure homogeneity of the alloy. Samples were then cut from the pancakes for metallographic studies and tensile property evaluation.

The microstructure of Ti-Si alloys with varying %~i is presented in Fig.l. It may be noted from Fig.la that the low Si-containing alloy exhibits a transformed ~-structure, whereas the 3wt.%Si alloy, corresponding to the solubility limit of Si in Ti shows a silicide precipitation within the prior P-grains and at the grain boundaries(Fig.lb). The latter is also associated with a precipitate free zone. This structure is thought to be responsible for the low tensile strength, low ductility (Table i) and predominantly intergranular mode of fracture of the alloy (not presented here). When Si% exceeded more than 3wt%, in the alloy, the microstructure consisted of dendrite grains of ~-Ti and a continuous eutectic aggregate (Fig.lc) and the alloy yielded improved strength (Table i) and exhibited a transgranular mode of fracture. As stated above, small additions of Bi were made to the Ti-Si alloy to further enhance the ductility of the alloy.

The microstructures, evolved as a consequence of small additions of Bi (0.02-0.5 wt%) to the Ti-6.5 wt% Si alloy, are presented in Fig.2. Fig.2 clearly brings out the effect of small additions of Bi on the distribution of dendrites and eutectic regions. It may be noted that the addition of Bi to the Ti-6.5wt%Si alloy results in a finer distribution of a dendrite and eutectic region along with increased volume fraction of dendrites. Examination of the microstructure at higher magnification (Fig.3) shows that silicides in the eutectic regions are coarse in nature and interconnected in the non-Bi containing Ti-6.5wt%Si alloy. However, an addition of bismuth up to ~ 0.1wt% results in microstructures with finer silicides and reduced/eliminated interconnectivity in the silicide needles (Figs. 3b and 3c). When the percentage of Bi addition exceeds ~ 0.1wt%, interconnectivity in silicides again seems to reappear(Fig.3d).

The tensile properties of the Bi-containing Ti-6.5 wt% Si alloy are listed in Table 2; apparently the variation in ductility follows the same behaviour as that of the microstructural variation (Figs.2 and 3) implying that the presence of fine silicides and reduced interconnectivity in silicides and high volume fraction of dendrites (Table 2) favours relatively better ductility.

The presence of a finer distribution of the dendrite and eutectic regions is more likely to occur if there is an increase in the number of nucleation events, which may be possible in the event that undercooling takes place. To confirm the validity of the aforementioned statement and simultaneously study if there is an increase in the Si-content in the eutectic region at the eutectic horizontal, differential thermal analysis (DTA) and electron probe microanalysis (EPMA) of the dendrite and eutectic regions was carried out. Differential thermal analysis data obtained under identical conditions of cooling for the Ti-Si-Bi alloys is presented in Fig.4. In Fig.4 the start of a peak on the right hand side represents the beginning of solidification (liquidus) whereas the eutectic horizontal (solidus) is represented by the end of the left hand side of the peak. The exact temperature of the eutectic horizontal was determined in the present study by drawing a tangent to the base line at the position of change in the slope of the curve and this solidus temperature was expected to remain unaltered by any compositional variations. It may, however, be noted that with the addition of small amounts of bismuth up to about 0.08 wt% in the Ti-6.5wt%Si alloy, the eutectic horizontal is shifted to slightly lower temperatures; at Bi% of 0.5 wt%, the eutectic horizontal is similar to that of the non-bismuth

Vol. 23, No. 1 EFFECT OF Bi IN A Ti-Si ALLOY 83

containing alloy. There is a maximum of-~10K shift towards the lower temperature in the eutectic horizontal of the Bi-containing Ti-Si alloys (Table 2). It is possible that the depression of the eutectic horizontal noticed in the bismuth-containing alloys may be one reason for obtaining a finer and homogeneous distribution of dendrites in the matrix as indicated in Fig.2. This observation is similar to that noted for modification of aluminium-silicon alloys by minor additions of sodium, in which case a depression of the eutectic horizontal by 5-7K is observed for the AI-Si alloys with a varying % of silicon(7,8).

The fact that the microstructural examination of the bismuth-containing Ti-Si alloys (Figs.l and 2) indicates an increase in the volume fraction of dendrites (Table i) with an increase in %Bi in the alloy (up to ~ 0.08wt% Bi) suggests that such an event may occur upon the increase of Si content in the eutectic region at the eutectic horizontal. Electron probe microanalysis data (Table 3) supports the aforementioned inference even if the overlapping in the microprobe data is taken into consideration. The increase in %Si in the eutectic region is indicative of a shift in Si-composition at the eutectic horizontal. Incidently, the increase in %Si in the dendrite and eutectic region with increase in %Bi in the alloy is accompanied by an increase in microhardness (Table 31).

Efforts were also made to study the distribution of Bi in the Ti-Si alloys through an electron probe microanalyser, in spite of the fact that the sodium distribution in the AI-Si system could not be detected by the investigation of the AI-Si alloy(8). The reason of non-detectability might have been a low atomic number of Na. It was therefore believed that Bi being a high atomic number element would make it easier to study its distribution. Contrary I;o this expectation, electron microprobe studies appeared to indicate a uniform distribution in the alloy. It is also possible that bismuth may be preferentially localised in certain regions at the atomic level, which could not be studied by the electron microprobe in view of the limitatien of the technique.

The study described here conclusively demonstrates for the first time the beneficial effect of a low surface tension element, bismuth, on the cast eutectic structure of a Ti-Si alloy. The modification (finer silicides and reduced interconnectivity in the silicides) induced in the microstructure of the alloy upon the addition of small amounts of Bi (up to ~ 0.08 wt%) is apparently associated with an improvement in ductility, a depression in the eutectic horizontal by ~ 10K, and a marginal increase in the % Si in both the dendrite and eutectic region. Efforts are now currently underway to gain an insight into the mechanism of modification and will be reported subsequently.

Acknowledgement

Authors are grateful to Dr.P.Rama Rao, Director, DMRL for his continued interest in the work and permission to publish this work.

References

l.H.R.Bomberger, G.S.Hall and S.R.Seagle, Titanium 1980 (Eds. H.Kimura and O.Izumi) The Met. Soc of AIME, Warrendale, p.1277 (1980).

2.J.V.Wood and B.Toloui, Semisolid processing of Ti alloys for medical and other applications, Proc. of Designing with Titanium, The Inst of Metals, London, p.50 (1986).

3.Handbook of Phase Diagrams, (Ed.T.B.Massalski) ASM, Metal Park, Ohio (1986). 4.M.Hansen, H.D.Kessler and D.J.McPherson, Trans. ASM 44, p.518 (1952). 5.H.W.Antes and R.E.Edelman, Trans. AFS 63, 662 (1955). 6.B.Lavelle and F.Dabosi, Titanium 80, (Eds. H.Kimura and O.Izumi), The Met Soc

of AIME, Warrendale, p.2275 (1980). 7.R.C.Plumb and J.E.Lewis, J.Inst.Metals 86, 393 (1957). 8.M.D.Hanna, Shu-Zu-Lu and A.Hellawell, Met.Trans. ]5A, 459 (1984).

84 EFFECT OF Bi IN A Ti-Si ALLOY Vol. 23, No. 1

Table I: Tensile properties of Ti-Si alloys with varying % Si

Si UTS 0.2% Y.S. Reference %Elongation

(Wt%) (MPa) (MPa) (for values reported in brackets)

1.0 746 (640) 608 (517) 18 (17) 5 2.0 (746) (685) (1.5) 5

3.0 (540) - (0.07) 5 4.5 711 687 0.7 -

6.5 834 736 0.8 - 8.5 775 (1000) 726 0.4 6

Table 2: Tensile properties, volume fraction of dendrites and depression in eutectic horizontal ofT i-6.5 wt% Si with varying % Bi

Bi UTS 0.2% Y.S. %Elongation vol. fraction Depression in (Wt%) (MPa) (MPa) of dendrites Eutectic horizontal(K)

0.0 834 736 0.8 38 0 0.02 814 716 1.4 - 0.04 834 736 1.7 49 10 0.06 824 736 1.8 45 0.08 814 716 1.6 42 8 0.20 795 628 0.9 - 0.50 765 628 0.5 37 2

Table 3: Microhardness and % silicon in dendrite and eutectic regions of Ti-6.5 wt% Si alloy with varying % Bi

Bi Dendrite Eutectic

(wt%) Si (wt%) Hardness(VPN) Si (wt%) Hardness (VPN)

0.0 1.77 ± 0.234 342 8.82 ± 0.251 393 0.04 1.93 ± 0.235 362 9.03 ± 0.252 455 0.08 2.26 ± 0.236 360 9.49 ± 0.233 453 0.5 1.73 ± 0.233 8.99 ± 0.250 -

Vol. 23, No. 1 EFFECT OF Bi IN A Ti-Si ALLOY 85

Fig.l: Microstructure of Ti-si alloys with varying % Si, (a) Ti-1.0 wt% Si; (b) Ti-3.0 wt% Si; (e) Ti-6.5 wt% Si.

Fig.2: Distribution of dendrite and eutectic regions in Ti-6,5 wt% Si alloy with varying % Bi (a) No Bi (b) 0.04 wt%; (c) 0.08 wt% Bi (d) 0.5 wt% Bi

86 EFFECT OF Bi IN A Ti-Si ALLOY Vol. 23, No. 1

Fig.3: Change in morphology of eutectic silicides with Bi additions (a) No Bi (b) 0.04wt% Bi (c) 0.08wt% Bi (d) 0.5wt% Bi.

COOLING RATE 70°C/min' / ~ ~

/ / / \ \ \ ~0.sB,

1300 1320 1340 1360 1380 1400 1420 1440

TEHPERATURE (°C)------"

Fig.4: DTA curves for Ti-6.5 wt% S~ alloy with varying % Eli