S T R U C T U R E A N D M E C H A N I C A L P R O P E R T I E S

OF N b - V - Ti A L L O Y S

E . M. L a z a r e v a n d E . V. V a s i l ' e v a UDC 620.17:669.293'292'295

In this work* we present the resul ts of an investigation of the s t ruc ture and mechanical proper t ies of niobium alloys containing 1-15% V and 1-10% Ti. For smelt ing the alloys we used cast ings of niobium 99.78% pure and vanadium and t i tanium iodides. The method of smelt ing the alloys was descr ibed in [1].

The density of the alloys de termined experimental ly was close to the calculated density, which in - dicates good alloying of the components , the absence of pores and flaws in the ingots, and the eonformity of the composi t ion obtained with that p rescr ibed .

The microstrueture of the east alloys was slightly dendritic, and the higher the vanadium concentra- tion the more clearly dendritic the structure. As the result of annealing in vacuum (5 �9 10 -4 mm Hg) for 25 h at 1400~ the grains became equi-axed, characteristic of the greater equilibrium of the structure. To reveal the structure we used a reagent consisting of two parts HNO3, one part HF, one part H2SO4, and one part H20.

To study the additional features of the structure we subjected polished samples to thermal etching in vacuum at 1400~ for 25 h. The microstructure revealed by this treatment differed sharply from the micro- structure of the annealed alloys revealed by the ordinary method (polishing and etching).

As the degree of alloying increased , the str ips and steps in the s t ruc ture of the gra ins , called "facets" or " t e r r a c e s " in the l i t e ra tu re , all developed more c lear ly and in g rea te r numbers in all the al loys. The aggregate of these etch f igures forms the rel ief . Apparently vacuum etching is an intr icate complex of phenomena, the essent ia ls of which are select ive chemical interact ions of parts of the surface of the metal with the active reagents of the a tmosphere (part icularly oxygen) and the select ive vaporization of these areas.

The intensity of these interactions is determined essentially by the atmosphere, temperature, and time of thermal etching, by the nature of the material (by the number and distribution of defects on the

surface), and by the crystal structure. The appearance of the etch figures is connected with the dislocation structure of the metal [2]. The step-shaped relief of vacuum etching is connected with the presence of screw dislocations [3]. It is well known also that the phenomenon of spiral growth of erystals is explained by the pre- sence of screw dislocations. In the process of vaporization the reverse processes occur, which is also appar- ently explainedby the presence of screw dislocations. Often the relief is connected with the nonuniform distri- bution of impuri t ies o rwi th elast ic displacement of dislocations [4].

The intensity of thermal etching differs in different crys ta l lographic planes. In metals with a bcc lat t ice the most c losely packed planes with low indices (100) and (111) are favorably oriented for intense etching. As the resul t of interact ions with the a tmosphere or vaporization in vacuum the atoms are de- tached f rom the sur face and part ial ly r ea r r anged , which is accompanied by local reduction of the surface energy. As the r e su l t of such r ea r r angemen t , p r imar i ly by means of surface diffusion, is formed the c h a r - ac te r i s t i c re l ief for the given conditions of the contour with the minimum surface energy.

Thermal etching is a useful method of reveal ing the dislocation s t ruc ture of metals and al loys.

In the alloys we investigated the intensity of the development of re l ief during thermal etching is de termined by the t ime of the p rocess and, under identical conditions, by the concentra t ion of titanium and

* This work was done under the direct ion of D. A. Prokoshkin.

Insti tute of Metal lurgy, Academy of Sciences of the USSR. Transla ted f rom Metallovedenie i Te rmi - eheskaya Obrabotka Metallov, No. 12, pp. 31-33, December , 1966.

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Fig. 1. 1VIicrostructure of the relief of N b - V - T i alloys thermally etched in vacuum (• a) 1% V, 1% Ti; b) 1~o V, 10% Ti; c) 5% V, 10% Ti; d) 15% V, 1% Ti; e) 1% V, 10% Ti; f) 15% V, 10% Ti.

HV 4 I

2~0300 ~

~ / 3 2 ! ZOO ~- ~ - - ~ - ~ - - -

a !

300

ZSO

200 - - - -

,oo f-4- o s ~o %V

Pig. 2. Var iat ion of the hard- ness of Nb- -V- -T i al loys with the concentration, a) As cast; b) annealed. 1) 0% Ti; 2) 1% Ti; 3) 5% Ti; 4) 10% Ti.

Ob, oo,2, kg/mrn s

b I A 2":_.kefJ. "..~. ,o ~ ,K/XT~...d-

t BO

e~

ZO 6" % ~0

0 s ~0 %v

Fig. 3. Var iat ion of the me- chanical propert ies of FIB- V - T i al loys with the concen- t rat ion. 1) 1% Ti; 2) 5~o Ti; 3) 10% Ti. '- ) O 'b ; - - - )Cro . 2.

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vanadium in the alloy. The higher the degree of alloying, the more distinguished the etch f igures and the higher the number of grains on which they a re observed. In the alloys containing 1% V the s t r ips form at f i r s t on the gra in boundaries and then on the grains (Fig. 1, c and d). As the concentrat ion of t i tanium is inc reased in alloys smel ted without a crucible the purity of the metal is higher , there are fewer impurit ies, and the same rule holds: the higher the vanadium content, the l a rge r the elements of the s t ruc ture of the re l ief (Fig. 1, e and f). Undoubtedly, alloying induces changes in the dislocation s t ruc tu re , and the higher the degree of alloying the g rea te r these changes. The change in the dislocation s t ruc ture is probably r e - f lected in the s t ruc ture of the mierosec t ion in the process of thermal etching.

The variat ion of the hardness of cas t (Fig. 2a) and annealed (Fig. 2b) alloys with the concentrat ion of vanadium for different concentrat ions of titanium is a smooth curve charac te r i s t i c of homogeneous solid solutions. As is evident, as the resu l t of annealing there is a very substantial reduction of the hardness because of the increase of the homogenization and equil ibrium s t ruc ture and the increase in the purity of the metal as a consequence of the part ial elimination of impuri t ies in vacuum.

An increase in the degree of alloying with ti tanium and vanadium induces an increase in the ha rd - ness of the cast and annealed s ta tes , in which the effect of vanadium is more substantial . Evidently, this re f lec ts the considerable difference in the atomic radius of niobium (1.45 A) and vanadium (1.34 A).

It is well known that alloying with vanadium or titanium has a favorable effect on the plasticity and s t rength of niobium. We determined the mechanical proper t ies of N b - V - T i alloys with tensile strength tes t samples with a d iameter of 1.5 mm and a gage length of 6.5 mm on the Chevenar mic rosys t em. The resul ts of the tests are shown in Fig. 3. Alloying with vanadium in the presence of titanium substantially i nc reases the s t rength. The s t rength of the alloys with 10% Ti is inc reased f rom 47.5 to 107.5 k g / m m 2, and that of alloys with 1% Ti f rom 34.3 to 83.6 k g / m m 2, as the concentrat ion of vanadium is increased f rom 0 to 15%.

Alloying has a similar effect on the nominal yield point of Nb-V-Ti alloys. It should be noted that vanadium strengthens the ternary Nb-V-Ti alloy to a greater extent than titanium.

CONCLUSIONS

1. We found a connection between the behavior of alloys in the process of thermal etching and their concentra t ion of alloying e lements .

2. I t was shown that an increase of the concentrat ions of vanadium and ti tanium increases the strength of the alloys and reduces the ductility with the limits of the concentrat ions investigated. Vanadium has a g r ea t e r effect on the s t rength, and titanium on the ductility.

L I T E R A T U R E C I T E D

1. D . A . Prokoshkin , E. V. Vas i l ' eva , and ]~. M. Lazarev , Zhurnal prikladnoi khimii, No. 2 (1966). 2. S. Amelincks and E. Votava, Naturwissenschaf ten, 41 (1954). 3. V . N . Danilov, Kr is ta l lograf iya , 6, No. 1 (1962). 4. E . K . Dukova, Kr is ta l lograf iya , 5, No. 5 (1960).

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