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Influence of manganese on properties of lanthanum-alloyed chromium alloys.

In this work study of influence of small quantities of transition metals on mechanical properties (hardness HV and brittle transition temperature [T.sub.b]) of high-purity chromium alloyed by lanthanum is continued.

Earlier influence on the Cr-La alloy of metals of the VIII group of periodic table (iron, cobalt, and nickel), which have close to chromium values of atomic numbers and dissolve in it within 0.6 wt.% was studied [1].

For further study of transition metals manganese was selected. In contrast to Fe, Co and Ni, Mn dissolves well in Cr. So, at 1310 [degrees]C its solubility makes up [approximately equal to] 70 wt.% [2]. On one hand, it is located in the periodic table between chromium and mentioned metals, and on the other hand it enters into the VII group like rhenium, which has similar chemical properties and influence of which on ductility of refined by lanthanum high-purity chromium is studied in [3].

Addition of manganese to chromium (several percent) causes drastic embrittlement of the latter [4, 5]. At the same time these alloys, especially provided they contain 40-60 wt.% Mn, are susceptible to formation of the surface layer subject to crack formation because of active absorption of nitrogen by them [6].

In [7] data are given on influence of alloying additions of manganese on elastic properties of chromium (modulus of shear G and Young's modulus E), and their influence on ductility is considered. It is noted that there is reverse relationship between change of the brittle transition temperature and change of the modules of elasticity and, respectively, strength of interatomic bond, which they represent.

So, increase of manganese content in chromium from 0 to 40 wt.% causes reduction of G from 1.1 * [10.sup.5] to 0.7 * [10.sup.5] Pa, and E--from 2.7 * [10.sup.5] to 1.7 * [10.sup.5], while [T.sub.b] noticeably increases.

It is registered that parameter of crystalline lattice of chromium after its alloying by manganese starts to significantly increase from [approximately equal to] 25 wt.% of the latter, and if the share of manganese is less the said parameter remains practically unchangeable.

The authors assume that the reason of worsening of the temperature of cold brittleness of chromium in case of its alloying by metals of the VII group, in particular manganese, is a not completely clarified character of change of a number of physical characteristics like dislocation structure, energy of stacking fault, solubility of interstitial elements, degree of twinning deformation development, etc.

Manganese, having comparatively weak chemical affinity to interstitial elements, can not, in contrast to lanthanum and other rare-earth metals, refine chrome matrix of these impurities. Contained in chromium carbides are peculiar for extremely high propensity to segregation on grain boundaries, where they are located in the form of lamellar precipitates and cause drastic embrittlement of chromium, while alloying of the latter by lanthanum does not allow changing morphology, shape and location of carbides and, as a result, affect ductility. That's why microadditions of manganese were introduced as the third component into binary Cr-La alloy for checking possibility of positive action on distribution in chromium structure of, first of all, such inclusions as carbides.

Ingots for the study were molten from high-purity charge materials. The alloys were based on electrolytic, refined in hydrogen, commercial chromium of ERKh grade with total content of interstitial element impurities at the level [approximately equal to] 0.017 wt.%, comprising besides 0.005 % N, 0.005-0.006 % C, 0.005 % O, sulfur, silicon and iron. Melting was performed in an induction furnace with copper water-cooled mould. Produced ingots had diameter 34 mm and mass up to 1.5 kg.

As far as chromium has rather high thermodynamic affinity to interstitial elements (oxygen, nitrogen and carbon), for preventing their getting into the alloys before each melting deep vacuum scavenging of the working space of the furnace down to (1.33-2.66) * [10.sup.-3] Pa and vacuum-heat treatment of the charge at 200-250 [degrees]C and vacuum not less than 6.65 * [10.sup.-3] Pa were performed, whereby oxygen, nitrogen and carbon, adsorbed by walls of the degassing chamber and surface of the charge materials, were removed.

Argon, which corresponded to commercial purity (GOST 10157-79), was used as protection environment in first series of meltings; in second series of meltings the same argon was used, but after it was additionally cleaned of impurities of moisture and foreign gases by passing it at 600-750 [degrees]C through special reaction columns with active metals (Ca, Ti-Zr alloy).

From produced ingots specimens were made according to standard technology for measuring temperature of cold brittleness and hardness. Values [T.sub.b] were determined during three-point bend tests of flat specimens, having size 1 x 4 x 30 mm, subjected to grinding and subsequent electrolytic polishing at speed of movement of the rod with bending blade 20 mm/min. Temperature of transition from brittle to ductile state was determined as mean value between minimum temperature, at which a specimen was bent at 90[degrees] without failure, and the maximum temperature, at which it failed. The number of specimens for these tests being selected in such way that maximally reduce this range.



On basis of the test results graphic dependences of HV and [T.sub.b] upon content of lanthanum and manganese in cast alloys of high-purity chromium were constructed. Content of manganese in the charge varied from 0 to 0.5 wt.%, and content of lanthanum in the charge of one group of alloys was 0.5 and in the charge of the other group 0.75 wt.%, which, taking into account its assimilation by chromium, in reality made up, approximately, 0.30 and 0.45 wt.% in molten ingots.

In Figure 1 the curve of dependence of [T.sub.b] on content of manganese in alloys Cr-La-Mn (0.5 wt.% La was introduced into charge of the alloy) molten in commercially pure argon and without using degassing operation before bleeding-in shielding gas, i.e. under so called <<dirty conditions>>, is shown. The alloys are characterized by increased value [T.sub.b], which proves their insufficient ductility. So, at 0.12 wt.% Mn temperature of cold brittleness exceeds 60 [degrees]C, and further increase of its content within studied concentration range practically does not effect its value. Such properties are, evidently, stipulated by increased content of interstitial element impurities in molten ingots.

The curves, which represent influence of manganese on [T.sub.b] of the alloys of high-purity chromium, ingots of which were produced under <<clean>> conditions, i.e. using operations of the charge degassing before melting and argon, which passed additional cleaning in special installation, as shielding environment, look differently (Figure 2).

So, alloys doped by 0.75 wt.% La (curve 1) have higher values of cold brittleness temperature in comparison with the ones, into which 0.5 wt.% La was introduced (curve 2). Most probably, it is explained by the fact that in the first group of alloys excessive lanthanum is present, i.e. not bound by nitrogen and oxygen, precipitated in the form of

interlayers over grain boundaries, which is the reason of a certain loss of ductility.


When comparing curves presented in Figure 2, one can note that influence of micro-additions of manganese on ductility of Cr-La alloys is more pronounced in the alloys, which contain more lanthanum, whereby by means of increase of manganese content in both groups of alloys [T.sub.b] first increases, achieves maximum at [approximately equal to] 0.25 wt.%, and then reduces and takes on the same values at 0.5 wt.% Mn. Taking into account shape of produced curves, it is possible to assume with high degree of probability that when manganese content exceeds 0.5 wt.%, alloys of high-purity chromium with 0.75 wt.% in charge will be more ductile than with smaller amount of lanthanum. For a time being we don't have satisfactory explanation of this phenomenon.

In Figure 3 graphic dependence of hardness of cast low-alloy alloys of the system Cr-La-Mn on manganese content is shown. Over the whole range of concentrations monotonous reduction of hardness and, therefore, strength is registered, which corresponds to the results of [T.sub.b] measurement of the alloys produced under <<clean>> conditions, but does not explain why ductility of alloys of the system Cr-La-Mn increases by means of manganese content increase, instead of reducing like in alloys Cr-Mn. It is possible to assume that reduction of [T.sub.b] should be connected with possible change of the character of the cast alloy failure from inter- to trans-crystalline one, which, evidently, does not happen in alloys of the system Cr-Mn. However, for confirmation of this hypothesis it is necessary to carry out further studies. In addition, of interest may be data on [T.sub.b] of the alloys, which contain more than 0.5 wt.% Mn.

[1.] Rudoj, A.P., Melnik, V.Kh., Portnov, A.P. (2002) Properties of alloys based on high-purity chromium. Advances in Electrometallurgy, 4, 39-40.

[2.] Sully, A.H., Brandes, E.A. (1971) Chromium. Moscow: Metallurgiya.

[3.] Rudoj, A.P., Zhuchenko, L.P., Melnik, V.Kh. et al. (2005) Effect of rhenium on properties of high-purity chromium and its alloys with lanthanum. Advances in Electrometallurgy, 1, 30-32.

[4.] Abrahamson, E.P., Grant, N.J. (1958) ASM Transact., 50, 705.

[5.] Sully, A.H., Brandes, E.A., Mitchell, K.W. (1952/1953) Int. Inst. Metals, 81, 585.

[6.] Sully, A.H. (1959) Manganese. Moscow: Metallurgizdat.

[7.] Kurdyumova, G.G., Milman, Yu.V., Trefilov, V.I. (1970) Influence of alloying elements on elastic properties of chromium. Metallofizika, 29, 101-106.


Institute of Material Sciences Problems, NASU, Kiev, Ukraine
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Author:Rudoj, A.P.; Zhuchenko, L.P.; Melnik, V.Kh.; Portnov, A.P.
Publication:Advances in Electrometallurgy
Article Type:Report
Date:Jan 1, 2006
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