INVESTIGATION OF ACOUSTICAL AND RESISTIVITY

PROPERTIES OF AUSTENITIC ALLOYS Fe-Cr-Mn

IN THE TEMPERATURE REGION 5 – 300 К

L.N. Pal-Val ¹, Yu.A. Semerenko ¹, G.N. Grikurov ²

^1. B.Verkin Institute for Low Temperature Physics and Engineering, National Academy of Sciences of Ukraine, Kharkov, Ukraine

^2. Institute of Metallurgy, Academy of Sciences of Georgia, Tbilisi, Georgia

In the temperature range 5¸300 K, the temperature dependences of decrement d , dynamic Young's modulus Е and a resistivity r in a perspective alloy for cryogenic engineering alloy Fe_100-x-yCr_xMn_y (x = 5; 10 and y = 30; 35; 40) are investigated. The main interstitials are: C, N, S, P < 0.1 % and Si < 0.3 %. Samples of two types were investigated: 1) rods cut from cold-rolled wires ~ 1 mm in diameter and ~ 30 mm in length; 2) rectangular parallelepipeds 3 x 3 x 30 мм³ cut out from massive swaged ingots. Samples had g-austenitic structure that was formed during 1 hour annealing at 1273 K and subsequent quenching into water.

Acoustic measurements were carried out using the two-component composite vibrator technique. Longitudinal standing waves were excited in the sample at frequencies f ~ 75 kHz. The ultrasonic strain amplitude had a value e₀ ~ 10^-7 corresponded to the amplitude-independent region. dc-resistivity measurements were carried out using standard four-point method at two opposite directions of a transport current. The temperature changes rate during thermocyclings was ~ 1 K / min.

Experimental results can be divided into two groups concerning to samples with fixed Cr content: 5 % and 10 %, accordingly. In the alloys with 5 % Cr, a series of internal friction peaks P₁, P₂ and P₃ is recorded in dependences d (T) which are localized at temperatures T_P1 » 20 К, T_P2 » 155 K and T_P3 » 255 K. When increasing Mn concentration, peak height of P₁ is reduced and peak is transformed to a plateau Parameters of peaks P₂ and P₃ change unsignificantly and the changes are not systematic. In samples with 10 % Cr, internal friction peaks on dependences d (T) are also registered and are localized at » 140 К and » 220 К. The greatest peak height is observed in samples with 40 % Mn, and the least - in samples with 35 % Mn. The nature of these peaks remains unknown.

Alloys investigated are characterized by low values of decrement d < 6×10^-4 and the considerable magnitude of a dynamic Young's modulus Е ~ 180 GPa which increases monotonically by 7 % when temperature decreases from 300 down to 5 K. Temperature dependences of a Young's modulus may be described by analytical expressions in the assumption of additivity of the electronic and phonon contributions. Variation of Mn concentration in an irregular manner changes absolute value Е and practically does not influence a shape of dependences Е (Т).

There is a series of the distinct differences in the results obtained for wires and massive samples. In massive samples over the whole temperature interval values of decrement are much lower than those recorded in wire samples. At the same time values of a dynamic Young modulus appeared to be noticeably higher. Supposed reason of these differences may be difference in initial defect structures of the massive and wire samples.

The alloys investigated are characterized by the considerable residual resistance r(T=0K) » 50 ¸ 70 mW·cm. In samples with 5 % Cr r increases systematically when Mn content is increased. In alloys with 10 % Cr the regular resistivity change with change in Mn and Fe concentration is not observed.