This paper reports work on a new variant of the high saturation Fe-27Co alloy. Ternary additions of niobium or tantalum are found to impart ductility to the hot formed material by means of grain refinement. The magnetic and tensile properties of the new alloy with the trade name VACOFLUX® 27 are compared to the standard chromium-type alloy. While the electrical resistivity is reduced, the saturation magnetization of VACOFLUX® 27 is superior.

Soft magnetic crystalline iron-cobalt alloys are known for their highest possible saturation magnetization. Even though the saturation maximum can be found around 35 wt.% cobalt, the most common representative is the Fe-50Co alloy. The Fe-35Co and the Fe-50Co alloys are brittle after final magnetic annealing due to an ordered state which develops starting from approximately 30 wt.% cobalt, so that additions like vanadium and further production steps need to be introduced to enable cold deformation.1 Niobium and tantalum in the Fe-50Co alloy are found to be more effective in suppressing the ordered state than vanadium, so that the substitution of 2 wt% vanadium by 0.35 wt% tantalum produces enhanced dc magnetic properties in the cold worked strip material.2 

To develop the excellent magnetic properties, the final shaped material needs to be heat treated above the order/disorder transition temperature so that after furnace cooling the material is brittle again. If better mechanical properties are desired, it is common practice to improve strength and ductility by further alloying elements at the expense of saturation magnetization and soft magnetic properties.

At 27 wt.% cobalt the ordered state is absent and the saturation is still very high. In commercially available Fe-27Co alloys, ductility and electrical resistivity are increased mainly by chromium but also manganese, silicon, and nickel in a total amount of approximately 0.5 – 2 wt%. However, these additions strongly decrease the saturation magnetization to the level of the Fe-49Co-2V alloy, which is then magnetically advantageous due to its higher permeability and lower coercive force.

Melts of 5kg weight, based on Fe-27Co with varying ternary additions, were prepared in a laboratory furnace via vacuum induction melting (VIM). The ingots were descaled and hot-rolled in several steps to a diameter of 12 mm. Magnetic permeameter samples and tensile test specimen were fine turned from the hot-rolled diameter. The permeameter samples were heat treated 10 hours at 850°C in pure dry hydrogen atmosphere. The magnetic properties were measured according to IEC standard3 and the tensile test was performed according to Ref. 4. The carbon content was determined by hot gas extraction and the content of other elements was analyzed by XRF spectroscopy.

Magnetic polarizations above 2T are shown in FIG. 1 for different ternary additions in a Fe-27Co alloy. The Fe-26.8Co-0.59Cr-0.60Ni-0.23Mn-0.29Si-0.009C is a typical chromium-type composition which can be found on the market. In comparison to the binary alloy, the ternary alloy polarizations decrease with increasing field strength and total amount of addition. For field strengths below 200A/cm, the polarization of a Fe-49Co-2V is superior due to a vanishing magnetocrystalline anisotropy at around 49 wt% cobalt.

FIG. 1.

Polarization above 2T of Fe-27Co alloys with different ternary additions in comparison to a typical Fe-49Co-2V alloy.

FIG. 1.

Polarization above 2T of Fe-27Co alloys with different ternary additions in comparison to a typical Fe-49Co-2V alloy.

Close modal

The polarization at 400 A/cm and the elongation to rupture is shown in FIG. 2 as a function of the niobium content in the binary alloy. Carbon is present in the shown Fe-27Co-Nb alloys as unavoidable impurity equal or below 0.0051 wt%. While the elongation shows a strong dependency and a maximum in the vicinity of 0.15 wt% niobium, the saturation magnetization is not strongly affected up to 0.30 wt% niobium.

FIG. 2.

Impact of niobium additions on the polarization and elongation of a binary Fe-27Co alloy.

FIG. 2.

Impact of niobium additions on the polarization and elongation of a binary Fe-27Co alloy.

Close modal

The strong impact of niobium can also be seen in Table I and FIG. 3. The small amount of 0.15 wt% niobium is sufficient to change the fracture mechanism of the Fe-27Co alloy from a brittle cleavage to a ductile cup-and-cone fracture. The higher ductility is caused by ball shape precipitations at the grain boundaries visible at the fractured surfaces in FIG. 3 (b). The precipitations were analyzed by EDX to be niobium rich and they exhibit a higher hardness than the surrounding matrix, which plastically deforms around these under mechanical stress. We assume that the precipitations are stable at high temperatures and keep the grain size small and the material ductile by impeding grain boundary movement in the hot forming process. The exact composition of the niobium rich precipitations could not be conclusively clarified. According to the TEM investigations in Ref. 5, two niobium rich phases can develop in a Fe-49Co-1.9V-0.3Nb alloy. The first one is an intermetallic Laves phase and the second one are niobium carbides.

TABLE I.

Fracture in the hot rolled condition and polarization after annealing 10h at 850°C.

Analyzed compositionElongation to rupture in %Type of fracturePolarization J in T at 400 A/cm
Fe-26.7Co 2.5/3.4 cleavage 2.389 
Fe-26.2Co-0.15Nb 27.7/33.0 cup and cone 2.384 
Fe-26.9Co-0.15Nb-0.05Ta 21.4 cup and cone 2.373 
Fe-26.9Co-0.17Nb-0.115C 3.8 cleavage 2.370 
Fe-26.8Co-0.59Cr-0.60Ni- 24.3/24.9 cup and cone 2.341 
0.23Mn-0.29Si-0.009C    
Fe-27.2Co-0.14Nb-0.990Cr 24.3 cup and cone 2.334 
Fe-27.0Co-1.95Cr 18.8 cup and cone 2.299 
Analyzed compositionElongation to rupture in %Type of fracturePolarization J in T at 400 A/cm
Fe-26.7Co 2.5/3.4 cleavage 2.389 
Fe-26.2Co-0.15Nb 27.7/33.0 cup and cone 2.384 
Fe-26.9Co-0.15Nb-0.05Ta 21.4 cup and cone 2.373 
Fe-26.9Co-0.17Nb-0.115C 3.8 cleavage 2.370 
Fe-26.8Co-0.59Cr-0.60Ni- 24.3/24.9 cup and cone 2.341 
0.23Mn-0.29Si-0.009C    
Fe-27.2Co-0.14Nb-0.990Cr 24.3 cup and cone 2.334 
Fe-27.0Co-1.95Cr 18.8 cup and cone 2.299 
FIG. 3.

SEM pictures of the tensile test specimen surface fractures at the same magnification. (a) Fe-27Co: The brittle cleavage fracture occurs along the grain boundaries. (b) Fe-27Co-0.15Nb: The ductile cup-and-cone fracture occurs around niobium rich precipitations.

FIG. 3.

SEM pictures of the tensile test specimen surface fractures at the same magnification. (a) Fe-27Co: The brittle cleavage fracture occurs along the grain boundaries. (b) Fe-27Co-0.15Nb: The ductile cup-and-cone fracture occurs around niobium rich precipitations.

Close modal

FIG. 4 indicates that an excess of carbon in a Fe-27Co-0.15Nb alloy results in a drastic decrease of ductility. While the degradation of polarization is negligible up to 0.12 wt% carbon, the elongation decreases in the range of 0.005 to 0.046 wt% from a value above 25% to below 5%. At the same time the fracture appearance changes from cup-and-cone to cleavage. This indicates that niobium carbides are responsible for embrittlement if too much carbon is alloyed.

FIG. 4.

Impact of carbon additions on the polarization and elongation of a ternary Fe-27Co-0.15Nb alloy.

FIG. 4.

Impact of carbon additions on the polarization and elongation of a ternary Fe-27Co-0.15Nb alloy.

Close modal

VAC’s solution of a Fe-27Co alloy with the trade name VACOFLUX® 27 includes microalloying additions and exhibits higher saturation and better ductility than the standard chromium-type alloy. Niobium and tantalum can be alloyed in smaller amounts than chromium, so that the saturation magnetization of the binary alloy is preserved. Carbon needs to be strictly controlled to prevent precipitations in undesirable size and amount. The electrical resistivity is reduced compared to the chromium type alloys. VACOFLUX® 27 thus offers the highest saturation polarization amongst commercially available soft magnetic alloys, and is especially suited for high magnetic field dc applications.

We thank Dr. Stephan Laßmann for the SEM investigations.

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