Researchers at Pohang University of Science and Technology (POSTECH) in South Korea have developed a new type of steel with improved tensile strength and lightness. In their approach, they effectively utilized a brittle intermetallic compound (B2) that metallurgists usually try to suppress by modifying B2 morphology and dispersion in the steel matrix.
The specific tensile strength and ductility of the developed steels improve on those of the lightest and strongest metallic materials known, titanium alloys, the researchers said. The results, reported in a paper in the journal Nature, demonstrate how intermetallic compounds can be harnessed in the alloy design of lightweight steels.
There is a growing demand for lightweight structural materials as an alternative to conventional steels which are heavy and impractical for future energy-efficient vehicles. The share by weight of steel and iron in an average light vehicle is now gradually decreasing, from 68.1% in 1995 to 60.1% in 2011, the authors of the paper noted.
This has been driven by the low strength-to-weight ratio (specific strength) of iron and steel, and the desire to improve such mechanical properties with other materials.
While high-aluminium low-density steels are actively under study as a means of increasing the specific strength of an alloy by reducing its density, increasing aluminium results in poor ductility, due to the formation of brittle intermetallic compounds forming in the resulting alloys.
Until now, low-density steel has been studied mostly in systems based on Fe-Al and Fe-Al-Mn-C. In particular, low-density steel with fairly high specific strength has been produced with alloys based on Fe-Al-Mn-C (the so-called TRIPLEX steels) using a microstructure consisting of austenite (face-centered cubic) matrix and finely dispersed nanometre-sized κ-carbides of the (Fe,Mn)3AlC type. However, the level of specific strength attainable by this microstructure was not comparable to those of light materials such as aluminium and titanium alloys. This was due to the low strain hardening rate of the Fe-Al-Mn-C alloys containing κ-carbides, which are easily shearable by gliding dislocations.
One of the general concepts employed until now in the alloy design of Fe-Al-Mn-C-based, high-aluminium, low-density steel has been the suppression of ‘brittle’ intermetallic compound formation by stabilizing the ‘ductile’ austenite matrix (this stabilization is achieved by alloying carbon and manganese). Instead, here we have actively utilized the brittle intermetallic compound B2 by modifying its morphology in the steel matrix. Despite their poor plasticity at ambient temperature in the bulk state, FeAl-based intermetallic compounds offer an attractive combination of physical and mechanical properties such as low density and good corrosion, oxidation and/or wear resistance.
To take advantage of B2, we devised an alloy design in which B2 is dispersed as a second phase in the austenite matrix on the basis of the ‘divide and rule’ principle, which is analogous to harnessing ‘brittle’ martensite as a strengthening second phase in the ferrite (body-centered cubic) matrix of dual-phase steels.—Kim et al.
To expand the stability domain of B2, the research team at the Graduate Institute of Ferrous Technology (GIFT) at POSTECH modified the alloying recipe of an austenitic low-density steel comprising iron, aluminium, manganese and carbon by adding 5 wt % nickel (Ni)—one of the most effective elements for forming B2 with aluminium.
The addition of Ni to low-density steel may appear to conflict with the collective wisdom of ferrous alloy design; Ni has been regarded merely as a well-known austenite stabilizer like Mn and C; and Ni has been little noticed in low-density steel design, mainly because it is not a critical determinant of the density in ferrous alloys.—Kim et al.
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They found that alloying of nickel catalyzes the precipitation of nanometer-sized B2 particles in the face-centered cubic matrix of high-aluminium low-density steel during heat treatment of cold-rolled sheet steel. The brittle but hard B2 particles thus were effectively used as a strengthening second phase in the high-aluminium, low-density steel, while alleviating the harmful effect of B2 on ductility by controlling its morphology and dispersion.
With this innovative approach, stronger and more ductile lightweight steels have been created. The team plans to work with South Korean steel manufacturer POSCO later this year to produce high-specific-strength steels that will be lightweight and strong enough to produce fuel efficient vehicles.
Sang-Heon Kim, Hansoo Kim & Nack J. Kim (2015) “Brittle intermetallic compound makes ultrastrong low-density steel with large ductility” Nature 518, 77–79 doi: 10.1038/nature14144