Effects of Mn additions on microstructure and properties of Fe–TiB₂ based high modulus steels

Highlights

• Additions of 0–30 wt.% Mn to Fe – TiB₂ high modulus steels result in broad spectrum of matrix microstructures

• Mn has little effect on density, but both Young's modulus and mechanical properties are strongly affected

• 20 and 30 wt.% Mn yield in γ and ε-martensite, and no gain in lightweight performance

• αʹ martensite and reverted γ with 10 wt.% Mn strikes an optimum balance between physical and mechanical performance

Abstract

We studied the effects of Mn additions from 0 to 30 wt.% on microstructure, mechanical and physical properties of liquid metallurgy synthesised high modulus steels in hypo- and hyper-eutectic TiB₂ concentrations. While Mn has little effect on density, both Young's modulus and mechanical properties were strongly affected by the achieved wide spectrum of matrix microstructures, ranging from ferrite to martensite, reverted austenite, ε-martensite and austenite. Mn additions of 20 and 30 wt.% did not translate into enhanced mechanical performance despite the higher inherent ductility of the predominantly austenitic matrix, and instead eliminate the intended weight saving potential by significantly reducing the Young's modulus. Martensitic matrices of Mn concentrations of 10 wt.%, on the other hand, are favourable for improved matrix/particle co-deformation without sacrificing too much of the composites' stiffness. In hypo-eutectic Fe – TiB₂ based steels, mechanical properties on the level of high strength dual phase steels could be achieved (~ 900 MPa UTS and 20% tensile elongation) but with an enhanced Young's modulus of 217 GPa and reduced density of 7460 kg m^− 3. These significantly improved physical and mechanical properties render Mn alloyed high modulus steels promising candidate materials for next generation lightweight structural applications.

Introduction

The ever-growing demand for weight reduction in transportation systems and machine components requires new pathways for structural material design. With steels as the most ubiquitously used structural materials, the main alloy design focus lies on increasing the yield (YS) and ultimate tensile strength (UTS), as well as on reduction of the density (ρ) [1]. The additional key factor stiffness, expressed by the Young's modulus (E), however, has only recently been started to be addressed [2], [3] with the implementation of stiff and low-density reinforcements embedded in ductile and tough Fe-based matrix, creating so called high modulus steels (HMS). The Fe – Titanium diboride (Fe – TiB₂) system has received considerable attention in this light, as TiB₂ is very effective with a specific modulus (E/ρ ratio) of about 125 MPa kg^− 1 m³ and can be synthesised in-situ from Fe – Ti – B melts in a pseudo-binary eutectic reaction [4], [5], [6], [7], [8], [9], [10].

However, as TiB₂ is not only stiff and light, but also brittle, the ductility especially of hyper-eutectic alloys with TiB₂ fractions above the eutectic concentration (~ 12 vol.%) suffers by the formation of sharp edged primary particles [11], [12]. Thus Fe – TiB₂ based HMS with superior physical properties (specific moduli of 30–35 MPa kg^− 1 m³) can be cost-effectively produced via liquid metallurgy routes [2], [10], [13], [14], [15], but their ductility (8–20% tensile elongation; TE) and strength (UTS below 650 MPa) has been so far unsatisfactory. One strategy to overcome this inverse relationship is an optimisation of the particle's microstructure (i.e. their size, morphology and dispersion) by adapted solidification kinetics [9] or alloying additions [8].

Another pathway – which of course may be combined with the above mentioned particle modifications – is to change the matrix microstructure away from the single phase ferrite (α) employed so far. This is readily enabled by the equilibrium and non-equilibrium phase transformation exploitable in steels, leading to a vast array of microstructures and corresponding mechanical property profiles [16], [17], [18], [19], [20], [21]. Among the multitude of alloying elements available, Mn appears most favourable – at least as a starting point – for HMS matrix design, as it is cost effective, has only slight (and beneficial) effects on the TiB₂ microstructure [8], and allows to achieve a wide range of matrix constitutions [18], [20], [22], [23]. Low concentrations (below ~ 5 wt.% Mn) lead to solid solution strengthened α. Intermediate concentrations (~ 10 wt.% Mn) result in αʹ and ε martensite, which can be coupled with the formation of reverted austenite (γ) during subsequent intercritical tempering, as it is used in medium Mn steels. Above Mn concentrations of ~ 20 wt.% retained γ matrices with decreasing stacking-fault energy (SFE) is obtained, which is utilised for TRIP and TWIP phenomena in high Mn steels [24], [25].

However, such changes in constitution are typically associated with a decrease of the stiffness of the matrix [26], [27], and thus the specific modulus increase achieved by the implementation of TiB₂ may be alleviated or even fully compensated. This highlights the need of establishing a balance between the mechanical and physical materials' performance, and motivates us to systematically study the effect of varying matrix microstructures as the basis for alloy design of HMS as next generation lightweight materials.