Combinatorial screening of the microstructure–property relationships for Fe–B–X stiff, light, strong and ductile steels
Graphical abstract
Introduction
Weight reduction is a major technical challenge for structural material design. An ideal pathway is blending strong and ductile metallic matrices for optimised mechanical performance with stiff and low-density ceramic particles for improved physical properties, thus creating metal matrix composites [1], [2], [3], [4], [5], [6]. Key material parameters are a high Young's modulus (E) for improved stiffness, high yield strength (YS) to allow for higher loads to be transferred, satisfactory ductility (such as tensile elongation TE) for forming operations during manufacturing, and a low density (ρ). High modulus steels (HMS; as the most common acronym for iron (Fe)-based composites) are especially attractive, as Fe exhibits a similar specific modulus (E/ρ) as established lightweight materials such as for example aluminium, magnesium or titanium alloys (about 26 GPa g− 1 cm3), but also a wide range of achievable mechanical properties due to its multitude of equilibrium and non-equilibrium phase transformations and low production costs [7].
Selecting suited particle phases however – from intrinsic properties alone – is difficult, as possible candidates are numerous and range from carbides to nitrides, oxides, intermetallics and borides [1]. Besides high efficiency (i.e. a high specific modulus), other aspects of critical importance are e.g. their thermodynamic stability (to prevent dissolution in the matrix), formation kinetics (possible floatation of low density particles forming in the liquid), interfacial properties (to ensure wetting of particles), as well as the availability and costs of the constituting elements. Furthermore, in-situ formation of such particles during synthesis greatly simplifies the production of such composite materials. All these factors are important for liquid metallurgy synthesis, and thus of key relevance for efficient mass production. When reviewing the wide range of stiff and low density particles under these engineering constraints, only a limited number of possible phases remain: Diamond for example has one of the highest specific moduli, but cannot be synthesised in-situ and will dissolve in Fe when added ex-situ. By contrast nitrides or oxides have high thermodynamic stability, but as they form rapidly in the melt and are typically of low mass density, they float and form a slag instead of rendering dispersed in the solidified material [8]. Carbides are typically less effective, and detrimental for both the mechanical properties (through preferential precipitation at grain boundaries) and the melt viscosity (formation in the liquid phase) [4].
Borides, on the other hand, fulfill most of the above listed criteria for HMS design. Specifically the Fe–Titanium diboride (TiB2) system has been intensely investigated [4], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], as TiB2 has a high specific modulus (~ 125 GPa g− 1 cm3) [26] and can be precipitated in-situ from a homogeneous Fe–Ti–B melt in a pseudo-binary eutectic reaction [9], with excellent interfacial properties and sufficient mechanical compliance with the matrix [11], [18], [23]. An Fe–10 vol.% TiB2 alloy for example typically exhibits a specific modulus of about 30 GPa g− 1 cm 3 at 400 MPa and ~ 30% TE [27], [28]. The increased costs associated with the comparatively large quantities of Ti required (about 5 wt.% for the above mentioned alloy) can be decreased by the in-situ reduction of Ti-oxides through aluminium additions in the melt [8]. More problematic is the pronounced embrittlement observed with increasing TiB2 fractions. This is caused by the unfavourable particle microstructure once the eutectic TiB2 concentration (about 12 vol.% [20], [25]) is exceeded and formation of large (several μm2) primary particles in addition to the already sharp edged eutectic TiB2 lamellae occurs. While the particle morphology can be successfully controlled by alloying additions and/or tailored solidification kinetics, leading to excellent property profiles [22], [24], [27], these measures further increase the associated efforts and costs, and thus may slow down the application of HMS as the next generation lightweight material on a broad scale.
However, as HMS still represent a relatively novel class of structural materials, the potential of other Fe–boride systems has not yet been investigated and exploited. Compared to the more thoroughly studied Fe–TiB2 based HMS, these alternative alloy systems may offer more effective boride particles (so that lower particle fractions are required for the same gain in properties), and/or improved boride microstructures even with established alloying and processing routes (allowing for improved mechanical performance at similar particle fractions). Identifying the most suitable Fe–B–X system for the design of HMS from existing literature alone is difficult though, for the following reasons: (i) The thermodynamics of the Fe-rich corners of ternary Fe–B–X systems are often not fully understood, thus making it difficult to reliably predict which phases are stable for a specific alloy composition and temperature [29]. (ii) Depending on the alloy system, a multitude of equilibrium phases occurs, with often closely spaced compositional ranges. In the Fe–B–Cr system for example, 9 binary and 6 ternary borides have been reported, not even counting metastable phases which are out of thermodynamic equilibrium [30]. (iii) Apart from the difficulties in predicting the formation of specific phases, data concerning their intrinsic properties (such as E and ρ) is rather scarce. Additionally, almost no information exists on what kind of particle microstructures (i.e. morphology, size and dispersion of the borides) will result for different processing conditions. Hence the prediction of the physical and mechanical performance of such an HMS alloy, i.e. its specific modulus, strength and ductility, is virtually impossible.
An experimental approach, i.e. screening the constitution, microstructure, mechanical and physical properties of ternary Fe–B–X systems, on the other hand, is extremely time consuming in view of the large number of possible alloy compositions and thermomechanical processing parameters, even with novel high throughput bulk metallurgical techniques [31]. Thin film combinatorial techniques may be substantially faster, but the correlation lengths of the property-dominating microstructural features (grain size, crystallographic texture, precipitate dispersion and topology etc.) typically exceed the dimensions accessible in thin films. It is therefore of high interest to use an alternative experimental strategy to efficiently obtain insight into which ternary Fe–B–X system has the highest potential for future HMS alloy design.
Section snippets
Objective and approach
The aim of this study is to evaluate ternary Fe–B–X alloy systems for the design of stiff, light, strong and ductile HMS. In view of the above listed difficulties with identifying the most suitable system exclusively from literature data, we follow here a property driven approach, i.e. first producing material based on the available data, then evaluate its mechanical and physical properties, followed by investigation of microstructure and constitution. This allows us to efficiently provide
Materials and methods
All alloys of this study are of the composition Fe–10 B–5 X (at.%; X = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W). This represents a hypo-eutectic concentration for the reference alloy with Ti, which is an optimum benchmark for judging the performance of the other material systems, as it does not undergo strong embrittlement through the formation of coarse primary particles, and represents the current state of the art in HMS design. Charges of ~ 600 g were synthesised by Arc melting pure materials (> 99.99%)
Physical and mechanical properties
The E and ρ data obtained for the Fe–10 B–5 X alloys are plotted in Fig. 1. In the as-cast state (Fig. 1a), the Mo alloyed samples exhibited with 221 GPa the lowest E values (black rhombi), and the Nb alloyed material with 246 GPa the highest ones. The bulk mass density ρbulk (blue circles) was found to be lowest for the Ti alloyed materials (7.38 g cm− 3) and highest for Ta-alloyed samples (8.28 g cm− 3). The ρflakes values (empty red circles) were almost identical to the ρbulk data except for Nb, Ta
Microstructure–property relationships of the different Fe–B–X alloys
We successfully screened the stiffness, density, strength and ductility of Fe–10 B–5 X alloys (at.%; X = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W ) after liquid metallurgy synthesis and thermo-mechanical processing. Our results allow for the first time to systematically evaluate and compare their microstructures and resultant properties, and thus to gain a first insight into their applicability for the alloy design of novel generations of lightweight structural materials. The reference Ti alloyed
Summary and conclusions
We systematically screened the mechanical, physical and microstructural properties of nine Fe–10 B–5 X alloy systems (at.%; X = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W), all synthesised and processed by standard liquid metallurgical techniques. As precise knowledge about the forming boride phases and their intrinsic properties is rather incomplete, we chose to follow a property driven approach in order to identify the most suitable systems for the alloy design of metal matrix composite steels as next
Acknowledgements
M. Kulse, B. Breitbach and L. Christiansen are gratefully acknowledged for their support with synthesis, XRD analysis, data evaluation and metallographic preparation.
References (46)
- et al.
Efficient liquid metallurgy synthesis of Fe–TiB2 high modulus steels via in-situ reduction of titanium oxides
Mater. Des.
(2016) - et al.
A comparison of the reciprocating sliding wear behaviour of steel based metal matrix composites processed from self-propagating high-temperature synthesised Fe–TiC and Fe–TiB2 masteralloys
Wear
(2002) - et al.
Synthesis and characterization of TiB2-reinforced iron-based composites
J. Mater. Process. Technol.
(2006) - et al.
Interface structure and chemistry in a novel steel-based composite Fe–TiB2 obtained by eutectic solidification
Acta Mater.
(2012) - et al.
Microstructural study and densification analysis of hot work tool steel matrix composites reinforced with TiB2 particles
Mater. Charact.
(2013) - et al.
Interfaces and defects in a successfully hot-rolled steel-based composite Fe–TiB2
Acta Mater.
(2015) - et al.
Microstructure refinement for high modulus in-situ metal matrix composite steels via controlled solidification of the system Fe–TiB2
Acta Mater.
(2015) - et al.
Damage mechanisms of a TiB2-reinforced steel matrix composite for lightweight automotive application
Metallurgical and Materials Transactions E
(2016) - et al.
Improving the mechanical properties of Fe–TiB2 high modulus steels through controlled solidification processes
Acta Mater.
(2016) - et al.
Effects of Mn additions on microstructure and properties of Fe–TiB2 based high modulus steels
Mater. Des.
(2016)
Rapid alloy prototyping: compositional and thermo-mechanical high throughput bulk combinatorial design of structural materials based on the example of 30Mn–1.2C–xAl triplex steels
Acta Materialia
Bulk combinatorial design of ductile martensitic stainless steels through confined martensite-to-austenite reversion
Mater. Sci. Eng. A
Combinatorial design of transitory constitution steels: coupling high strength with inherent formability and weldability through sequenced austenite stability
Mater. Des.
Solidification paths in the iron-rich part of the Fe–Ti–B ternary system
J. Alloys Compd.
Elastic constants of AlB2-type compounds from first-principles calculations
Comput. Mater. Sci.
A new metastable phase in Fe–Nb–B system
J. Alloys Compd.
A review of recent research on mechanics of multifunctional composite materials and structures
Compos. Struct.
High modulus steels: new requirement of automotive market. How to take up challenge?
Can. Metall. Q.
Particulate reinforced metal matrix composites — a review
J. Mater. Sci.
Particle-reinforced aluminum and magnesium matrix composites
Int. Mater. Rev.
Strengthening of Steels by Ceramic Phases
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