High Modulus Steels Research Articles

Effects of TiB2 particles on deformation behavior, softening mechanisms and recrystallization texture of hot-compressed Fe-TiB2 composites

Fe-TiB2 composites, also termed as high modulus steel, offer a promising solution to the challenge of achieving both lightweight and high stiffness materials. However, the presence of TiB2 particles in Fe-TiB2 composites results in poor hot-workability. Therefore, understanding the effects of TiB2 particles on the hot deformation behavior, dynamic recrystallization (DRX), and microstructural evolution of Fe-TiB2 composites is crucial for optimizing their hot-working process. In this study, we elucidated the effects of TiB2 particles on deformation behavior and dynamic softening behavior by conducting a series of isothermal compression tests on as-cast Fe-TiB2 composites and as-cast base alloys (control group) at temperatures of 800–1200 °C, strains of 0.36–1.2, and strain rates of 0.01–1 s⁻1. Using electron backscatter diffraction, we characterized the microstructures of composites and base alloys, showing that TiB2 particles induce a DRX process through particle stimulated nucleation (PSN) at low temperatures and promote continuous dynamic recrystallization (CDRX) at high temperatures. The presence of TiB2 particles have significantly affected the dislocation movement and distribution, which changes the deformation energy distribution and thus facilitates different DRX behaviors under various thermal deformation conditions. Additionally, the microstructure resulting from DRX through PSN exhibits significant texture weakening and grain refinement, presenting a promising method for fabricating ultrafine-grained Fe-TiB2 composites.

Surface hardening of high modulus steels through carburizing and nitriding: First insights into microstructure property relationships

High modulus steels are promising materials for future lightweight design solutions, as their embedded boride particles in a ductile steel matrix increase the stiffness/density ratio. One key requirement for maturing them towards industrial application is their suitability for improving the surface hardness. In this study we investigated the effects of low-pressure carburizing and plasma nitriding on the microstructure and mechanical properties of selected Fe-TiB2- and Fe-Cr-M2B-based high modulus steels. Nitriding resulted with the formation of expanded ferrite in the strongest hardness increase to about 1100 HV0.05 from the alloy system's base hardness of 240 and 500 HV0.05, respectively, albeit with different hardness depth profiles. The Fe-Ti-B alloy indicated deformation phenomena in the ferritic matrix after nitriding, whereas nitriding of Fe-Cr-B-C resulted in a diffusion-controlled particle transformation of M2B borides into CrN nitrides of lower stiffness. Carburizing on the other hand led to a slightly lower maximum hardness value of about 800 HV0.05 over an increased depth for Fe-Cr-B, as martensite and additional M23C6 carbides were formed in the surface zone. The surface hardness of the Fe-TiB2-based alloy could not be increased by the deployed carburization parameters, most likely due to excessive Ti dissolved in the matrix. Consequences for the transfer to engineering applications as well as the refinement of both, thermochemical processing parameters and designated alloy concepts, of high modulus steels are outlined and discussed.

Special Issue on Advanced Metal Cutting Technologies

Cutting technologies are utilized in many industrial sectors, such as automobile, aircraft, and medical-device manufacturing as well as dies and molds. Thus, the requirements for higher geometric accuracy, better surface integrity, and longer component service lifetimes have substantially increased. In addition, maintaining high machining efficiency while simultaneously reducing power consumption and CO2 emissions during machining is important for sustainable development. This special issue features 11 papers on the most recent advances in cutting technologies for metals and composite materials. - Reverse finishing characteristics of drilling surfaces - Machining error simulation for end milling - Visualization of cutting phenomena using a single-crystal diamond tool - Milling of TiB2 particle-reinforced high-modulus steel - Electrodischarge-assisted turning of carbon fiber-reinforced polymers - Suppression of chatter vibration during double-insert turning - Prediction of surface roughness components during turning - Microtome blades for high-precision tissue sectioning - Boiling of coolant near the cutting edge in high-speed machining - Residual stress during drilling of aluminum alloy - Effect of strain hardening on burr control during drilling This issue provides an understanding of recent developments in cutting technologies, aiming to inspire further research in this field. We deeply appreciate the careful work of all the authors and thank the reviewers for their diligent efforts.

Effect of substrate temperature on in-situ precipitation during laser powder bed fusion of Fe-TiB2 high modulus steel

ABSTRACT Fe-Ti-B high modulus steel (HMS) fabricated via laser powder bed fusion exhibits in-situ precipitation of nanostructured TiB2 particles within a ferritic Fe-matrix. However, porosity and cracking are common challenges associated with this process. This study systematically varies process parameters, specifically volume energy density and substrate temperature, to analyse macroscopic defects formation and propose methods to prevent their occurrence through detailed microstructure characterisation. For substrate temperatures of 400, 600, and 800 °C, an optimal combination of laser power and scan velocity was determined, resulting in minimised specimen porosity (< 1%). Yet, pronounced cracking occurred at 400 and 600 °C substrate temperature, most likely attributed to the presence of hard and brittle non-equilibrium microstructure constituents. Increasing the substrate temperature to 800 °C further reduces porosity and promotes the formation of the equilibrium constituents Fe-α and TiB2. These phases are desirable as they improve the stiffness-to-density ratio while reducing hardness and brittleness. By mitigating thermal gradient and resulting lower stresses, the successful fabrication of HMS samples with the desired microstructure and defect-free macrostructures becomes feasible. Potential future steps, such as incorporating in-situ heat treatments between layer depositions, are outlined and discussed as means to lower the substrate preheating temperature.

Precipitation of ceramic particles in Fe-TiB2 and Fe-Ni-TiB2 cast steels during the sub-rapid solidification process

Titanium diboride (TiB2) particle reinforced steels were prepared by a sub-rapid solidification process using the in-situ method. Effects of the Ni concentration on the precipitation of TiB2 ceramic particles were studied and modified in detail. The results show that the mechanically unfavorable-block TiB2-morphologies and the matrix constitutions change with the addition of alloy Ni. The Ni concentration in the iron matrix closes the primary δ-Fe ferrite phase zone, retards the precipitation of the primary TiB2 particles, changes the residual interfacial lattice strain in α-Fe ferrite from tension to compression and promotes the ordered adoption of Fe atoms in (Ti, Fe)B2, which results in the ceramic particles growth into the vermicular-shaped morphologies. Strongly coherent interfaces between a-Fe matrix and TiB2 particles form in Fe-Ni-TiB2 high-modulus steels (HMS) according to the generation of the compression lattice strains at α-Fe ferrite interfaces. The mechanical favorable interfaces are believed to be responsible for the obvious improvements in the mechanical properties, especially the tensile elongation, of Fe-Ni-TiB2 HMS.

Mechanical properties of hybrid fiber reinforced ternary-blended alkali-activated materials

This study employed fly ash, blast furnace slag and steel slag to synthesize ternary-blended alkali-activated materials (AAMs) in order to make full use of the industrial solid wastes and improve the mechanical performance by synergistic effects of the precursors. However, peudo-brittle nature is still the main problem of ternary-blended AAMs. To solve this problem, the effect of hybrid fibers consisting of high-modulus steel (ST) fiber and low-modulus Polyvinyl Alcohol (PVA) fiber on the mechanical properties of the ternary-blended AAMs was evaluated by testing their setting time, flowability, uniaxial compressive strength (UCS), indirect tensile strength (IDT), uniaxial tensile strength (UTS), and three-point bending strength (3PBS). Also, the microstructure of the matrix and fibers were analyzed using a scanning electron microscope. The results show that the flowability and setting time increased with the increasing replacement of PVA fiber by ST fiber. At the PVA/ST fiber volume ratio of 1:1, the AAMs cured for 28 days achieved the highest UCS, IDT, UTS, and 3PBS, which were 32 %, 91 %, 80 % and 114.7 % higher than the AAMs without fiber reinforcement, respectively. The fracture pattern and microstructure illustrated that the best synergistic effect of hybrid fibers was achieved at the PVA:ST fiber ratio of 1:1, at which the small-sized PVA fibers and large-sized ST fibers can inhibit the propagation of micro- and macro-cracks at early and late deformation processes. Furthermore, the ultimate elongation of the composite was improved from 2.62 % to 4.21 % and 5.3 % by modifying the PVA fiber with acrylic polyurethane copolymer and replacing the PVA fiber with polyethylene fiber, respectively, implying the significance of surface hydrophilicity of low-modulus fiber on the ductility of AAMs. This study provided a guide for synthesizing and tailoring the mix design of hybrid-fiber-reinforced ternary AAMs with better strength and ductility.

Feasibility of using contactless electromagnetic cavitation for steel composite manufacturing

This work investigates the feasibility of using contactless electromagnetic sonication for dispersing and refining the strengthening phase in iron and steel for ferrous metal matrix composite production. An oscillating pressure field is generated under superimposed alternating (0.13 T) and steady magnetic fields (up to 8 T). The processing employs a static floating-zone technique and is crucible-free. This approach allows to reach an around 0.8 MPa pressure oscillation amplitude that is sufficient to initiate acoustic cavitation in high melting temperature liquid metals. The viability of different reinforcement dispersions, both ex situ and in situ using this electromagnetic sonication, is explored in the context of the production of oxide dispersion strengthened steel and high modulus steel. Figs 3, Refs 9.

High Modulus Steel Produced by Cold Spray Additive Manufacturing

High Modulus Steel Produced by Cold Spray Additive Manufacturing

Novel multi-metal stainless steel (316L)/high-modulus steel (Fe-TiB2) composite with enhanced specific modulus and strength using high-pressure torsion

A novel multi-metal composite with 316L stainless steel matrix and Fe-TiB2 high-modulus steel reinforcement was successfully fabricated using high-pressure torsion (HPT). After post-HPT annealing at 800 °C for 60 min, the composite demonstrated a superior combination of high specific modulus (~25.5 GPa·cm3·g−1), high yield strength (881 MPa), and good ductility (31%). The improved properties can be ascribed to the high relative density, sound metallurgical bonded 316L/Fe-TiB2 interface, ultra-fine grains and heterogeneous microstructures.

Nanoparticle enabled high performance high modulus steels

Here we present a novel concept that uses a low volume fraction of nanoparticles to nano-treat Fe-Ti-B melts for a new high performance high modulus steel (HMS). Microstructures of the nano-treated Fe-Ti-B HMS were investigated. The nano-treated HMS consists of fine TiB2 particles and a strengthening phase of unexpected ultrafine Fe2B lamellas. While Fe2B phase is traditionally considered to be detrimental in steel making due to its inherent embrittlement effect, this study shows a successful conversion of the detrimental Fe2B phase into an advantageous one. The nano-treated HMS has significantly higher tensile strength compared to its conventionally made counterpart, 950 MPa vs. 510 MPa, while retaining the high Young's modulus, low density, and ductility. This new concept effectively demonstrates the successful structural refinement in Fe-Ti-B system using nanoparticles, without changing the solidification rate, which eliminates the dilemma between achieving enhanced mechanical properties versus manufacturing for mainstream applications and enables the use of stiff HMS for lightweighting of practical applications. A new mechanism of nanoparticle-induced solute enrichment at nanoscale in alloy melts was also proposed and has implications for new manufacturing routes of metals.

On the effect of carbon content and tempering on mechanical properties and stiffness of martensitic Fe–18.8Cr–1.8B–xC high modulus steels

We studied the effects of C concentration (0.1, 0.3 and 0.5 wt%) and tempering temperature (100–700 °C) on the dynamic Young's modulus, mass density, hardness and impact toughness of quenched and tempered martensitic Fe–18.8Cr–1.8B (wt.%) based high modulus steels. Increasing C concentration lead to consistent hardness levels above 600 HV5 for tempering temperatures below 500 °C. While the tensile strength could be adjusted tailored from about 1000 MPa to more than 2000 MPa depending on the tempering temperature and respective C concentration, dynamic Young's modulus and mass density were left virtually unaffected. The E/ρ ratio shows little variability between approximately 31.2–33 GPa g−1 cm3, which is superior to conventional high strength steels and comparable to ferritic Fe–Cr–B steels without carbon additions. Particles identified within the martensitic matrices are mainly (Fe,Cr)2B type borides stemming from solidification, as well as an increasing fraction of (Fe,Cr)23(C,B)6 carbo-borides with increasing C content and tempering temperatures.

On the fatigue crack propagation mechanism of a TiB2-reinforced high-modulus steel

The fatigue crack propagation mechanism of a TiB2-reinforced high-modulus steel fabricated by eutectic solidification process was investigated for the first time. The fatigue crack growth rate (FCGR) of the TiB2-reinforced steel was measured by using in-situ scanning electron microscope (SEM) and it was compared with that of a low carbon ferritic steel. It showed that the FCGR of TiB2-reinforced steel was always lower than that of the low carbon steel. It can be mainly attributed to excellent intrinsic fatigue crack resistance due to improved Young's modulus and the extrinsic reasons ascribed to multiple fatigue crack propagation mechanisms resulting in different crack closure level. In addition, fractographic analysis revealed that almost no interfacial debonding occurred during fatigue crack propagation. It was found that the four types of fatigue crack propagation behavior existed in the studied TiB2-reinforced steel, which contribute to its excellent fatigue crack growth resistance in a synthetical way.

Additive manufacturing of high modulus steels: New possibilities for lightweight design

This work demonstrates the feasibility of fabricating bulk nanostructured high modulus steels in-situ by additive manufacturing. This ideal match of novel processes and alloy concepts opens up new pathways for lightweight design by producing light, stiff, strong and ductile components with minimal geometric restraints. On the example of an Fe – Ti – B alloy, a conventional processing sequence of melting and casting pre-alloys, gas-atomisation and laser powder bed fusion (selective laser melting) led to finely dispersed metastable particle and matrix phases. A simple annealing step transformed them into the desired equilibrium constituents of ductile ferrite (matrix) and light and stiff TiB2 (particles), with only minimal changes in particle size (about 20–150 nm in diameter) and distribution (mainly on the matrix grain boundaries). This nano-scaled composite structure promises an extremely attractive property profile, i.e. an increased stiffness/ratio at elevated strength and without deteriorated ductility. However, the not yet optimized parameters of the laser fusion process led to the formation of few pores and cracks, which prevented the complete assessment of the property profile of the manufactured samples. Material and processing strategies for the further development of this promising lightweight design approach – including the suitability of other powder metallurgy processing routes – are outlined and discussed.

Solidified microstructures and elastic modulus of hypo-eutectic and hyper-eutectic TiB2-reinforced high-modulus steel

The solidified microstructures and elastic modulus of both hypo-eutectic and hyper-eutectic TiB2-reinforced high-modulus steels (HMS) with different TiB2 fractions were investigated. The solidification sequence for hyper-eutectic liquid HMS was as follows: (i) primary Ti(CN) particles, (ii) primary TiB2 particles and iron halos, (iii) first eutectic TiB2 particles having a radial pattern or screw shape and fine TiC particles, (iv) second eutectic TiB2 particles having a lamellar morphology, and (v) Fe3C particles that precipitated in the solid. The screw-shaped and fiber-like TiB2 particles corresponded to two different sectional views of the hexagonal TiB2 tube. Both density and Young's modulus were measured from the hot-rolled HMS having different fractions of TiB2. It was found that density decreased but elastic modulus remained almost constant when the TiB2 fraction increased from 12.1% to 33.3%. The latter is because coarse primary TiB2 particles were formed at the middle of specimen and their aggregates at the top due to the floating up of such a high fraction of TiB2 particles in the experimental condition. Local plastic deformation might be generated near them, leading to the decreased elastic modulus measured for the entire specimen.

On the effect of Ni additions to Fe – Cr – B high modulus steels

We studied the effect of Ni additions up to 15 wt% on microstructure and properties of cast and hot-rolled Fe – 24 Cr – 1.8 B (wt%) steels. The base alloy without Ni addition reached a stiffness/density ratio of 32.7 GPa g−1 cm3, 400 MPa yield strength, 800 MPa ultimate tensile strength and 20% ductility. Increasing the Ni concentration resulted in a change of lattice parameters and the mean average size of the in-situ formed M2B type boride particles. It also altered the matrix from a ferritic towards a martensitic and then austenitic constitution, which led to a decrease in the stiffness/density ratio but an increase in strength. Salt spray testing indicated that Fe – Cr – B – Ni alloys can reach a similar performance against wet corrosive attacks as conventional stainless steels. The results of this systematic study provide the basis for balancing the mechanical, chemical and physical properties, and thereby broadening the property profile of this novel alloy design concept for lightweight design.

Particle-induced damage in Fe–TiB2 high stiffness metal matrix composite steels

Fe–TiB2 metal matrix composites, termed high modulus steels, have great potential for lightweight design applications due to their high stiffness/density ratio. However, the observed embrittlement, caused by the TiB2 particles, critically limits application of these steels. Experimental studies to identify the influence of particle microstructure on ductility and toughness are complex in view of the multitude of parameters affecting microstructural damage. We therefore pursue instead an integrated computational materials engineering approach to gain understanding and derive guidelines for optimizing the particle microstructure and thus improve the mechanical properties, particularly the damage tolerance of these high modulus steels. Key microstructural parameters such as particle clustering degree, size and volume fraction were investigated. Model geometries were statistically and systematically generated with varied particle configurations from random to clustered distributions. Simulations were performed using a crystal plasticity Fast Fourier transformation method coupled with a novel phase field damage model. The influence of particle configuration on damage initiation and evolution was evaluated from the simulation results, and it was observed that microstructures with homogeneous particle distributions of 7 to 15 vol% TiB2, devoid of large TiB2 particles stemming from primary solidification, appear most favorable for obtaining high modulus steels with optimized mechanical properties.

Flexural Performance of Hybrid Engineered Cementitious Composite Layered Reinforced Concrete Beams

This paper presents the experimental investigation to evaluate the flexural performance of newly developed hybrid Engineered Cementitious Composite (ECC) layer at tension zone around the main reinforcement of beam. Four different ECC mixes are used in the beam to evaluate the flexural performance, hybrid ECC based on the low modulus poly vinyl alcohol (PVA) and high modulus steel short random fibre reinforcement. The aim of hybridation is to improve the flexural, energy absorption and ductility performance of reinforced concrete beams. In addition to the compressive strength, young’s modulus, uniaxial tensile strength and bond strength of ECC mixes are determined. ECC with PVA fibre with 2.0% volume fraction mix is kept as reference mix, hybridation is made with PVA (1.35%) and steel (0.65%), PVA (1.00%) and steel (1.00%) and finally with PVA (0.65%) and steel (1.35%). This hybridization has a remarkable achievement in mechanical properties and in the flexural behavior in ECC layered RC beam. From the results, it has been observed that mono fiber ECC reinforced with PVA of 2.0% and hybrid fiber ECC reinforced with 1.35 % of PVA fiber and 0.65% of steel fiber has reasonable flexural characteristics than the conventional beam.

Development of high modulus steels based on the Fe – Cr – B system

We present a novel alloy design strategy for cost-efficient high modulus steels with an increased stiffness / mass density ratio. The concept is based on the liquid metallurgy synthesis of Fe – Cr – B based alloys, straightforward processability, and well tuneable mechanical properties via plain heat treatments. The base alloy Fe – 18 Cr – 1.6 B (wt%) contained 14–17 vol% of (Cr,Fe)2B particles of ellipsoidal morphology in a ferritic matrix. Hot rolled materials revealed a specific modulus of 32.8 GPa g−1 cm3, exceeding that of conventional Fe-Cr steels by almost 30%. Mechanical properties obtained are comparable to TiB2 based high modulus steels. Addition of 1 wt% Cu to the base alloy did not interact with the formation, fraction, size and morphology of (Cr,Fe)2B particles, and allowed to mildly increase the strength values by ageing treatments, however at the price of a reduction of the specific modulus. C additions of 0.2 wt% did not affect the (Cr,Fe)2B particle microstructure greatly, but free C dissolved in the matrix enables for the first time to utilize the wide range of microstructures and mechanical properties of established C-containing high strength steels also in high modulus steels.

Stiff, light, strong and ductile: nano-structured High Modulus Steel

Structural material development for lightweight applications aims at improving the key parameters strength, stiffness and ductility at low density, but these properties are typically mutually exclusive. Here we present how we overcome this trade-off with a new class of nano-structured steel – TiB2 composites synthesised in-situ via bulk metallurgical spray-forming. Owing to the nano-sized dispersion of the TiB2 particles of extreme stiffness and low density – obtained by the in-situ formation with rapid solidification kinetics – the new material has the mechanical performance of advanced high strength steels, and a 25% higher stiffness/density ratio than any of the currently used high strength steels, aluminium, magnesium and titanium alloys. This renders this High Modulus Steel the first density-reduced, high stiffness, high strength and yet ductile material which can be produced on an industrial scale. Also ideally suited for 3D printing technology, this material addresses all key requirements for high performance and cost effective lightweight design.

Strengthening Fe – TiB2 based high modulus steels by precipitations

We systematically studied the microstructure, mechanical and physical properties of hyper-eutectic Fe – TiB2 high modulus steels (20vol% TiB2) with (Si, Mn, Ni) and Cu additions for the formation of G-phase and Cu precipitates during ageing treatments. Alloying with Si, Mn and Ni led predominantly to pronounced solid solution strengthening, reaching tensile strength (UTS) values up to 1100MPa after quenching. While G-phase formation could be observed in aged materials, its preferential formation on grain boundaries significantly deteriorated ductility. Its effects on strength were partially balanced by a decrease of grain boundary density. Additions of 1 and 2wt% Cu, respectively, led to lower strength in the as quenched state, but also to significant strengthening via ageing. The peak ageing conditions as well as the Cu particle structure and size are comparable to values reported for Cu strengthened interstitial free steels and Fe-Cu alloys. Both alloying additions slightly lowered the specific elastic modulus of the HMS, most pronounced for Cu addition with a drop of about 3GPacm3g−1 per wt% and also promoted embrittlement. Microstructure-property relationships and consequences for future alloy design, especially towards more ductile hypo-eutectic HMS, are outlined and discussed.