Lupine Publishers| An Alternative Strategy for the Use of a Low-Cost, Age- Hard enable Fe-Si-Ti Steel for Automotive Application

 Lupine Publishers| Modern Approaches On Material Science

Abstract

For High Strength Low Alloy (HSLA) steels or for age-hard enable steels (miraging) the strengthening by precipitation is done before forming operation to increase the yield stress as much as possible. In this publication the advantages of a hardening thermal treatment after forming operation are investigated in a low-cost age-hard enable steel Fe-Si-Ti consistent with automotive application.

Keywords: Age-hard enable, Precipitation, Forming, Ductility

Introduction

The high strength low alloy (HSLA) steels are a group of low carbon steels with small amounts of alloying elements (as Ti, V, Nb, etc.) to obtain a good combination of strength, toughness, and weldability [1-3]. By the addition of micro-alloying elements (less than 0.5 wt.%), HSLA steels are strengthened by grain refinement strengthening, solid solution strengthening and precipitation hardening [4-8]. Regarding automotive applications, the entire range of HSLA steels are suitable for structural components of cars, trucks, and trailers such as suspension systems, chassis, and reinforcement parts. HSLA steels offer good fatigue and impact strengths. Given these characteristics, this class of steels offers weight reduction for reinforcement and structural components [9-11]. Despite the interest of HSLA, the precipitation hardening is about 100MPa. So the new targets concerning the CO₂ emission of vehicles push the steel-makers to develop Advanced High Strength Steels (Dual-Phase, Transformation Induced Plasticity) hardened by multiphase microstructures containing 10 to 100% of martensite [12] which offer a better combination between strength and ductility for an acceptable cost Let’s notice that the Ultimate Tensile Strength (UTS) increases more that the Yield Stress (YS). YS is crucial for anti-intrusive aspect during a crash [12]. In the other side, because there is no phase transformation in Aluminum, age-hardening by precipitation is widely used in Aluminium alloys hardened by various additive elements [see [13] for a review]. The volume fraction of precipitates is several percent’s, the hardening can be increased up to 400MPa, but the ductility decreases rapidly.
In steel, there is only miraging steels which are strengthened by a massive precipitation of a martensitic matrix [see [14] for a review]. Despite the impressive YS (up to 2.5GPa), the uniform elongation is less than 1%. Consequently, no forming operation is possible. In addition, the high contents of nickel (about 18%), of molybdenum and of cobalt make these steels very expensive and so they are never used in automotive industry. On the contrary, in the 60’s authors investigated other kind of steels suitable to be strongly hardened by massive intermetallic precipitation without extracost [15,16]. Among the different investigated system, the Fe-Si-Ti alloys are the most promising. This is the reason why a series of publications has been dedicated to this system in the 70’s by Jack&al. [17-19]. Unfortunately, hardness and compression behaviour have been only reported and a lot of discussions concerned the nature of the precipitates (Fe₂Ti, FeSi, or Fe2SiTi) have been reported. More recently a systematic study of precipitation kinetics in a Fe_-2.5%Si- 1%Ti alloy in the temperature range 723 K to 853 K (450°C to 580°C), combining complementary tools transmission electron microscopy (TEM), atom probe tomography (APT), and Small-Angle-Neutron Scattering (SANS)) have been carried out [20]. It has been shown that the Heusler phase Fe2SiTi dominates the precipitation process in the investigated time and temperature range, regardless of the details of the initial temperature history.

Figure 1: Summary of the different strategies for the use of precipitation hardened steels: The green arrow is the usual one, the orange is the strategy developed in this publication.

Considering that the ductility decreases regularly as a function of the hardening up to 1.2GPa and because it is targeted to obtain a steel suitable for deep drawing the strategy showed in (Figure 1). (orange arrow) has been investigated. The objective is to form a material as soft as possible and as ductile as possible and to obtain the hardening by a thermal treatment after forming. This publication presents the characterization of this way to manage the situation. It is noticed that as Si and Ti promote ferrite, there is no phase transformation whatever the thermal treatment. (Figure 1) Summary of the different strategies for the use of precipitation hardened steels: The green arrow is the usual one, the orange is the strategy developed in this publication.

Results

In order to assess for the first time in steel for automotive applications, the hardening have been chosen followed a thermal treatment at 500°C for two hours consistent with the kinetic determined by SANS [20]. As shown in (Figure 2). illustrating the tensile behavior of the steel consisting only in solid solution (i.e. only after recrystallization) is very ductile as for IF steels but with a higher yield stress due to solid solution hardening. After treatment and hardening of 300MPa is obtained with promising ductility. As shown in (Figure 3) the treatment has induced the expected massive precipitation of Fe2SiTi (3.8% weight percent with a radius of 4nm determined by TEM and SANS [20]). To assess the metallurgical route, the (Figure 4). shows that it is possible to severely bend the steel consisting only in solid solution without any defect up to an angle of 180°. Hardness trough the thickness has been measured before and after the thermal treatment (Figure 5). The value confirms the hardening by precipitation. It is noticed that tis precipitation hardening is not sensitive to the strain hardening induced by bending trough the thickness. Because the alloy is dedicated to automotive industry, the drawing must be investigated. This the reason why a cup obtained by deep drawing of 5cm diameter has been manufactured using the steel consisting in solid solution without any problem as illustrated in (Figure 6). It confirms the very high ductility before heat treatment. One another problem in automotive industry is the increase in spring-back with an increase in strength. In addition, it is very difficult to predict or to model this phenomenon. As shown in (Figure 7). this aspect has been studied by a standard test base the forming of a hat-shaped part. It is highlighted that the spring-back during the treatment is weak. That is probably because there is no phase transformation during precipitation and so no internal stresses.

Figure 2: Tensile curves before and after the ageing treatment.

Figure 3: TEM micrography showing the massive precipitation of Fe₂SiTi after 2 hours at 500°C (the composition have been determined by APT [20]).

Figure 4: Fully bent specimen before heat treatment (bending angle of 180°).

Figure 5: Hardness measurement trough the thickness of the fully bent specimen (i.e. angle of 180°) before and after heat treatment.

Figure 6: Cup drawing before the hardening heat treatment (5cm diameter).

Figure 7: Evaluation of the effect of heat treatment on the spring-back.

Conclusion

For the first time in steel industry for automotive industry a low cost age-hard enable steel has been studied following a strategy based on forming operations before the heat treatment. The bending, the drawability and the spring-back have been investigated highlighting promising results. In addition, the alloy exhibits a cost acceptable for automotive industry. In the future crashworthiness and weldability should be assessed after heat treatment. One of the last advantages is that a lot of parts can be treated at the same time in a furnace usually dedicated to tempering treatment.

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Lupine Publishers| Monitoring Time-Progression of Structural, Magnetic Properties of Ni Nano Ferrite During Synthesis

 Lupine Publishers| Modern Approaches On Material Science

Abstract

We present time-progression of structural, magnetic properties of NiFe₂O₄ nano ferrite during its synthesis via sol-gel auto combustion technique, monitored by x-ray diffraction XRD, and magnetic measurements. XRD patterns of the samples collected between 18-52 minutes shows the formation of the nano spinel phase (grain diameter: 15.4 nm-28.6 nm), presence of a-Fe2O₃phase was also detected. Samples collected between 8-14 minutes show the amorphous nature of the samples. Time-progression studies show: a) sample taken after 20 minutes shows a sharp decrease of specific surface area (range between 39.01 m²/g to 72.73 m²/g), b) non-equilibrium cationic distribution for samples taken between 16-20 minutes with a continuous increase of Fe³⁺ ions population on B-site with simultaneous decrease of Ni²⁺ population, c) for samples taken after 22, 52 minutes, cationic distribution is close to its ideal value of (Fe³⁺) [Ni²⁺Fe³⁺], d) alteration of a degree of inversion (d), oxygen parameter (u), modification of A-O-B, A-O-A, B-O-B super-exchange interactions, e) ferrimagnetically aligned core, and spin disorder on the surface with a thickness between 1.9 nm to 3.6 nm, reducing the saturation magnetization (ranging between 11.7 - 25.5 Am²/kg), as compared to bulk Ni ferrite (55 Am²/kg), f) low squareness ratio values (0.15-0.22) shows the presence of multi-domain nanoparticles, with coercivity between 111-157 Oe.

Keywords: Time-evolution of properties; Sol-gel auto combustion synthesis; XRD; Nano Ni ferrite; Cationic distribution; Magnetic properties

Introduction

Spinel ferrites with general formula Me²⁺O.Fe³⁺₂ O₃, [Me: Divalent metal ion e.g. – Ni²⁺, Zn²⁺, Mg²⁺ Co²⁺ etc.], display face-centered cubic (fcc) structure, with two inter-penetrating sub-lattices: tetrahedrally coordinated (A site), octahedrally coordinated (B site) [1]. Nickel ferrite (NiFe₂O₄) has inverse spinel structure expressed as: (Fe³⁺) [Ni²⁺Fe³⁺] [1]. Allocation of cations on A, B site is crucial in determining properties of spinel ferrites [2,3], can be effectively used to achieve desired properties. Literature gives Ni ferrite synthesis using various methods including mechanical milling [4], coprecipitation [5], hydrothermal synthesis [6], sol-gel auto combustion method [7], showing the effect of the technique on structural, magnetic properties. Literature also reports real-time monitoring (in-situ studies) of properties [8,9], require special, sophisticated equipment, may not be available in all laboratories. Ex-situ monitoring of properties [10], describing the time-evolution of structural, magnetic properties, is a rather simple, more convenient way to perform experiments by utilizing standard laboratory equipment available in many laboratories. Ni ferrite is used in magnetic resonance imaging (MRI) agents [5], photocatalysis for water purification, antimicrobial activity [11], etc.) hence tuning its properties are preferred for improved efficiency.So, in this work, we present the time-development of structural, magnetic properties of NiFe₂O₄ nano ferrite during its synthesis via sol-gel auto combustion technique. Prepared samples are investigated via x-ray diffraction 'XRD,' vibration sample magnetometry, to get complimentary information on structural, magnetic properties.

Experimental Details

NiFe₂O₄ ferrite samples were synthesized by the sol-gel auto-combustion protocol, as described in detail in [12], by utilizing AR grade -nitrate/acetate-citrate precursors: Nickel acetate - Ni(CH₃CO₂)₂·4H₂O, Ferric nitrate (Fe(NO₃)₃.9H₂O), Citric acid - C₆H₈O₇]. The precursors were mixed in the stoichiometric ratio, were dissolved in 10 ml de-ionized water by keeping metal salts to fuel (citric acid) ratio as 1:1. At the same time, the solution pH was maintained at 7. Now the solution was heated at ~110 ̊C. As dry gel starts to form (taken as 0 minutes) small part of the sample is taken out from the reaction vessel (in an interval of 8, 10, 12, 14, 16, 18, 22, and 52 minutes), and were immediately ice-quenched to room temperature. Powder samples were used for Cu-K- X-ray diffraction 'XRD' measurements (Bruker D8 diffractometer), hysteresis loops by vibrating sample magnetometer. Full-profile XRD analysis was done by MAUD Rietveld refinement software [13] to obtain the lattice parameter (apex.). XRD analysis gives Scherrer's crystalline size D (calculated by the integral width of 311 peak, corrected for instrumental broadening), specific surface area (S), inversion parameter (d), oxygen parameter (u). XRD data was also analysed to get cationic distribution via Bertaut method [14], This provides cationic distribution by comparing experimental and computed intensity ratio of planes I(220)/I(400) and I(400)/I(422), susceptible to cationic distribution [12]. Cationic distribution was used to calculate theoretical or Néel magnetic moment at 0K (Ms(th)), theoretical lattice parameter (ath.), bond angles (θ₁, θ₂, θ₃, θ₄, θ₅) as shown in [3]. Coercivity (Hc), saturation magnetization (Ms), remanence (Mr), squareness ratio (Mr/Ms) was obtained from hysteresis loops. (Figure 1) gives the schematic of sample synthesis and characterization.

Figure 1: Schematic of sample synthesis and characterization.

Results and Discussion

(Figure 2) (a) gives XRD-patterns of the studied NiFe₂O₄ samples collected after 18, 20, 22, and 52 minutes, confirm the formation of the spinel phase. XRD patterns also show the presence of a-Fe2O3 phase, ascribable to sample synthesis at a reasonably lower temperature (~110̊C), as reported in [15], while its disappearance is seen after higher sintering temperature. Figure 1(a) inset shows XRD patterns of samples collected after 8, 10, 12, 14 minutes show the amorphous nature of the samples. Only in the sample collected after 14 minutes, there is the start of spinel phase formation (indicated by a dotted circle). Illustrative Rietveld refined XRD pattern (Figure 2) b) of NiFe₂O₄ sample taken after 20 minutes also validates the cubic spinel ferrite phase formation. (Figure 2)(c) shows a variation of D (range between 15.4 nm to 28.6 nm) and S (range between 39.01 m²/g to 72.73 m²/g) for NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes. A perusal of (Figure 2) (c) shows a well-known inverse relationship shown by the expression: [S = [6/(D ´r_XRD)], where r_XRDis x-ray density, as was also reported in[2]. (Figure 2) (c) shows that for samples taken after 22, 52 minutes, D sharply increases with concurrent reduction of S, is ascribable to significant changes in cationic distribution via migration of Ni²⁺ions to B site with simultaneous migration of Fe³⁺ ions on A site(as can be seen in Table 1). (Figure 2 )(c) inset display linear relation between d and u as was also observed earlier [3], shows that reduction of the degree of inversion (d) leads to a reduction of oxygen parameter (u), a measure of disorder in the studied system, is expected to affect the properties of the studied samples. Table 1 depicts the variation of experimental and theoretical lattice parameter (a_exp., a_th. ), inversion parameter (d), oxygen parameter (u), Cation distribution (for A, B site), and calculated, observed intensity ratios for I400/422, I220/400 plane for the studied samples. The observed variation of a_exp. is consistent with changes in cationic distribution, and variation of the degree of inversion (d). Close agreement between observed, calculated a_exp., a_th. suggests that the computed cationic distribution agrees well with real distribution [16]. Close matching of calculated, observed intensity ratios for I400/422, I220/400 signifies an accurate cationic distribution among A, B site [17]. Cationic distribution illustrates that as we go from NiFe₂O₄ samples taken after 16, 18, and 20 minutes, the population of Fe³⁺ ions on B site increases from 1.2 to 1.5 with a concurrent decrease of Ni²⁺ ions from 0.80 to 0.50. For samples taken after 22, 52 minutes Fe³⁺ population on B site decreases, while Ni²⁺ ion population increases up to 0.98, which is close to the ideal inverse cationic distribution of (Fe³⁺) [Ni²⁺Fe³⁺] [1].

Figure 2: (a): XRD patterns of the studied NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes showing the formation of the spinel phase. Inset: XRD patterns of the studied samples taken after 8, 10, 12, 14 minutes. (b): Illustrative Rietveld refined XRD pattern of NiFe₂O₄ sample taken after 20 minutes (* - Experimental data, Solid line - theoretically analyzed data, |- Bragg peak positions, Bottom line- Difference between experimental, and fitted data). (c) variation of grain diameter (D) and specific surface area (S) for NiFe₂O₄samples taken after 16, 18, 20 22, 52 minutes. Line connecting points guide to the eye. Inset: variation of inversion parameter (d) with oxygen parameter (u). The straight line is a linear fit to the experimental data.

Figure 3 depicts the variation of bond angles between cations, cation-anion q₁, q_2,q₃, q₄and q₅, for the studied samples taken between 16 - 52 minutes. In samples taken after 8, 10, 12, and 14 minutes, due to the absence of the spinel phase, bond angles could not be computed. Bond angles provide information on super-exchange interaction (A-O-B, A-O-A, B-O-B), mediate by oxygen. (Figure 3) shows that for samples taken after 16, 16, 20 minutes q₁, q₂, q₅, decreases while q₃, q₄increases, indicates a weakening of A–O–B, A– O–A and strengthening B–O–B super-exchange interaction as is also observed earlier [16]. For samples taken after 22, 52 minutes q₁, q₂, q₅, increases, and q₃, q₄decreases reveals strengthening of A-O-B, A-O-A, and weakening of B-O-B super-exchange interaction, reported in the literature with compositional changes [3]. Samples taken after different times, there is a modification of A-O-B, A-O-A, B-O-B super-exchange interactions, are attributed to changes in dand u as shown in(Table 1), observed with compositional changes [3,16]. Observed A-O-B, A-O-A, B-O-B super-exchange interactions should mirror in magnetic properties, matches well with reported literature [3,16]. Thus, collecting samples after different times during synthesis is analogous to compositional changes in spinel ferrites, affects structural, magnetic properties [3, 12, 16, 18].

Figure 3: Dependence of bond angles (q₁^A-O-B,q₂^A-O-B, q₃^B-O-B,q₄^B-O-B, q₅^A-O-A) for NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes. Line connecting points guide to the eye.

Figure 4 depicts hysteresis loops, reveal changes in Ms_(exp.)samples taken after 16, 18, 20 22, 52 minutes, attributable to alteration of B-O-B, A-O-B, and A-O-A interaction, depends on bond angles, as shown in (Figure 3), and cationic distribution, as shown in (Table 1). (Figure 4) inset displays hysteresis loops of the samples taken after 8, 10, 12, 14 minutes, showing very low magnetization, attributable to the fact that in these samples ferrite phase is not formed, as was also observed in XRD data shown inset of (Figure 2) (a). Observed lower values of M_s(exp.) (ranging between 11.7 - 25.5 Am²/kg) as compared to the multi-domain bulk Ni ferrite (55 Am²/kg) is attributed to the two-component nanoparticle system as described in [19]consisting of a spin-disorder on the surface layer and ferrimagnetically aligned spins within the core. Computed magnetic dead layer thickness as described in [20,21]for NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes are respectively 2.3, 1.8, 1.9, 2.5 and 3.6 nm. They confirm the contribution of 'dead layer thickness' in the reduction of M_s(exp.), apart from B-O-B, A-O-B, and A-O-A super-exchange interaction and cationic distribution.

Figure 4: Hysteresis loops of the studied NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes. Inset: Hysteresis loops of the samples taken after 8, 10, 12, 14 minutes.

Table 1: Variation of experimental and theoretical lattice parameter (a_exp., a_th.), inversion parameter (), oxygen parameter (u), Cation distribution (for A, B site), and observed, calculated intensity ratios for I400/422, I220/400 plane for the studied samples.

Figure 5(a) depicts Variation of M_s(exp.), M_s(th.)for NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes. A perusal of figure 5(a) shows that observed behaviour is attributable to alteration of B-O-B, A-O-B, and A-O-A super-exchange interaction, depends on bond angles (see figure 3), and cationic distribution (see Table 1). Non-similar trend of M_s(exp.), M_s(th.)in (Figure 5)(a), shows that the magnetization behaviour is governed by Yafet-Kittel three sub-lattice model, described in [22], confirmed by the computed canting angle (a_Y-K) values for NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes, which are respectively 52.7, 56.6, 46.2, 55.7, 46.9 ̊. The canting angle provides information on spin canting on the surface, is so-called 'magnetic dead layer,' leads to a reduction of M_s(exp.), which is lower than bulk saturation magnetization of Ni ferrite (55 Am²/kg). Inset of Figure 5 (a) shows the variation of M_s(exp.) with oxygen parameter 'u'(which is a measure of disorder in the samples [1]). Figure 5 (a) shows the disorder-induced enhancement of M_s(exp.),as was also reported in [3]. (Figure 5) (b) depicts the Coercivity(H_c) variation for NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes. Obtained H_cand related Dvalues imply that studied samples lie in the region with overlap between single or multi-domain structures, as reported earlier [3]. (Figure 5) (b) Inset depicts the variation of Mr/Ms for NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes. Mr/Ms values ranging between 0.15-0.22 reveal enhanced inter-grain interactions suggesting isotropic behavior of the material [23] reveal multi-domain particles with no preferential magnetization direction. Time-dependent tunable structural, magnetic properties during synthesis are valuable in achieving optimal properties of Ni ferrite for their usage in magnetic resonance imaging [5], hyperthermia [24] for cancer treatment, photocatalysis for water purification [11].

Figure 5: (a) Variation of M_s(exp.), M_s(th.)for NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes. Inset: Dependence of M_s(exp.)on oxygen parameter (u), line connecting points in Inset are linear fit to the experimental data.; (b) Coercivity(H_c) variation for NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes. Inset: Variation of M_r/M_sfor NiFe₂O₄ samples taken after 16, 18, 20 22, 52 minutes.

Summary

To summarize, the sol-gel auto combustion technique is used to observe the time-development of structural, magnetic properties of Ni ferrite. Changes in cationic distribution lead to modification of structural properties, magnetic interactions, responsible for observed magnetic properties. Time-progression of properties are of use to alter structural, magnetic properties of Ni ferrite as a material for its prospective usage in heterogeneous catalysis, water purifications, biomedical applications.

Acknowledgments

Authors thank Dr. M. Gupta-L. Behra, UGC-DAE CSR, Indore for XRD measurements. Work supported by UGC-DAE CSR, Indore project (No.: CSR-IC-ISUM-25/CRS-308/2019-20/1360, dated March 5, 2020).

Conflicts of Interest

Author

Author Contributions

Conception: SNK; Sample synthesis, RV, SNK; Measurements, analysis of data: SNK, RV; Supervision, Resources, supervision, project management: SNK; Writing the manuscript: SNK, RV. All authors approve the draft and participate in reviewing.

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