Influence of Nb on the Microstructure and Fracture Toughness of (Zr_0.76Fe_0.24)_100−xNb_x Nano-Eutectic Composites

Abstract

The present study demonstrates the evolution of eutectic microstructure in arc-melted (Zr_0.76Fe_0.24)_100−xNb_x (0 ≤ x ≤ 10 atom %) composites containing α-Zr//FeZr₂ nano-lamellae phases along with pro-eutectic Zr-rich intermetallic phase. The effects of Nb addition on the microstructural evolution and mechanical properties under compression, bulk hardness, elastic modulus, and indentation fracture toughness (IFT) were investigated. The Zr–Fe–(Nb) eutectic composites (ECs) exhibited excellent fracture strength up to ~1800 MPa. Microstructural characterization revealed that the addition of Nb promotes the formation of intermetallic Zr₅₄Fe₃₇Nb₉. The IFT (K_IC) increases from 3.0 ± 0.5 MPa√m (x = 0) to 4.7 ± 1.0 MPa√m (x = 2) at 49 N, which even further increases from 5.1 ± 0.5 MPa√m (x = 0) and up to 5.9 ± 1.0 MPa√m (x = 2) at higher loads. The results suggest that mutual interaction between nano-lamellar α-Zr//FeZr₂ phases is responsible for enhanced fracture resistance and high fracture strength.

1. Introduction

Eutectic composites (ECs) are extensively studied because their properties are superior to those of monolithic single-phase alloys and single crystals. They have good chemical compatibility and exhibit high strength in ambient atmospheres and at elevated temperatures [1,2]. ECs have a low melting temperature, high castability, a controllable microstructure, and widespread engineering applications. Recently, high strength (2 GPa) and a high plasticity of 20–25% was achieved in Ti–Fe-, Ni–Zr-based lamellar ultrafine eutectic composites (UECs) by tuning the composition of bulk metallic glass (BMG) to synthesize a bulk specimen via arc melting and solidifying at low cooling rates [2,3,4,5,6,7,8,9,10,11,12,13,14,15]. In such cases, a high degree of constitutional super-cooling restricts the growth of eutectic phases within the nanometer scale towards the synthesis of nano-lamellar eutectic composites (NECs) [2,3,4,5,6,7,8,9,10,11,12,13,14,15]. The microstructural features of lamellar composites are crucial for tuning the deformation behavior of ECs. Several NECs, such as Ti–Fe–(Sn) [2,3,4,5,6,7], Ni–Zr–(Al) [8,9,10], Al–Cu–(Sn) [11], Al–Cu–(Al) [12,13], Fe–Nb–(Al/Mn/Ni) [14], Fe–Si–Ti [15], and Fe–Nb–(Al) [15], have been reported to have outstanding fracture strength (>2 GPa) along with large plastic strain (>15% at RT) [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. It has been suggested that enhanced mechanical properties have been obtained through impingement of the propagating shear bands (SBs) and through the impingement of the movement of dislocations along with the pile-up of dislocations at the nano-/ultrafine lamellae interface, which promotes a strong work hardening behavior [14,15,16,17]. The lamellar eutectic provides nucleation of a large number of SBs, which accommodate the applied shear strain at the lamellae interfaces, eutectic colonies and slip band transfer. All the above said mechanisms are responsible for the significant tensile ductility of the eutectic microstructure [8,9,10,11,12,13,14,15,16,17].

Considering a critical survey of the current state of understanding, further exploration in the broad field of the mechanical properties of NECs, particularly in the context of processing, as well as the structure–property relationship seem to be essential. On the other hand, Zr-based alloys possess high-temperature properties, good mechanical and radiation damage resistance, low thermal neutron capture cross section, and excellent corrosion resistance, which makes them useful in nuclear power plant applications and chemical processing plants [17,18,19]. Zr–Fe eutectic composites are very attractive because of their high abundance of natural resources and corrosion-resistant properties [17,18,19]. However, very few studies have been reported on the structure–property relationship and phase formation, the deformation mechanism, and the fracture behavior of the Zr-base lamellar eutectic composites [20,21]. The present work attempts to reveal the microscopic deformation behavior of the Zr–Fe-based NECs. The effect of Nb addition to eutectic Zr₇₆Fe₂₄ on the compressive mechanical properties and the deformation behavior have been evaluated using the indentation fracture toughness method at various loads.

2. Materials and Methods

A series of (Zr_0.76Fe_0.24)_100−xNb_x (x = 0, 2, 4, 6, and 10 atom %) alloy ingots were prepared by arc-melting process in an Ar atmosphere. The arc-melted ingots (AMIs) were re-melted repeatedly at least three times for homogenization. Parallelepiped specimens were cut from the AMIs using electro-discharge machining (EDM). The constituent phases and their structure were identified using a Philips PW3373 PANalytical high-resolution X-ray diffraction unit (XRD, Cu-K_α radiation, Philips, Kassel, Germany). The Vickers macro-hardness (H) of the specimens were measured using Leco LV-700, USA Vickers hardness tester (LECO, Saint Joseph, MO, USA) according to ASTM E-384 standard for a dwell time of 15 s. Indentations at each test load of P_max = 49 N up to P_max = 294 N were conducted for an average hardness H [4,8]. The measurement of IFT is sensitive to surface preparation; therefore, the samples were polished carefully to eliminate the residual stress zones on the specimens’ surface. The IFT values denoted as K_IC was measured on highly polished specimen surfaces, which were free from any pre-cracks, using a Vickers diamond pyramid indenter at P_max values of 49 to 294 N and the Vickers hardness tester mentioned above. K_IC deals with the critical value of stress intensity factor K_I in crack opening mode when fracture initiates and an unstable crack propagates. The K_IC values of Zr–Fe–(Nb) eutectic composites alloys were calculated using an equation proposed by Niihara et al. [22,23]:

$$K_{IC}=0.018(E^{0.4})\left(H^{0.6}\right)\left(l^{0.5}\right)\left(a\right)$$

(1)

$$K_{IC}=0.0123(E^{0.4})\left(H^{0.1}\right){\left(\frac{P}{l}\right)}^{0.5}$$

(2)

where H is the macrohardness in GPa, E is Young’s modulus in GPa, P is the applied indentation load (P_max), a is half of the indentation diagonal length, and l is the Palmqvist crack length, i.e., the extent of the cracks that emerge from the edges of the indentations only. A Leica DM 2500M optical microscope (OM) and a SUPRA 40 field emission scanning electron microscope (FESEM, Carl Zeiss SMT AG, Oberkochen, Germany) equipped with an Oxford ISIS300 energy dispersive X-ray spectrometer (EDS) (Oxford Instruments plc, Oxfordshire, UK) were used to investigate the geometry of the indentation impressions, the median, and Palmqvist crack lengths. The volume fraction of the phases were calculated from the microscopy images. At least five images were used to obtain average values. Compression tests (CTs) were performed at room temperature using a TINIUS Olsen H50KS universal testing machine (TINIUS Olsen, Kolkata, India) at an initial strain rate of 8 × 10⁻⁴ s⁻¹. A detailed microstructural investigation was performed using a JEOL JEM-2100 transmission electron microscope (TEM) (JEOL, Tokyo, Japan). Thin TEM samples were prepared by mechanical polishing followed by ion-beam milling in liquid N₂ using Gatan PIPS691 precision ion polishing system. A 35 DLP Olympus Panametric ultrasonic pulser receiver was used to measure the elastic properties.

3. Results and Discussion

3.1. X-ray Diffraction and Phase Analysis of As-Solidified Composites

Figure 1 shows the XRD patterns for the Zr–Fe–(Nb) with varying Nb content up to x = 10 atom %. The XRD pattern of x = 0 shows sharp diffraction peaks of hcp α-Zr, bcc FeZr₂; however, additional reflections of intermetallic Zr₅₄Fe₃₇Nb₉ (JCPDS #00-046-1095) has been observed from x = 2 [24]. It has been observed that the peak intensity of FeZr₂ phase gradually decreases with the addition of Nb content, whereas the peak intensities of intermetallic Zr₅₄Fe₃₇Nb₉ phase increases with the increase in the amount of Nb. This indicates that the phase fraction of intermetallic Zr₅₄Fe₃₇Nb₉ is higher in the alloys with higher Nb content. Therefore, it may be concluded that the addition of Nb assists in the formation and stabilization of the intermetallic phase Zr₅₄Fe₃₇Nb₉.

3.2. Microstructural Characterization of the As-Solidified Composites

Figure 2a displays the presence of alternate eutectic lamellae of brighter FeZr₂ and dark α-Zr phases in x = 0 as marked by arrows and have been identified by EDS analysis. A few α-Zr dendrites have been noted to be present along with a eutectic matrix. Similarly, the microstructures of x = 2 and x = 4 samples show a eutectic matrix, as shown in Figure 2b,c, respectively.

Table 1. Phase constituents (including their volume fraction) and the corresponding mechanical properties of (Zr_0.76Fe_0.24)_100−xNb_x composites.

Alloys	Phases	α-Zr	FeZr2	Zr54Fe37Nb9	σy (MPa)	σf (MPa)	εf (%)
x = 0	α-Zr + FeZr2	69	31	-	1015	1025	2.1
x = 2	α-Zr + Zr54Fe37Nb9 + FeZr2	74	18	8	1055	1060	2.0
x = 4	α-Zr + Zr54Fe37Nb9 + FeZr2	72	12	16	1510	1510	2.4
x = 6	Zr54Fe37Nb9 + α-Zr	72	-	28	1800	1800	3.7
x = 10	Zr54Fe37Nb9 + α-Zr	61	-	39	1025	1025	1.8

summarizes the constituent phases in different Nb containing composites, as identified using XRD and EDS analyses. A new phase with chemical composition Zr₅₄Fe₃₇Nb₉, (atom %) evolved in the x = 4 sample, and the morphology was modified from the lamellar eutectic to a complex heterogeneous microstructure, consisting of mainly α-Zr and Zr₅₄Fe₃₇Nb₉, as also observed in x = 6 (Figure 2d) and x = 10 (Figure 2e). The volume fraction (vol %) of the phases present in these samples are shown in Table 1. The amount of α-Zr phase remained constant around 69–74 vol % when the Nb content varied from 0 to 6 atom %. The α-Zr showed a decrease when Nb content increased to 10 vol %. On the other hand, the amount of FeZr₂ phase decreased from 31 vol % in the x = 0 sample to 12 vol % in the x = 4 sample and finally disappeared when Nb content increased to 6 atom %. The ternary intermetallic phase is not observed in the sample without Nb content. With the addition of Nb (x = 2), the ternary intermetallic phases form and its fraction increased from 8 to 39 vol % when the Nb content increased from 2 to 10 atom %, respectively. Therefore, the addition of Nb helped to destabilize the FeZr₂ phase and promoted the formation of the Zr₅₄Fe₃₇Nb₉ intermetallic compound, as corroborated by the XRD data.

3.3. TEM Analysis

Figure 3 shows bright field (BF) TEM images of the x = 0 and x = 2 samples, respectively. Figure 3a shows the alternating two-phase lamellar eutectic structure of the x = 0 sample. The darker lamellae were identified as hcp α-Zr, and the brighter lamellae as tetragonal FeZr₂, as deduced from SAED patterns. Similarly, a eutectic microstructure is revealed in Figure 3b for the x = 2 sample. The interlamellar spacing (λ = (λ_α-Zr + λ_FeZr2)/2) was determined by measuring the lowest possible λ values at 10 different locations. The lowest values of λ were measured to be 120 nm, 135 nm, and 300 nm in the x = 0, x = 2, and x = 6 samples, respectively. Therefore, the addition of Nb increases the λ value and causes a coarsening of the microstructure.

3.4. Compressive Deformation Behavior

The compressive engineering stress–strain plots of Zr–Fe–(Nb) NECs as obtained during the uniaxial compression tests (CT) under constrained geometry at room temperature are plotted in Figure 4. The yield strength (σ_y), fracture strength (σ_f), and fracture strain (ε_f) of the investigated NECs are measured and summarized in Table 1. The sample without Nb had a high value of σ_y = 1015 MPa and σ_f = 1025 MPa, with a low fracture strain of ε_f = 2.1%. The stress–strain curves of the investigated NECs show primarily elastic deformation with very limited plastic strain, as evident in Figure 4. Therefore, their σ_y values of all the NECs are close to their σ_f values. The compression test revealed an increase in fracture strength σ_f for the composites with an increase in Nb content. In the x = 2, x = 4 and x = 6 NECs, the measured σ_f was found to be increased up to 1060 MPa, 1510 MPa, and 1800 MPa, respectively. The fracture strain ε_f gradually increased from 2.1% up to 3.7% upon the addition of Nb. A sudden drop in σ_f was observed in the x = 10 NEC, which had a low fracture strength of σ_f = 1025 MPa with a reduced strain of ε_f = 1.8%. The presence of the eutectic microstructure and an increase in the volume fraction of the ternary intermetallic phase improved the mechanical properties of these samples (with increasing addition of Nb). The best mechanical properties were obtained in the x = 6 condition, where there was an optimum mix of α-Zr and Zr₅₄Fe₃₇Nb₉ phases. Further increase in the amount of brittle Zr₅₄Fe₃₇Nb₉ phase degraded the properties of these (Zr_0.76Fe_0.24)_100−xNb_x composites.

3.5. Elastic Modulus Measurement

The density of Zr–Fe–(Nb) composites were measured by the Archimedes principle and are listed in

Table 2. The density (ρ), Poisson’s ratio (ν), Young’s modulus (E), bulk modulus (K), and shear modulus (G) of (Zr_0.76Fe_0.24)_100−xNb_x composites.

Alloys	ρ (g/cc)	ν	E (GPa)	K (GPa)	G (GPa)
x = 0	6.74	0.3787	68	25	94
x = 2	6.82	0.3540	87	32	99
x = 4	6.88	0.3396	97	36	101
x = 6	6.90	0.3396	101	38	105
x = 10	7.03	0.3499	96	36	107

. Nb addition increased the density (ρ) of the composites from 6.74 g/cc (x = 0) to 7.03 g/cc (x = 10). The estimated Poisson’s ratio ν, Young’s modulus E, bulk modulus G, and shear modulus K is summarized in Table 2. The addition of Nb gradually increased the Young’s modulus E from 68 (x = 0) to 101 GPa (x = 6), the shear modulus K from 25 (x = 0) to 38 GPa (x = 6), and the bulk modulus K from 94 (x = 0) to 105 GPa (x = 6), respectively. However, E = 96 GPa and K = 36 GPa was observed in x = 10. Nb addition increased the density of the composites and caused significant changes in their elastic moduli due to the alteration in the phase constituents and subsequently the microstructure.

3.6. Vickers Bulk Hardness

The macro-hardness (H) data at different load P_max values is summarized in

Table 3. Vickers bulk hardness (H), indentation fracture toughness (K_IC), and Palmqvist crack length (l) at different applied P_max values.

Alloys	Pmax (N)	H (GPa)	l (µm)	KIC (MPa√m)
x = 0	49	3.82 ± 0.17	33.09 ± 10	3.02 ± 0.5
	98	3.56 ± 0.10	33.68 ± 5	4.08 ± 0.2
	196	3.54 ± 0.05	64.22 ± 16	4.27 ± 0.6
	294	3.42 ± 0.13	67.43 ± 17	5.05 ± 0.5
x = 2	49	4.44 ± 0.12	18.69 ± 9	4.67 ± 1.0
	98	4.21 ± 0.04	21.70 ± 1	5.70 ± 0.1
	196	4.13 ± 0.04	59.48 ± 19	5.07 ± 1.0
	294	4.20 ± 0.02	65.26 ± 23	5.90 ± 1.0
x = 4	49	5.30 ± 0.07	34.08 ± 14	3.71 ± 1.0
	98	5.00 ± 0.05	87.63 ± 22	3.15 ± 0.7
	196	4.72 ± 0.10	166.27 ± 30	3.12 ± 0.3
	294	4.70 ± 0.05	201.66 ± 50	3.50 ± 0.4
x = 6	49	5.20 ± 0.05	26.38 ± 9	4.18 ± 1.0
	98	5.06 ± 0.05	52.55 ± 17	4.21 ± 1.0
	196	4.93 ± 0.06	82.73 ± 8	4.48 ± 0.2
	294	4.88 ± 0.06	122.4 ± 18	4.52 ± 0.4
x = 10	49	5.06 ± 0.05	34.47 ± 3	3.56 ± 0.8
	98	4.87 ± 0.01	72.60 ± 3	3.33 ± 0.2
	196	4.67 ± 0.04	112.5 ± 4	3.78 ± 0.4
	294	4.70 ± 0.06	144.9 ± 4	4.08 ± 0.4

. Figure 5 shows the variation of H with the addition of Nb. A high value of H = 3.82 ± 0.17 GPa was obtained in the x = 0 sample at P_max = 49 N. However, H further increased up to 5.20 ± 0.05 GPa at P_max = 49 N in x = 6. Thus, H gradually increased upon the addition of Nb, which suggests an increase in composite strength. However, a sudden drop in hardness of 5.06 ± 0.05 GPa was observed in the x = 6 sample at P_max = 49 N, and the intermetallic Zr₅₄Fe₃₇Nb₉ phase had a big impact on the higher hardness values. The high volume fraction of Zr₅₄Fe₃₇Nb₉ was present in the x = 10 sample, as revealed in XRD analysis and SEM micrographs, lowering the H values. Therefore, the H of the investigated NECs depended on the relative volume fraction of α-Zr, FeZr₂, and Zr₅₄Fe₃₇Nb₉, as shown in Table 3.

3.7. Indentation Fracture Toughness

The Vickers IFT measurements refer to a complex state of three-dimensional crack system with significant deformation residual stress and damage around the cracks. In this investigation, l/a data was considered to be in the range of 0.25–2.5. The values of calculated K_IC of the investigated NECs are summarized in Table 3. The length of the cracks extending from the four corners (l) and the size of the indentation diagonal (d) was measured for the desired Palmqvist crack lengths and the size of indentation with an standard error in the range of 2–10%. The IFT measurements were performed at various P_max values within a range between 49 and 294 N in order to study the effect of load variation and Nb content on the K_IC in Zr–Fe–(Nb) NECs. Figure 6 shows the variation of the estimated K_IC measured at different P_max values. K_IC gradually increased from 3.0 ± 0.5 in the x = 0 sample at P_max = 49 N to 4.2 ± 1 MPa√m in the x = 6 sample at P_max = 49 N; thereafter, it dropped down to 3.6 ± 0.8 MPa√m at x = 10 at P_max = 49 N. In addition, the estimated K_IC gradually increased with the increase in applied indentation load P_max. Such as in case of the x = 0 AMI, K_IC increased from 3.0 ± 0.5 √m at P_max = 49 N up to 5.1 ± 0.5 MPa√m at P_max = 294 N. Therefore, the fracture resistance of the investigated composites increased with the increase in Nb content in the NECs up to x = 2. The evolution of intermetallic Zr₅₄Fe₃₇Nb₉ decreased the IFT values. By increasing the Nb up to x = 6 atom %, the fracture strength was enhanced due to the presence of nanolamellar α-Zr and FeZr₂ phases and solid-solution strengthening. However, the presence of homogeneous NECs (x = 0 and 2) with a higher volume fraction of FeZr₂ phase led to IFT values higher than the other Nb-containing specimens.

3.8. Fractrographic Investigation

To understand the IFT behavior of the NECs with the nanolamellar phases, the indented and fractured NECs were further investigated in detail. Figure 7 shows the SEM images of the lateral surfaces of the indented and fractured specimens. Palmqvist cracks were observed emerging from the edges of the indentation impressions. The geometry of Palmqvist cracks at the edges of the indentations at an operating load of 49 N is shown in Figure 7, which shows the SEM micrograph of the indented impression along with the Palmqvist cracks emerged from the edge of the indentation diagonals in the x = 0 and x = 6 NECs. It is interesting to note that the plastic flow lines in the α-Zr phase were observed near the vicinity of the indented plastic zone in the x = 6 samples, while the other phases do not show plastic flow lines. These results suggest that the mutual interaction between lamellar FeZr₂//α-Zr phases is responsible for an enhanced fracture strength and fracture toughness.

4. Summary

A series of eutectic nano-lamellar α-Zr//FeZr₂ eutectic composites was developed in Zr–Fe–(Nb) at low cooling rates (10 K/s). Nb addition resulted in the formation of intermetallic Zr₅₄Fe₃₇Nb₉ and destabilized the FeZr₂ phase. Even though Nb addition coarsened the interlamellar spacing λ, a compressive fracture strength up to 1800 MPa was achieved. On the other hand, Nb addition increased the density, the Young’s modulus, and the hardness of the composite. The fracture resistance of the NECs increased with the increase in Nb from 3.1 ± 0.5 (x = 0) up to 4.7 ± 1.0 MPa√m (x = 2) at P_max = 49 N. These results suggest that the mutual interaction between nanolamellar FeZr₂//α-Zr phases is responsible for an enhanced IFT and a high fracture strength, whereas a higher volume fraction of Zr₅₄Fe₃₇Nb₉ intermetallic phase in x = 10 is responsible for reduced IFT values.