Microstructure, Mechanical Properties, Deformation Behavior, and Crystallographic Texture of the Al-Gd-Cr-Ti Quaternary Alloy for Thermal Neutron Absorption

Abstract

In this work, we report the identification of a novel quaternary intermetallic phase (Al₂₁GdCrTi) formed during the solidification of a novel Al-Gd-Cr-Ti alloy, which has not been previously documented in the literature to the best of our knowledge. The study also provides a detailed analysis of microstructure evolution, texture behavior, and the mechanical strengthening effect of rolling processes, along with neutron absorption performance. XRD analysis reveals that the intensity of (022), (113) planes of the as-hot-cold-rolled sample is higher than that of the as-cast due to the change in the direction of some grains in these planes during rolling. The results indicate that the studied alloys scatter neutrons about 100 times less than a nearly pure aluminum alloy. The hardness of the as-cast alloy increased from 36 to 53 HV after cold rolling and to 50 HV after hot rolling-cold rolling. Hot-cold-rolled alloy has a yield strength of 160 MPa and an ultimate tensile strength of 181 MPa, while maintaining an elongation of 11.3%. The studied alloys, containing 4.2 wt.% of the alloying elements 3.8Gd, 0.2Cr, and 0.2Ti (Al-3.8Gd-0.2Cr-0.2Ti), exhibited a yield strength 28 MPa higher than those containing 21 wt.% of the alloying elements 5Cu, 6Gd, and 8Bi (Al-5Cu-6Gd-8Bi). The studied alloys form the basis for the development of high-technology Al-Gd alloys for neutron shielding.

1. Introduction

Materials utilized in designing applications can be strong or ductile, but they are seldom consistent with each other. Numerous auxiliary applications, such as transportation, spacecraft, and nuclear plants, require stronger and tougher materials for more noteworthy vitality, productivity, and progressed performance to meet the wants of the industry and its progressing advancement [1,2]. From past information derived from literature readings, a weak lattice is fortified with a dispersion hardener, the second phase of crystallization origin, or solid solution strengthening [3]. The more imperative address is whether there are unused techniques, however, to be investigated to form the following era of metals and alloys with a “quantum leap” in strength and ductility, rather than the conventional improvements we have seen over the past century.

One of the most important materials used in the automotive, aerospace, and other industries is aluminum alloys due to their resistance to high temperatures and their mechanical properties [2,4,5,6,7,8]. Some researchers have recently suggested that it is possible to enhance tensile strength, heat resistance, and corrosion resistance by adding rare earth elements to aluminum alloys [9,10,11,12,13]. Al-Gd alloy may be a ternary, quaternary, or higher composition that incorporates copper, magnesium, silicon, zirconium, and manganese [4,14]. The aluminum and gadolinium may be primary ingredients (e.g., components) of aluminum alloys. Al may function as a solid solution matrix in the aluminum alloys, and Gd may function as a eutectic structure with Al [15]. The Al-Gd phase diagram shows that the solubility of Gd in aluminum is very low, and it mainly forms the second phases Al₃Gd, Al₂Gd, and AlGd along the grain boundaries. Moreover, Gd forms a variety of phases ideally with Al-Gd alloy elements [16]. By including 0.13 Gd and 0.07 Y wt.% to the Al-Zn-Mg-Cu-Zr alloy, the dispersed Al₃CuGd altogether progressed the microstructure and anticipated advanced recrystallization, resulting in changes in both strength (639 MPa) and elongation (16.8%) [17].

Crystallographic texture studies currently cover polycrystalline materials of various natures, namely metals, alloys, ceramics, rocks, and biological objects [18,19,20]. In metallurgy, the term texture is used to refer to the preferred crystal orientation in polycrystalline aggregates [21]. The study of material texture has become essential due to its significant impact on evaluating the properties and engineering of materials over the past few decades [22,23]. The fatigue behavior of 2xxx aluminum alloys is significantly affected by different textures, including crack initiation, closure, and propagation. Rolling is a complex thermo-mechanical process, and especially during hot rolling, deformation is always accompanied by recrystallization, which also improves the structure and texture [24,25,26]. The texture development of aluminum alloys during the rolling and annealing process has been a subject of research for a long time, as the anisotropy of mechanical properties is greatly affected by the texture developed during processing [27,28,29,30,31]. Thermal neutron absorption is essential in reactors such as space and vehicular reactors used in engineering applications [15]. In these systems, a significant portion of the mass is attributed to radiation-absorbing materials. In viable applications, reactor working temperatures within the fundamental coolant lines ordinarily extend between 280 and 320 °C in long-term benefit [32]. Gadolinium (Gd) is an element known for its high thermal neutron absorption cross section (48,800 b). In any case, adding a high percentage of Gd to aluminum alloys results in the precipitation of excess Gd atoms at grain boundaries, forming an intermediate phase known as Al₃Gd [33].

The main idea of this study is to analyze and evaluate the microstructure, crystallographic texture, and mechanical properties of Al-Gd-Cr-Ti alloys for thermal neutron absorption.

2. Materials and Methods

Al-3.8Gd-0.2Cr-0.2Ti alloy was prepared by casting metallurgy in a graphite crucible in an electric resistance furnace, poured into a copper water-cooled mold with a cavity size of 20 × 40 × 120 mm³, with a cooling rate of about 15 Ks⁻¹. The raw materials for casting were pure Al grade A99 and master alloys of Al-12Gd, Al-10Cr, and Al-5Ti. The melt temperature before casting was 800 °C. The nominal composition of the Al-Gd-Cr-Ti alloy is shown in

Table 1. Chemical composition of the studied alloy, wt.%.

Alloy	Gd	Cr	Ti	Al
Al-Gd-Cr-Ti	3.8	0.2	0.2	bal.

.

The solidus temperature is determined based on the differential scanning Calorimeter (DSC) results. The experiments were performed in an air atmosphere with a heating and cooling rate of 5 °C/min. X-ray diffraction (XRD) data (using Cu-Kα radiation) were collected at the laboratory of FLNP, JINR (Dubna, Moscow region, Russia) using an Empyrean (Malvern PANalytical, Dubna, Moscow, Russia) (Figure 1a). Evaluations were carried out on specimens with a plane surface in reflection geometry. Prior to the evaluation of the texture experiment, X-ray phase analysis was conducted utilizing the High Score Plus software package, version 4.1 (PANalytical, Dubna, Moscow, Russia), and the PDF-2 database. The X-ray scanning parameters are X-ray tube with nickel-filtered copper Kα radiation (λ = 1.54 Å), voltage 40 kV, current 40 mA, and total exposure time 33 min. The X-ray texture experiment is the most common type of texture experiment and is used for mass texture studies [34,35,36]. X-ray pole figure measurements are based on diffraction that occurs when X-rays are scattered by the electron shells of atoms in a crystal lattice. The neutron diffraction used in this research was carried out at the laboratory of FLNP, JINR (Dubna). We utilized the SKAT (from Russian: Spektrometer Kolitshestvennogo Analiza Tekstury, Spectrometer for the Quantitative Analysis of Textures) setup situated at the 7A-2 beamline of the high-flux pulsed nuclear reactor IBR-2 (Figure 1b). Spektrometer Kolitshestvennogo Analiza Tekstury (SKAT) is composed of a detector ring (2 m in diameter), on which are pinpointed 19 detector-collimator complexes at the same scattering angle 2θ = 90°. Keppler et al. [37] reported that the long flight path of over 100 m and the collimator system supply a relatively high resolution of Δd/d = 5 × 10⁻³ at d = 2.5 Å and 2θ = 90°.

Homogenization of Al-Gd-Cr-Ti alloy was carried out at 615 °C for 1h, followed by quenching in water. Heat treatment of the Al-3.8Gd-0.2Cr-0.2Ti ingot was carried out in Nabertherm and SNOL furnaces with a fan and a temperature control accuracy of 1 °C. The heat-treated ingot was rolled utilizing two diverse frameworks. To begin with, it was cold rolled from a thickness of 20 to 1 mm, moment hot rolled from a thickness of 20 to 10 mm at 440 °C, and after that cold rolled to a thickness of 1 mm at room temperature.

A Struers LaboPol-5 polishing machine (Struers APS, Ballerup, Denmark) was utilized to prepare specimens for microstructure analysis before and after deformation, including the stages of mechanical grinding and polishing. The surface of specimens was ground on sandpaper with different grain sizes of 220, 320, 800, 1200, 2400, and 4000, respectively. The specimens were then mechanically polished using OP-S suspension.

Microstructural examination and phase recognition were implemented on a TESCAN VEGA 3LMH scanning electron microscope (SEM) (Tescan, Brno, Kohoutovice, Czech Republic) utilizing an energy-dispersive X-ray detector X-Max 80 (EDX) (Oxford Instruments Advanced AZtecEnergy, High Wycombe, UK), and a Bruker D8 Advance X-ray diffractometer (Bruker, Billerica, MA, USA).

A Zwick/Roell Z250 universal testing machine (Zwick/Roell, Kennesaw, GA, USA) with an automatic longitudinal strain sensor was used to measure the tensile properties at a tensile rate of 4 mm/min. The tensile test specimens were cut out from 1.5 mm-thick rolled sheets. Ultimate tensile strength (UTS), yield strength (YS), and elongation (EL) to failure were evaluated. The hardness was measured by the standard Vickers procedure. The Vickers hardness experiment was performed with a load of 5 kg and a dwell time of 10 s. The hardness number HV was chosen as the number-crunching pitiless of five estimations for rolled cases and of ten estimations for cast. The botch inside the confirmation did not outperform 3 HV.

3. Results and Discussion

Microstructure (SEM) of the as-cast Al-3.8Gd-0.2Cr-0.2Ti alloy is shown in Figure 2. The microstructure (SEM) of the as-cast alloy is shown in Figure 2a,b. Aluminum solid solution, eutectic structure of aluminum solid solution and Al₃Gd phase, and a novel quaternary (Al, Gd, Cr, Ti) phase with possible composition Al₂₁GdCrTi are identified in the as-cast alloy. Point EDX SEM analyses demonstrate the content of 19 wt.% Gd, 6.5 wt.% Ti, and 5.3 wt.% Cr (Al–bal.) in the white particles (Figure 2c, left image). Recalculating the phase composition in the atomic fraction with a content of 4.1 at.% Gd, 4.2 at.% Ti, and 4.2 at.% Cr (Al-Bal.) allows us to write the phase formula as Al₂₁GdCrTi. The novel quaternary Al₂₁GdCrTi phase was first identified in the present study. The key element in the casting’s microstructure was dendrites of aluminum solid solution, with a volume fraction of 80%. Fine crystallization-origin Al₃Gd phase was observed organized into eutectic structure on the periphery of the dendrite cells of the aluminum solid solution. The eutectic structure consists of Al₃Gd platelets with approximately 0.2 μm in thick and compact primary crystals of Al₂₁GdCrTi phase with a size of about 10 μm. Figure 2c (right image) shows XRD of the as-cast Al-3.8Gd-0.2Cr-0.2Ti alloy. Unidentified peaks (marked with “?”) were found in the XRD patterns. These peaks probably correspond to the quaternary (Al, Gd, Cr, Ti) phase. The atomic rotation of Al/Gd/Cr/Ti in this phase is 21/1/1/1 in accordance with the point EDX SEM results.

The solidus temperature of the Al-Gd-Cr-Ti alloy (Figure 3a) is 619 °C as examined by DSC experiment. The alloy was homogenized at 615 °C for 1 h with subsequent water quenching. The microstructure (SEM images) after homogenization treatment is shown in Figure 3b. The homogenization annealing for 1 h leads to fragmentation, spheroidization, and growth of the Al₃Gd eutectic phase particles; the particle’s mean size increased from 0.2 μm (Figure 2a) to about 1.8 μm (Figure 3b) after homogenization. But what is interesting and discussed is that the novel Al₂₁GdCrTi particles appear in two forms: First, the particles are surrounded by Al₃Gd phase, similar to the Al₂₁GdCrTi core and Al₃Gd shell (Figure 3b-points A). Second, the Al₂₁GdCrTi particles without the Al₃Gd phase (Figure 3b-points B). There was no need to increase the homogenization time because the fragmentation had already occurred, and rolling would have made the structure more uniform.

The ingot was cold-rolled from 20 mm to 1.5 mm, and hot-rolled from 20 to 10 mm at 450 °C, then cold-rolled to 1.5 mm at room temperature. The intermetallic particles were more uniformly distributed in the aluminum matrix after cold and hot-cold rolling compared to the homogeneous state at 615 °C for 1 h (Figure 4). The average Al₃Gd particle size was 1.1 ± 0.5 μm, with a volume fraction of 20% in both cold- and hot-rolled sheets. The results indicate no significant change in microstructure in both regimes. Furthermore, the novel Al₂₁GdCrTi particles appear in two forms: first, particles surrounded by Al₃Gd (points A), and second, Al₂₁GdCrTi particles without Al₃Gd (points B), as seen after homogenization at 615 °C. On top of that, the particle size of Al₂₁GdCrTi at point A is about 7 ± 2 μm, while the particle size at point B is about 3 ± 2 μm after cold rolling (Figure 4a,b). This is probably because the Al₂₁GdCrTi particles at points A are harder, making them more resistant to crushing during rolling. It is also observed that the Al₂₁GdCrTi particle size after hot rolling is relatively smaller than after cold rolling and is more uniformly distributed in the aluminum matrix. This is contrary to expectations and may be because hot-cold rolling caused greater crushing of the aluminum particles (Figure 4a,c).

Figure 5 shows XRD patterns of four regimes: as-cast, as-cast with homogenization at 615 °C for 1 h, as-cold-rolled, and as-hot-cold-rolled. XRD patterns of four regimes were prepared to interpret the crystallographic texture of an aluminum solid solution based on the intensity ratio of crystallographic planes. The intensity of diffraction peaks from different crystal planes is not always equal, even in the same material. This difference in intensity, or intensity ratio, is often used to identify preferred orientations or "texture" within the material [38]. XRD analysis of the alloy reveals solid solution peaks of four planes (111), (002), (022), and (113), in addition to the Al₃Gd and peaks possibly corresponding to the quaternary phase Al₂₁GdCrTi. This figure clearly shows the difference in the relative intensity of the corresponding reflections in the diffraction patterns for the four regimes. The results showed that the intensity of (111) and (002) planes of as-cast and as-cast with homogenization at 615 °C for 1 h are higher than the (022) and (113) planes. This is due to the larger volume percentage of crystals (preferred orientation) of the (111) and (002) planes than the (022) and (113) planes. On top of that, the intensity of the (022) and (113) planes increased significantly after rolling, especially after hot-cold rolling, indicating that the crystal volume percentage increased during the rolling process. This may be due to the change in the direction of some grains in these planes during rolling, whether hot or cold. Furthermore, the intensity of (022), (113) planes of as-hot-cold-rolled are higher than that of as-cast and as-cast with homogenization at 615 °C for 1 h, This is due to the larger volume percentage of crystals (preferred orientation) of the (022), (113) planes than the (111) and (002) planes after hot and cold rolling.

Figure 6 represents the pole figures of the samples obtained by means of four regimes. The pole figure values are traditionally expressed in multiples of the random orientation distribution (mrd), which means that the pole figure of a random standard sample will have an intensity of 1 mrd at all points. The maximum pole density of as-cast specimens for planes (111), (002), (022), and (113) is 2.074, 3.248, 1.405, and 1.705 mrd, respectively. The texture of the plane (002) reveals a higher maximum intensity of 3.24 mrd than other planes, which can be interpreted as the sharpness of the crystallographic texture is large (Figure 6a,

Table 2. Summary of pole figure results for as-cast, as-cast with homogenization at 615 °C for 1 h, as-cold-rolled, and as-hot-cold-rolled specimens.

	Regime	As-Cast				As-Cast with Homogenization at 615 °C for 1 h				As-Cold-Rolled				As-Hot-Cold-Rolled
Intensity		(111)	(002)	(022)	(113)	(111)	(002)	(022)	(113)	(111)	(002)	(022)	(113)	(111)	(002)	(022)	(113)
Min		0.131	0.438	0.381	0.677	0.116	0.338	0.462	0.606	0.255	0.403	0.392	0.627	0.364	0.506	0.462	0.670
Max		2.074	3.248	1.405	1.705	3.184	5.941	2.578	2.244	2.139	2.139	2.126	2.664	2.828	2.126	2.664	2.353

). Moreover, maximum pole density of planes (111), (002), (022) and (113) increased to 3.184, 5.941, 2.578 and 2.244 mrd, respectively, after homogenization at 615 °C for 1 h, indicating that the homogenized sample has a strong preferred orientation [39] (Figure 6b). This may be due to the increase in grain size during the heating process. In addition to this, the grain orientation evolves during the subsequent solution heat treatment and plastic deformation. The homogenized sample at 615 °C for 1 h shows a coarser grain size compared to other regimes, as shown in Figure 6b. Moreover, other researchers presented evidence that the homogenization influence on cast Al alloys can decrease their hardness, coming approximately from the increase in grain measure during the midst of the heating process [40,41]. The present study revealed that the coarse-grain structure in Al-3.8Gd-0.2Cr-0.2Ti alloy can lead to a strong texture [24,35].

Furthermore, a typical deformation texture was formed after rolling, as one can see in Figure 6c,d. The maximum pole density of as-cold-rolled and as-hot-cold-rolled samples of four planes (111), (002), (022), and (113) ranges from 2.126 to 2.828 mrd, indicating a texture strength advantage compared to the cast samples.

The thermal neutron absorption property of the as-cast Al-3.8Gd-0.2Cr-0.2Ti alloy compared to pure Al alloy is shown in Figure 7. The horizontal axis represents interplanar spacing (d, Å), while the vertical axis represents the neutron scattering intensity (arbitrary unit, a.u.). The intensity scattered by the studied alloy is very low and almost negligible compared to pure Al alloy due to the absorption of neutrons by the alloy. The results indicate that the studied alloys scatter neutrons about 100 times less than a nearly pure aluminum alloy.

The hardness of the as-cast Al-3.8Gd-0.2Cr-0.2Ti alloy is 36 HV, while the hardness of the as-cast with homogenization at 615 °C for 1 h is 31 HV. The hardness increased to 53 HV after cold rolling and to 50 HV after hot-cold rolling.

The as-cold-rolled specimen has mechanical tensile properties: yield strength of 159 MPa, ultimate tensile strength of 177 MPa, and elongation of 10.5%, while the as-hot-cold-rolled specimen has mechanical tensile properties: yield strength of 160 MPa, ultimate tensile strength of 181 MPa, and elongation of 11.3% (Figure 8). Contrary to expectations, the as-hot-cold-rolled sample shows slightly higher yield and tensile strength. With hot-cold-rolled deformation, the size of the Al₂₁GdCrTi particles decreased, and they were more uniformly distributed in the matrix. The studied alloys, containing 4.2 wt.% of the alloying elements 3.8Gd, 0.2Cr, and 0.2Ti (Al-3.8Gd-0.2Cr-0.2Ti), exhibited a yield strength 28 MPa higher than those containing 21 wt.% of the alloying elements 5Cu, 6Gd, and 8Bi (Al-5Cu-6Gd-8Bi) [33]. This is attributed to the addition of 0.2 wt.% chromium and 0.2 wt.% titanium, which improved tensile strength while maintaining elongation (Figure 8).

4. Conclusions

The microstructure of Al-3.8Gd-0.2Cr-0.2Ti alloys was observed during the casting, homogenization stages, and rolling, along with the mechanical properties, crystallographic texture, deformation behavior, and neutron diffraction. The microstructure revealed the presence of the aluminum solid solution, Al₃Gd, and a novel (Al, Gd, Cr, Ti) quaternary phase with possible composition Al₂₁GdCrTi was found in the as-cast Al-3.8Gd-0.2Cr-0.2Ti alloy. The novel quaternary Al₂₁GdCrTi phase was first identified in the present study. X-ray diffraction and crystallographic texture analysis in samples after rolling showed a marked difference in volume percentage of crystals and texture strength compared to the cast samples. The results showed that the pole density of (022) and (113) planes of the as-hot-cold-rolled sample is higher than that of the as-cast. The Al-3.8Gd-0.2Cr-0.2Ti alloy scatters neutrons about 100 times less than a pure Al alloy. The hardness of the as-cold-rolled alloy is 53 HV, while the as-hot-cold-rolled alloy is 50 HV. Mechanical tensile properties of cold-rolled alloy: yield strength of 159 MPa, ultimate tensile strength of 177 MPa, and elongation of 10.5%, while Mechanical tensile properties of hot-cold-rolled alloy: yield strength of 160 MPa, ultimate tensile strength of 181 MPa, and elongation of 11.3%.

The alloy is a promising combination for the development of novel high-technology neutron shielding alloys due to its high thermal neutron absorption. The alloy’s mechanical properties can be improved by adding zirconium and magnesium, the effect of which on the alloy’s properties is still being studied. Further research is also needed on the effect of silicon on the phase composition and alloy properties.