In-situ heating observations on microstructure relaxation of ultrafine-grained high-entropy alloys using neutron diffraction and laser-scanning confocal microscopy

. The thermal stability of ultrafine-grained metals can be fully understood when observing time-resolved microstructural changes over multiple-length scales. The global microstructural relaxation behavior upon heating of an ultrafine-grained (UFG) CoCrFeNi high-entropy alloy (HEA) was characterized by in-situ heating neutron diffraction measurements. Before heating, the nanocrystalline microstructure was introduced by applying high-pressure torsion (HPT), leading to severe lattice distortion by excess dislocations and defects. The sequential information on the structural relaxation of recovery, recrystallization, and grain growth are identified by in-situ heating neutron diffraction analysis defining the texture development, linear thermal lattice expansion


Introduction
The thermal stability of metallic materials is a critical ability and design characteristic that can decide the specific applications of engineering metals.It is especially important for ultrafinegrained (UFG) metals and bulk nanostructured materials, whose stored free energy at the grain boundaries is high leading to accelerated microstructural recovery and grain growth [1,2].Such changes in microstructure directly influence the mechanical properties, i.e., deterioration of strength and hardness, of UFG metals.Thus, the understanding of thermal behavior including microstructural evolution and changes in mechanical properties is significantly important along with the successful development of severe plastic deformation (SPD) techniques for microstructural refinement of numerous metals including multiple-principal element alloys often called high entropy alloy (HEA) [3].This study describes a recent result on the in-situ heating microstructural relaxation behavior of a nanostructured CoCrFeNi alloy examined using neutron diffraction.Moreover, the direct observation of the relaxation microstructure at large length scales is demonstrated on a nanocrystalline CoCrFeNiMn alloy through the application of hightemperature laser-scanning confocal microscopy.

Formation of nanostructure
The present study uses the CoCrFeNi alloy produced by additive manufacturing (AM) of the powder bed fusion technology.The detailed conditions for AM are described earlier [4].After machining to have disk samples having 10 mm diameter and 0.85 mm thickness, where the disk surfaces are perpendicular to the building direction, high-pressure torsion (HPT) processing was applied to the first batch of the as-built alloy disks for 1/2, 1, 2, 4 and 8 turns under 6 GPa at room temperature.Figure 1 shows the micrographs of the alloy after AM at (a) normal to the built direction and (b) the mid-radius of the disk sample after HPT for 8 turns [5].The as-built microstructure shows a coarse, columnar grain microstructure.The HPT process refined the microstructure to have reasonably equiaxed grains with an average grain size of d = 90 nm.The microstructural changes during HPT processing occur gradually with increasing numbers of turns, and it is confirmed by taking the hardness contour maps across the disk cross-sections as shown in Fig. 1(c).The initial Vickers microhardness values before HPT was HV = 260.Due to the imposed shear strain by torsion being proportional to the disk radius [6], high hardness, thus grain refinement, is developed from the disk periphery towards the disk center with increasing HPT turns, while the hardness is homogeneously developed across the disk height.After 8 turns, the hardness distribution became homogeneous in the nanostructured AM CoCrFeNi alloy.

Figure 1. Micrographs of the alloy (a) in as-built and (b) after HPT for 8 turns, and (c) colorcoded hardness contour maps after HPT for 1/2 turn through 8 turns of AM CoCrFeNi alloy.
A series of samples including AM powder before AM, after AM (as-built), and after HPT for 1/2 turn through 8 turns were examined by the X-ray diffraction (XRD) method.All sample conditions showed diffraction peaks of only face-centered cubic (f.c.c.) structure, and the estimated microstructure parameters including crystallite size, microstrain, and lattice constant are listed in Table 1 [5].While there is a slight change before and after AM, significant changes in these microstructural parameters are initiated by the application of HPT even for 1/2 turn.The changes in the parameters are maintained reasonably through 8 turns.It is common to observe a smaller domain size of 75 nm by XRD after 8 turns than by TEM (Fig. 1(b)) due to the high populations of subgrains and dislocation walls as well as crystallographic orientation [7].

In-situ heating neutron diffraction analysis
Microstructural evolution upon heating at 4 K/min from 300 K to 1273 K followed by quenching was examined for the as-built and HPT-processed CoCrFeNi alloy through neutron diffraction experiments at the iMATERIA, beamline BL20, at the Japan Proton Accelerator Research Complex (J-PARC) [8].The obtained time-resolved neutron diffraction patterns from Banks 1-3 are plotted into diffraction contour maps with temperature changes as shown in Fig. 2 for the CoCrFeNi alloy in (a) as-built and (b) AM and HPT for 15 turns [8].HPT for 15 turns was applied to ensure the homogeneous nanostructure of the alloy.The peak profiles at t = 0 min of heating imply the textures of the alloy conditions immediately after AM (a) and HPT (b).Both materials showed diffraction peaks of a single-phase f.c.c.crystal structure throughout heating followed by cooling, thereby proving no phase transformation.In addition, two features are noticeable in the contour plots of neutron diffraction profiles with time and temperature for the CoCrFeNi alloy: the continuous and concurrent changes in the peak shifting and peak width.The former indicates lattice expansion and contraction with heating and cooling, respectively, and the latter involves relaxation in microstrain and growth in crystallite size, indicating microstructural relaxation.These parameters are further analyzed, and Fig. 3 shows the evolution of (a) peak lattice expansion and (b) relative peak width at the plane coordinate of 111 of the neutron diffraction peaks with temperature for both as-built (AM) and HPT-processed (AM + HPT) CoCrFeNi alloy [8].The relative lattice expansion and contraction in Fig. 3(a) were estimated using the base lattice parameter of a0 = 3.60 Å taken at the minimum strain condition after the completion of a heatingcooling cycle in the diffraction measurement.Ex-situ hardness measurements after heating to the specific target temperatures followed by cooling were conducted for the samples, where the results are shown in Fig. 3(c).

Figure 3. Evolution of (a) relative peak width, (b) relative lattice expansion, and (c) Vickers microhardness upon heating for the as-printed (AM) and HPT-processed (AM + HPT)
CoCrFeNi alloy [8].For both AM and AM + HPT samples, there is a consistent linear lattice contraction of ~1.6 × 10 -5 K -1 at <900 K, while a high lattice expansion of 2.2 × 10 -5 K -1 is observed over 960 K with some plateau at 900-960 K for AM + HPT.Such inconsistency in relative lattice expansion without phase transformation can be explained by the inhomogeneous distribution of thermal vacancies leading to heterogeneous lattice strain in CoCrFeNi and CoCrFeNiMn high-entropy alloys at >973 K [9].The changes in relative peak width and hardness values with heating towards 1273 K in Fig. 3(b) and (c) provide microstructural relaxation behavior of the nanostructured CoCrFeNi alloy, which are not visible in the AM sample.In practice, the gradual reduction in relative peak width and a hardness increase from HV of 550 to 650 are shown up to 873 K, followed by a dramatic decrease in both relative peak width and hardness to HV ≈ 450 through 973 K, and these achieved the minimum saturated values of peak width and hardness of HV ≈ 370 with further heating thought 1273 K.These three stages of microstructural relaxation are considered as recovery, recrystallization, and grain growth.
Annealing-induced hardening (AH) [10] is achieved upon heating to 873 K in the present UFG CoCrFeNi alloy.AH has been observed in other SPD-processed UFG metals, including stainless steel [11], single-phase CoCrNi-based medium entropy alloys [12], Ni-Mo alloy [13], and Ti [14].AH must be differentiated from transformation hardening, including the nucleation of Guinier-Preston zones in Al alloys and the formation of NiMn-, FeCo-, and Cr-rich phases in the CoCrFeNiMn Cantor alloy.Moreover, AH is achievable without solute segregation strengthening [15].Specifically, UFG metals processed through a top-down strategy, such as SPD, achieved AH by defect relaxation involving (i) relaxation of mobile dislocations inside the grains by clustering into low energy configurations without any significant loss in dislocation density; (ii) relaxation of non-equilibrium grain boundaries leading to a difficult emission of dislocations; and (iii) clustering of excess vacancies into planer defects, impeding the motion of dislocations.The ductility of UFG metals after AH is anticipated to be improved because of the microstructural relaxation during annealing.Accordingly, the AH effect on the UFG metals differs from the "yield strength anomaly", which strengthens materials at elevated temperatures by dislocation locking and dynamic strain aging strengthening leading to lower ductility [16].
The in-situ neutron diffraction experiments provide a knowledge base to minutely design crystalline microstructures and improve the desired properties of a UFG material by an accurate post-processing heating treatment.A recent report summarized currently available examples of SPD-processed nanostructured materials which are characterized by synchrotron X-ray and neutron diffraction techniques [17].

In-situ heating laser scanning confocal microscopy
The present neutron diffraction analysis as well as lab-scale X-ray and synchrotron diffraction analyses can capture the non-monotonous distribution of diffraction peak broadening, indicating strain anisotropy, due to the heterogeneous distributions of excess numbers of dislocations, vacancies, and other defects within crystals of SPD-processed metals [18].The microstructural inhomogeneity and heterogeneous evolution of a UFG microstructure can be observed by microscopy techniques that are capable of scanning over large-length scales.A recent experiment demonstrated a heterogeneous microstructural relaxation behavior of an HPT-processed nanocrystalline CoCrFeNiMn alloy having saturated grain sizes of 40-50 nm through laser scanning confocal microscopy (LSCM) (Model VL2000DX-SVF18SP) with a heating facility [17].The principles of LSCM including how the observed surface information represents the bulk behavior of materials are explained elsewhere [19].Figure 4 shows a series of the LSCM micrographs taken at cross-sections of the alloy disk edges upon heating from 373 K to 923 K at 10 K/min.

Figure 4. A series of LSCM micrographs taken at a disk edge region of the HPT-processed
CoCrFeNiMn HEA at arbitrarily selected temperatures between 373 K and 923 K.The micrographs are adopted from [17].
Overall homogeneous microstructure until 580 K started to show coarser multi-phase elongated phases aligning shear directions only at the sample surfaces except for the middle-thickness without coarsening through 650 K.The separated phase evolution with phase coarsening was maintained until 800 K. Above this temperature, all regions are coarsened, and the equiaxed phases are homogeneously distributed across the sample thickness.While it is a preliminary result without characterization of the coarsened phases, the heterogeneous microstructure evolution during heating was visualized within the HPT-processed CoCrFeNiMn HEA disk sample by the application of high-temperature LSCM for the first time.The phase transformation and agglomerations of the nanostructured CoCrFeNiMn alloy were observed at 723 K [20], but careful consideration is required for the local measurements for microstructural analysis and small-scale mechanical testing, i.e. nanoindentation, due to the possible heterogeneous microstructure and texture introduced by SPD processing.These diffraction and microscopy techniques which analyze the large sample volumes and areas are essential and complementary to the other local measurements to fully understand the evolution of microstructure and mechanical properties of the SPD-processed UFG metals.

Summary
The novel characterization techniques of in-situ heating neutron diffraction and high-temperature laser scanning confocal microscopy provided a comprehensive view of the continuous microstructural relaxation of bulk nanocrystalline HEAs.With the application of different techniques, the heterogeneous UFG microstructures developed during heating can be revealed and understood precisely.

Figure 2 .
Figure 2. Contour plots showing variation in neutron diffraction profiles from three detector banks 1-3 with time and temperature for the AM CoCrFeNi HEA after (a) printing and (b) HPT for 15 turns [8].

Table 1 .
[5]mary of estimated crystallite size, microstrain, and lattice parameters for the AM CoCrFeNi alloy in the conditions of powder, after AM, and HPT for up to 8 turns[5].