Effect of Temperature and Strain rate on Flow behavior of Al0.3CoCrFeNi HEA

Srinivasu, Mr. Raju, Mr. Rangaiah, Mr. Ramu for their technical assistance during SEM characterization and EDM cutting. Finally, I would like to thank my family members, who have been a great support to me. Super plasticity is a major concern for the design industries to produce complex and curved components for use in automotive, aerospace and other applications.

Because superplasticity requires a very small grain size, typically <10 μm, it is feasible to introduce significant grain refinement through thermomechanical processing. 2 Schematics of (a) perfect lattice with similar atoms (b) severe lattice distortion of different elements ……….5 Figure 2.3 Lattice constant and hardness of the AlxCoCrFeNi alloys as functions of Al.

Introduction

Overview: ........................................................................................................................... 1-2

Most of the literature suggests that good stable barazal grain size is the microstructural prerequisite along with high strain rate sensitivity, cavitation resistance to achieve superplastic flow [29]. And the fine-grained structure can also be achieved by thermo-mechanical processing which includes cold rolling with subsequent annealing. The presence of the second phase at grain boundaries and triple junctions help to stabilize the fine grain size during high temperature deformation which is essential for superplastic deformation.

So the choice of an alloy composition which can produce a microstructure with fine grain size and stable second phase at the grain boundaries can exhibit super plasticity at high temperature. Deformation behavior at room temperature was investigated using tensile tests at quasi-static strain rate (1x10-3s-1).

Background

Four Effects
The high entropy effect:............................................................................................... 3-4
Microstructural prerequisites

The XRD pattern in figure 2.1 shows the influence of a high entropy effect in the formation of solid solutions with simple crystal structures [14] and it is clear that all alloy systems of quinary. The lattice deformations increase the strength of the alloy in the form of solution hardening [14]. As an example, Figure 2.3 illustrating the phase transformation in AlxCoCrFeNi HEA with increasing Al content.

However, the void diffusion of substitutive elements is limited in high entropy alloys compared to the conventional alloys because a void in the matrix is surrounded and competes with various components during the diffusion. As shown in Figure 2.4, the elements in the HEA system show the lowest diffusion coefficients compared to other alloy systems. Furthermore, it is suggested that the sluggish dispersion of HEAs will not only produce extraordinary high temperature strength and structural stability, but also aid in the formation of nanoprecipitates because the nuclei are easier to generate but will be difficult to grow.

One of the several examples is shown in Figure 2.3, where the hardness of the high-entropy alloy can be significantly changed by adjusting the Al content in the AlxCoCrFeNi HEAs [23]. There has been a rapid increase in the number of publications on HEAs in the last decade and studies relevant to the present work are summarized below. Stress-strain curve showed the yield drop and low strain-hardening rate, suggesting that the nano-precipitates observed in the matrix are coherent GP zones and dislocations cutting through the precipitates as the primary strengthening mechanism.

Qiao et al., [25] studied deformation behavior of single phase BCC AlCoCrFeNi HEA in compression mode in the temperature from 278 K to 77 K and showed that the yield strength and fracture strength increase by 29.7% and 19.9% respectively with decreasing the temperature of 298 to 77K. However, there is a change in the fracture mode from intergranular at 298 K to transgranular at 77 K. Laktionova et.al., [26] reported deformation studied on Al0.5CoCrFeNi alloy in the temperature range from 300K to 4.2 K shown in figure 2.7 and showed yield stress of 450 MPa and 750 MPa at temperatures 300K and 4.2K respectively.

Figure 2.3 Hardness of the AlxCoCrFeNi alloys as a function of Al content [14] [16] [17]

Superplasticity in HEAs………………………………………………………………..15-18

In this scenario, it is necessary to study the possibility of superplastic behavior in these HEAs. Fine and uniform distribution of the second phase or particles along the grain boundaries can pin the grain boundaries and thus inhibit the grain growth. Materials containing low-angle grain boundaries can also behave superplastically by converting the low-angle grain boundaries to high-angle grain boundaries through thermal or thermomechanical processing.

In these alloys, continuous recrystallization takes place during deformation, such that low-angle grain boundaries are transformed into high-angle grain boundaries in the initial phase of deformation, and these high-angle grain boundaries contribute to subsequent grain boundary sliding (GBS), leading to superplastic flow. Kuznetsov et al., [33] investigated the microstructure and deformation behavior of hot-forged AlCoCrFeNi alloy in tensile condition at high temperatures shown in Figure 2.10. Tensile strength tests were performed in the temperature range 773 K to 1023 K and at different strain rates as shown in Figure 2.11.

Post-deformation microstructure suggesting that the superplastic elongation is due to the formation of fine precipitates along the grain boundaries that inhibit the grain growth during superplastic deformation. Samples are tested in temperatures from 873 to 1000K and showed the maximum elongation to failure 830% at 973K temperature and 10-2s-1 strain rate. Reddy et al., [36] investigated the high temperature plastic deformation behavior of thermomechanically processed quasi-single phase CoCrFeMnNi alloy with a grain refinement of 1.4 µm grain size.

While the strain at 1023K and strain rate of 10-3s-1 has relatively very low yield strength, but the ductility increases to 160%. From the literature study, it can be seen that HEAs have the ability of superplastic flow after grain refining. To understand the effect of temperature and strain rate on the flow behavior of thermomechanically treated Al0.3CoCrFeNi HEA.

Figure 2.11 Flow curves for samples processed by HPT AT 973 K and at different strain rates [34]

Why Al 0.3 CoCrFeNi HEA ……………………………………………………………..18-19

To study the microstructure evolution and stability of cold-rolled Al0.3CoCrFeNi HEA annealed at different temperatures and annealing times. Cold rolling of single phase (annealing at temperature above 1200 C produces single phase FCC) FCC phase followed by recrystallization can produce fine eqi-as grain size. The subsequent formation of the B2 phase along grain boundaries and triple junctions stabilizes this fine grain size at high temperature which can be a suitable condition for superplastic deformation and this is the strategy followed in the present work [38] [39].

Currently, 30-40% of the components in these applications are made from aluminum and titanium alloys due to their light weight and high specific strength, which will increase fuel efficiency. Recent research on HEAs shows that the addition of Al reduces the density of AlxCoCrFeNi alloys (density approximately ⁓ 6.5 g/cc). Ashby Map; The yield strength is plotted against density, shown in Figure 2.13, which clearly suggests that AlxCoCrFeNi alloys have high specific strength, comparable to aluminum and titanium alloys, and could be a good alternative structural material for automotive applications.

Materials and Methods

Sample preparation
Compositional analysis

The surface of the hot-rolled sheet was cleaned by surface grinding and cut into small pieces with dimensions: 40 x 20 x 5.3 mm using the Secotom precision cutting machine and then cold-rolled to an 80% thickness reduction using a laboratory scale with two high rollers with a roll diameter of ~140 mm (SPX Precision Instruments, Fenn Division, USA) in multiple passes, approximately 2% reduction with each pass. Samples for chemical composition analysis are collected from the received hot-rolled sheet in the form of drill chips and the composition analysis was carried out using inductively coupled plasma mass spectrometry (ICP-MS), which is capable of detecting metals and various non-metals in concentrations. as low as parts per billion on unhindered low background isotopes.

Microstructural studies

The microstructure of the annealed samples was characterized using SEM-EBSD (Scanning Electron Microscopy-Based Electron Backscattered Diffraction) which is shown in Figure 3.3. Flat dog-bone shaped, as shown in Figure 3.4, tensile specimens of the annealed strip were cut using electro discharge machine (EDM) parallel to the rolling direction. To perform a high temperature tensile test, three zone ATS furnace is attached to the same Instron (U.S. model number:5967) universal testing machine as shown in the figure 3.6, Port hole with 10 x 2 cm dimension is provided in the center of the furnace. to capture the sample images during high temperature deformation to measure the strain using DIC.

The as-received (hot-rolled and homogenized) microstructure shown in Figure 4.1 consists of a significant amount of annealed twins. The cold-rolled samples were annealed at 900 °C for 2 h in an argon atmosphere and the corresponding EBSD phase map is shown in Figure 4.4a. Grain size measurements were made using the average linear intercept method shown in Figure 4.2 using sigma scan pro image analysis and a minimum of 500 grains were measured in each condition.

The grain size (dm) variation with annealing time and temperature is shown in figure 4.7 which indicates that there is no significant increase in the FCC matrix grain size (dm) with either increasing the annealing temperatures from 900 to 1000oC or with annealing time from 1 to 16 hour The average chemical composition of FCC parent phase and BCC phases was measured using EDS (corresponding BSE image shown in figure 4.9) as Al3.7Co23.6 Cr26.9Fe26.6Ni19.3. Average Vickers hardness as a function of grain size is shown in Figures 4.11 (a) and 4.11(b), and indentation made by diamond pyramid indenter is shown in Figure 4.12.

The spot pattern created on the sample using the spray gun is shown in Figure 4.14 for stress calculation. VIC 2D uses the digital image correlation technique to provide strain measurements on a two-dimensional contour map as shown in Figure 4.15. Tensile experiments at room temperature were carried out for samples aged at different times and temperatures having different grain sizes at strain rate 10-3 s-1 and the corresponding engineering strain curves are shown in Figure 4.16.

The yield strength, ultimate tensile strength along with ductility were calculated from the stress-strain curves and are presented in Figure 4.17 and the values are summarized in Table 4. SEM images of fracture surface samples deformed at 600, 700 and 800oC in 10-4s- 1 degree of deformation are shown in figure 4.23. and clearly showing dimple fracture, which is a clear indication of ductile fracture.