High entropy conventional alloy (HECA) via randomization of matrix

• A cost-effective design strategy by introducing configurational randomization (ΔSmix  ≥ 1 R) in a compositionally lean f.c.c. matrix to develop a HECA, Ni-12.5Fe-7.5Al-4.75Cr-4.75Mn-0.5Si (at%).

• Ni-rich HECA with a lean random matrix was developed, exhibiting superior strength (∼900 MPa), ∼50 % ductility, and enhanced strain hardening compared to conventional Ni-CA and dual-phase Ni-HEA.

• Electrochemical tests show improved corrosion resistance (Ecorr: −0.27 ± 0.007 V, and Icorr: ∼1.24 × 10−7 (± 0.2) A/cm2), due to lower galvanic corrosion susceptibility.

   Effect of alloying element on properties of complex alloy

• Microstructural evolution with increasing V content in Co30Ni45-xVxCr15Fe5Si5 (at. %) alloys shows a transition from a single-phase FCC structure to a multi-phase structure beyond 15 at.% V, while still maintaining good compressive formability (ε > 50%).

• Oxide layer thickness and weight gain increase significantly beyond 10 at.% V, with thicker oxide layers (>350 μm) and higher weight gain (>45 mg/cm²) due to the greater reactivity of V with oxygen at elevated temperatures.

   ​ Additive Manufacturing

   Damage tolerant alloy printing

• Damage-tolerant alloys are those that exhibit negligible decrease in mechanical properties despite a high defect content.

• Fe38.5Mn20Co20Cr15Si5Cu1.5 (Cu-HEA) is a damage-tolerant alloy because its ductility is approximately 60% that of Fe40Mn20Co20Cr15Si5 (CS-HEA), in spite of having 15 times more defects.

• The activation of TRIP at the crack tip has provided Cu-HEA with the ability to delay the crack propagation and thus gave it a damage tolerance.

HEA printing

• AM-printed Fe40Mn20Co20Cr15Si5 (CS-HEA) exhibited greater strength due to its high work hardenability, while its substantial uniform ductility is a result of a combination of transformation- and twin-induced plasticity during deformation.

• The figure depicts 3D reconstructed X-ray micrograph for CS-HEA showing the defect distribution whereas the BSE image depicts mixed grain morphology having predominantly columnar grains.

Grain boundary architecture

• Grain boundary architecture refers to the arrangement and distribution of various structural features along the grain boundaries of a material. These structural features include defects such as vacancies, dislocations, and stacking faults, as well as impurities and segregation of elements

• Electron Backscatter Diffraction  phase map for as-built Fe40Mn20Co20Cr15Si5 (CS-HEA) doped with 0.5 wt.% B4C (termed CS-BC)  as-built CS-BC 120-800 specimens are represented, where the phase fraction of γ-f.c.c. is increased from 26% to 98%. The segregation of B at grain boundary for CS-BC specimen is evident by Energy-dispersive X-ray spectroscopy elemental mapping.

• The impact of grain boundary segregation on  mechanical properties is shown in tensile stress-strain in comparison with the alloys that are not grain boundary architectured.  

   Additive overview

• Damage-tolerant alloys are those that exhibit negligible decrease in mechanical properties despite a high defect content.

• Cu-HEA is a damage-tolerant alloy because its ductility is approximately 60% that of Fe40Mn20Co20Cr15Si5 (CS-HEA), in spite of having 15 times more defects.

• The activation of transformation induced plasticity (TRIP) at the crack tip has provided Fe38.5Mn20Co20Cr15Si5Cu1.5 (Cu-HEA) with the ability to delay the crack propagation and thus gave it a damage tolerance.

   HEA Design

   HEA design overview

• Non-equiatomic HEAs are designed based on metastability engineering and high entropy approach.

• Flexible microstructural evolution in designed HEAs is evident with the variation of Si content.

• (E–H) figure shows the microstructural evolution for conventional rolling and (I–K) laser powder bed fusion (LPBF) assisted 3D printing.

Strength ductility synergy in HEA designing

• Unexpected strength-ductility synergy was obtained by unconventional phase evolution i.e., γ → ɛ during annealing for Fe39Mn20Co20Cr15Si5Al1 (Al-HEA).

• Precipitation of Al and change in martensite morphology played a crucial role in tailoring the strength-ductility synergy.

Corrosion resistance 

• Microstructural evolution in Fe38.5Mn20Co20Cr15Si5Cu1.5 (Cu-HEA) for D-pass condition, which shows the bimodal microstructural evolution, with very fine grains.

• High strength-ductility combination was attained due to controlled transformation induced plasticity effect irrespective of grain sizes

• With reduction in grain size, corrosion is found to be slower both thermodynamically and kinetically.

Superplasticity

• Super plasticity of as-friction stir processed dual phase-HEAs are shown by the help of engineering stress-engineering strain curves at 700°C with a maximum ductility of 550.

• Superplastic flow is maintained due to concurrent occurrence of  γ  → σ  transformation at high temperature

• The specimens show diffusional cavitation leading to failure during testing.

   Friction Stir Processing

   Strength ductility synergy in FSP

• The γ-phase dominated HEAs showed slower yet sustained work hardening (WH) rates (~ 2000 MPa) owing to transformation induced plasticity (TRIP) effect whereas ε-phase dominated microstructure showed exceptionally high and sustained work hardening rates (2300–2700 MPa) due to formation of nano-plates or twins in ε-phase upon deformation.

• Therefore, MF-HEA design leads to exceptional strength–ductility synergy (1100-1250 MPa, 30-43%) irrespective of the dominance of either γ- or ε-phase in the starting microstructure owing to selective operation of deformation mechanisms in the dominant phase.

Multi pass friction stir processing

• FSP was carried out on Fe40Mn20Co20Cr15Si5 (CS-HEA) with overlapping passes of high and low rotational rates for a given transverse speed

• Through mechanical testing it is concluded that multi-pass FSP gives good combination of both strength and ductility due to formation of dual phase microstructure.