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.

Fatigue in friction stir processing

• The combination of severe plastic deformation and annealing produced a ultrafine grained (UFG) - Cu-HEA with an average grain size of approximately 100 nanometers and an unprecedentedly high fatigue limit, indicating that it could withstand a large number of stress cycles without failing.

• This high fatigue limit is supported by the S-N curve, and comparison of fatigue endurance limit vs Ultimate tensile limit.

FSP for conventional alloys

• Friction stir processing for enhancing the microstructure and mechanical properties of conventional magnesium alloys, such as the newly designed AXM541 alloy, by means of friction.

• The microstructural improvement in Mg-5Al-3.5Ca-1Mn (AXM541) alloy is substantiated by SEM images depicting the refined eutectic particles of nugget after friction stir processing.

• The engineering stress-engineering strain curves, true stress-true strain curves, and work hardening curves for AXM541 alloy along the processing direction demonstrate an improvement in mechanical properties.

  Mg - Alloy 

   Ultralight structural applications             

• As the density of LX41(Mg-4Li-1Ca) is below 1.6 g/cm3, it can be regarded as an ultralight alloy. This makes it suitable for both lightweight engineering structures and medical implant applications.

• Development of ultralight Mg based alloy was done which shows high strain ductility.

• Extreme strain ductility synergy in the alloy due to complete randomization of basal due to particle stimulated nucleation.

   FSP of Mg alloy

• In Mg-5Al-3.5Ca-1Mn (AXM541) Mg alloy after friction stir processing EDS scan of the large particles revealed that they were rich in Al and Mn and may correspond to the D810 phase.

• Dynamic recrystallization was the primary mechanism for the formation of fine grains after FSP. The       presence of fine C36 particles assisted Dynamic recrystallization via Particle-stimulated nucleation. Most of           the particles refined during FSP were larger than 1 µm. 

Biocompatibility

• Biocompatibility of LX41 alloy was found to be function of microstructure evolved as a result of thermo-mechanical processing.

• Among all processing conditions, TA30 displayed good biocompatibility by showing happy and healthy morphologies of L929 and osteoblast cells after 14 days of incubation.

• A microstructure-biocompatibility relationship was established for LX41 alloy demonstrating the pathway for designing biodegradable Mg alloys.

   Friction Stir Welding

   Conventional Dissimilar welding (butt joint) 

• Welding surfaces indicate clearly that welding flash at advancing side (AS) increased with welding heat input.

• Differences in hardness values are due to competing effects of lower thermal softening and higher grain refinement in stir zone with increase in traverse speed.

   HEA similar welding

• Electron backscattered diffraction phase map shows the different zones like stir zone, thermo-mechanically affected zone and heat affected zone after but welding of Fe38.5Mn20Co20Cr15Si5Cu1.5 high entropy alloy Cu-HEA

• The BM depicts grain size more than 50 μm with phase distribution of γ (f.c.c) 79% and ε (h.c.p) 21% whereas SZ shows where γ (f.c.c) is 92% and ε (h.c.p) is 8%, phases with a refined grain size of less than 15 μm.