Microstructure and Physical Property Optimization in High Entropy Alloys: Microstructure and Physical Property Optimization in High Entropy Alloys
Sponsored by: TMS Phase Transformations Committee
Program Organizers: Bharat Gwalani, North Carolina State Universtiy; Arun Devaraj, Pacific Northwest National Laboratory; Eric Lass, University of Tennessee-Knoxville; Rajarshi Banerjee, University of North Texas

Monday 8:00 AM
November 2, 2020
Room: Virtual Meeting Room 37
Location: MS&T Virtual

Session Chair: Bharat Gwalani, Pacific Northwest National Lab; Tianhao Wang, Pacific Northwest National Lab; Abhishek Mehta, University of Central Florida


8:00 AM  
3D Twinning in High Entropy Alloys: Scott Mao1; Zhengwu Fang1; 1University of Pittsburgh
    The combination of high strength and high ductility is hard to attain in metallic materials. However, materials that show twinning-induced plasticity are exceptional. To understand the intrinsic mechanisms by which the usual strength-ductility trade-off can be defeated, we apply quantitative in situ transmission electron microscopy (TEM) to examine deformation twinning in CrMnFeCrNi and CrCoNi that exhibits one of the highest combinations of strength, ductility and toughness on record. A three-dimensional (3D) hierarchical twin network is formed when the alloy is deformed that serves a dual function: conventional twin boundary (TB) strengthening involving blockage of dislocations impinging on the TB, coupled with the high-density network of twins which offers additional pathways for dislocation motion by gliding along the TB-matrix interfaces.

8:30 AM  
Short-range Atomic Order Drives Exceptional Mechanical Properties of Multi-principal Element Alloys: Sezer Picak1; Prashant Singh2; Yuriy Chumlyakov3; Duane D Johnson2; Raymundo Arroyave1; Ibrahim Karaman1; 1Texas A&M University; 2Ames Laboratory; 3Siberian Physical Technical Institute
    High and medium entropy alloys (HEAs/MEAs) are a new class of multi-principal element alloy (MPEA) systems with unique mechanical and functional behavior. Here, we reveal, both theoretically and experimentally, the complex deformation mechanisms responsible for the exceptional mechanical response of FeMnCoCrNi, Fe40Mn40Co10Cr10 and NiCoCr, which are model MPEAs. Our first-principles thermodynamic analysis unequivocally proves the existence of a short-range order (SRO). Although the existence of SRO in MPEAs has been correlated with planar-slip in the literature, our theoretical and experimental approaches show that SRO is responsible for the simultaneous deformation twinning-induced-plasticity and martensitic transformation-induced plasticity (TWIP/TRIP), and thus, is responsible for unusual deformation in these MPEAs. The bulk single-crystal experiments establish the role of SRO in the simultaneous observation of TWIP/TRIP, with nanometer range twin and martensite thicknesses. These concurrent strengthening mechanisms triggered by SRO point towards strategies to further improve or discover new classes of MPEAs with tunable properties.

8:50 AM  
On the Low Cycle Fatigue Response of CoCrNiFeMn High Entropy Alloy with Ultra-fine Grain Structure: Sezer Picak1; Thomas Wegener2; Seyedvahid Sajjadifar2; Julia Richter2; César Sobrero2; Thomas Niendorf2; Ibrahim Karaman1; 1Texas A&M University; 2University of Kassel
    High Entropy Alloys (HEAs) such as CoCrFeMnNi are a new class of multi-component metallic materials. However, their relatively low yield strength level is a major drawback. In the present work, Equal Channel Angular Pressing (ECAP) was employed to improve the initial yield strength level of CoCrFeMnNi HEA. In light of envisaged fields in engineering, the HEA was also studied under cyclic loading as this might show different microstructural evolution as compared to quasi-static monotonic loading. To allow for meaningful comparison, both strain-controlled low cycle fatigue tests under fully reversed push-pull loading (R=-1) and quasi-static (tensile and compression) tests were conducted at room temperature. Very high tensile and compressive yield strength levels (1.1 GPa) and superior fatigue life were obtained after ECAP processing due to grain-refinement and high defect densities. Finally, the formation of cell structure in these kinds of alloys is, for the first time, rationalized by the Copley-Kear effect.

9:10 AM  
Hierarchical Eutectoid Nano-lamellar Decomposition in an Al0.3CoFeNi Complex Concentrated Alloy: Sriswaroop Dasari1; Bharat Gwalani1; Abhinav Jagetia1; Vishal Soni1; Stephane Gorsse2; Rajarshi Banerjee1; 1University of North Texas; 2University of Bordeaux, France
    This talk reports a novel eutectoid nano-lamellar (FCC+L12) / (BCC+B2) microstructure that has been discovered in a relatively simple Al0.3CoFeNi high entropy alloy (HEA) or complex concentrated alloy (CCA). This eutectoid microstructure is a result of solid-state decomposition of the FCC matrix and therefore distinct from the commonly reported eutectic microstructure in HEAs which results from solidification. This novel nano-lamellar microstructure exhibits a tensile yield strength of 1074 MPa with a reasonable ductility of 8%. The same alloy can be tuned to form a more damage-tolerant FCC+B2 microstructure, retaining high tensile yield stress (~900 MPa) with appreciable tensile ductility (>20%), via annealing at 700°C. Such tunability of microstructures with dramatically different mechanical properties can be effectively engineered in the same CCA, by exploiting the complex interplay between ordering tendencies and lattice distortion.