Grain boundary engineering: Development of a strong and tough CoNiCr alloy

A research team from Zhejiang University has developed a strong and tough CoNiCr alloy by overcoming intermediate temperature brittleness (ITB). By using the strategy of grain boundary engineering (GBE), the strength and ductility of the alloy are simultaneously improved at elevated temperature. The solution here might provide a pathway to overcome the GB brittleness, which currently restricts practical application of many metallic alloys.

Precipitation strengthened alloys are widely used in high temperature structural components for aerospace, petrochemical, energy, power industries. Unfortunately, Severe brittleness during processing or service at the intermediate temperature range (600-900 °C) seriously threatens the reliability for this kind of alloys (ITB), which is normally related to GB brittleness.

To resolve the ITB in CoNiCr alloy, a strategy through grain boundary engineering (GBE) is purposed, including introducing either the heterogeneous γ′ phase (GBE I) or grain boundary pinning TCP phase (GBE II) at the grain boundaries. Both GBE methods successfully refine the grains and achieve the better grain boundary structure, so that the fracture mode of the alloys changed from intergranular brittle fracture to ductile fracture at 800 ℃. The elongation to fracture (EF) of the alloy increases from 1% to ~10%, and the yield strength simultaneously increases from ~650 MPa to 770~850 MPa.

By using advanced microscopy, deformation mechanism has been explored and a cracking model has been developed. For the GBE treated alloy, in addition to dislocation shearing, the deformation mechanism also involves stacking faults and deformation twins, which might contribute the strong and tough CoNiCr alloy after GBEs.

The Future

ITB is a relatively complicate issue, which attracts many attention ever since the dawn of modern metallurgical research. There are still many controversies about the interpretation of the causes and remedies for the ITBs. Although fundamental understanding the ITB mechanism is important, it should be noted that engineering approach to provide reliable and economic pathway to resolve this long lasting issue is the key to alloy development for real practical applications.

The impact

By using the GBE strategy, the strength and ductility of the alloy are simultaneously improved by overcoming ITB in a CoNiCr alloy.The solution here might provide a pathway to overcome the GB brittleness for many high strength metallic alloys, including superalloys, heat-resistance steelS and high entropy alloys, etc.  

The research has been recently published in the online edition of Materials Futures, a prominent international journal in the field of interdisciplinary materials science research.

Reference:

Zhouqing Zhang, Qingqing Ding, Yanfei Gao, Xiao Wei, Ze Zhang, Hongbin Bei. Evading the intermediate temperature brittleness of a precipitation strengthened CoNiCr alloy by grain boundary engineering[J]. Materials Futures. 2024. DOI: 10.1088/2752-5724/ad9bce

---

---