Researchers produce high strength steel with increased fracture resistance and ductility

Scientists from the University of Hong Kong and the Lawrence Berkeley National Laboratory have managed to create a steel product that presents high strength, cracking resistance and ductility.

In a study, recently published in Science Magazine, researchers present a technique to produce Dual-phase steel that can be used in the industry. The produced material has increased ductility and fracture resistance while preserving the steel's high strength.

Steel is a material widely used in civil engineering. It can be utilized alone to form steel structures or alongside with concrete to form most of the modern buildings (reinforced concrete). Concrete behaves great in compression but lacks significant tensile strength. Therefore, it is reinforced with steel to bear the tensile stresses. Steel also provides durability and sustainability while it is a cost-efficient material.

Nevertheless, in industry, engineers constantly aim at developing new materials that could satisfy 3 major criteria:

1. Strength

2. Ductility

3. Fracture resistance (toughness)

The efforts to accomplish such a task have not always been successful due to a major reason. In most of the techniques used, when one of those attributes is enhanced, the others will be undermined. Therefore, an increase in ductility will result in a deterioration in strength and the opposite. The phenomenon is known as strength-ductility trade-off dilemma.

Dual-phase steel is a type of steel that acquires a ferritic–martensitic microstructure. When it is stressed, the strain is concentrated in the ferrite parts which are generally soft and present ductility. The islands of martensite have increased tensile strength ultimately providing the material with higher load capacity. However, dual-phase steel has not fully tackled the strength-ductility trade-off and new efforts to produce better combinations are underway.

Scientists created the new steel material through a unique deformed and partitioned method. According to the authors, their technique resulting in a material with great performance considering all factors examined (strength, ductility and toughness).

“In this latest breakthrough in super D&P steel, we attained an unprecedented strength-toughness combination which can address a major challenge in safety-critical industrial applications – to attain an ultra-high fracture toughness so as to prevent catastrophic premature fracture of structural materials. The breakthrough also changes the conventional view that attaining high strength will be at the expense of deteriorating toughness, which invariably leads to the embrittlement of structural materials and greatly limits their application,” Huang Mingxin, co-author of the study and a Professor at the Department of Mechanical Engineering of the University of Hong Kong, stated.

The behavior of the material is associated with the propagation of multiple micro-cracks which are formed below a main fracture, absorbing energy and providing a "toughening mechanism".

Laboratory tests conducted in Lawrence Berkeley National Lab on the new steel, yielded a tensile strength of 2000 MPa, deformability of 19% and an intensity factor of 102 MPa*m^1/2. In comparison, typical steel has a tensile strength of 350 MPa, deformability of 15% and intensity factor of 50 MPa*m^1/2.

Source: University of Hong Kong