Effects of pulsed laser surface treatments on microstructural characteristics and hardness of CrCoNi medium-entropy alloy

ABSTRACT

A hot-swaged/annealed CrCoNi medium-entropy alloy (MEA) was surface-treated by pulsed laser at two different powers (400 and 200 W). Microstructural characteristics of the laser-modified zones were characterised and analysed by energy dispersive spectroscopy, electron backscatter diffraction and electron channelling contrast imaging techniques. Results show that melting and rapid solidification occur on the surfaces of both laser-treated specimens, and profuse annealing twins existing in the initial microstructures are essentially eliminated in the melting zone (MZ) with plentiful low-angle boundaries appearing. Meanwhile, the initial equiaxed grains are replaced by new grains in the MZ with either granular or columnar appearance (in the 2D cross-sectional views). These grains are comprised of fine cellular structures with relatively uniform sizes (∼1–2 μm in width/diameter), the formation of which is related to the segregation of Cr during solidification. With the laser power decreasing from 400 to 200 W, the volume of the MZ and grain sizes in its interior are reduced. This is due to less heat supply and faster cooling caused by reducing the laser power. Hardness tests reveal that the surfaces of both the laser-treated specimens are slightly softened (by 10–20%), and quantitative analyses suggest that this is mainly related to grain coarsening and the disappearance of annealing twins in the MZ.

KEYWORDS:

• CrCoNi MEA
• laser surface treatment
• annealing twin
• hardness
• electron backscatter diffraction

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Linjiang Chai http://orcid.org/0000-0001-9750-6259

Additional information

Funding

This work was financed by the Fundamental and Cutting-Edge Research Plan of Chongqing [grant number cstc2018jcyjAX0299], the National Natural Science Foundation of China [grant number 51601147], the Natural Science Foundation of Shaanxi Province [grant number 2017JQ5010] and the Ministry of Science & Technology of China [grant number 2017YFA0700700].