Size effects of lamellar twins on the strength and deformation mechanisms of nanocrystalline hcp cobalt

Wen Wang,Fuping Yuan,Xiaolei Wu,Ping Jiang

doi:10.1038/s41598-017-09919-2

Abstract

Twins play an important role in the deformation of nanocrystalline (NC) metals. The size effects of {10bar{1}2} tensile/{10bar{1}1} compressive lamellar twins on the tensile strength and deformation mechanisms of NC hcp cobalt have been investigated by a series of large-scale molecular dynamics simulations. Unlike the size effects of twins on the strength for polycrystalline fcc metals, the strength of NC hcp cobalt with lamellar tensile/compressive twins monotonically increases with decreasing twin boundary spacing (TBS) and no softening stage is observed, which is due to the consistent deformation mechanisms no matter TBS is large or small. These consistent deformation mechanisms can be categorized into four types of strengthening mechanisms: (i) Partial basal dislocations nucleated from grain boundaries (GBs) or twin boundaries (TBs) intersecting with TBs/GBs; (ii) Phase transformation from hcp to fcc; (iii) <c + a> partial edge dislocations nucleated from TBs intersecting with basal partial dislocations; (iv) Growth of the newly formed secondary tensile twins inside the primary compressive/tensile twins. The observed multiple twinning in MD simulations has also been confirmed by TEM after tensile testing in NC cobalt processed by severe plastic deformation.

Highlights

Stronger and tougher metals and alloys have been the pursuit of scientists for structural applications for centuries strength and ductility are in general mutually exclusive[1,2,3,4,5,6,7,8,9,10,11]
In order to study the size effects of twins, the twin boundary spacing (TBS) is varied while the grain size is fixed at d = 60 nm in the present study
With the same grain size, it should be noted that the average flow stress monotonically increases with decreasing TBS for NC cobalt with both lamellar {1012} tensile twins and lamellar {1011} compressive twins, and there is no observed softening stage when the TBS is small

Summary

Introduction

Stronger and tougher metals and alloys have been the pursuit of scientists for structural applications for centuries strength and ductility are in general mutually exclusive[1,2,3,4,5,6,7,8,9,10,11]. The corresponding close-up views for Fig. 2 showing two following deformation mechanisms are displayed in Fig. 4: (i) Partial basal dislocations nucleated from GBs/TBs interacting with other TBs/GBs and formation of basal SFs; (ii) Phase transformation from hcp phase to fcc phase by basal SFs at adjacent planes.

Results

Conclusion