ALBERT

Description: 〈p〉Publication date: Available online 20 August 2019〈/p〉 〈p〉〈b〉Source:〈/b〉 Acta Materialia〈/p〉 〈p〉Author(s): Hao Chen, Valery Levitas, Liming Xiong〈/p〉 〈div xml:lang="en"〉 〈h5〉Abstract〈/h5〉 〈div〉〈p〉Molecular dynamics (MD) simulations of the amorphous band nucleation and growth ahead of the tip of a shuffle 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.svg"〉〈mrow〉〈msup〉〈mrow〉〈mn〉60〈/mn〉〈/mrow〉〈mrow〉〈mi〉o〈/mi〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 dislocation pileup at different grain boundaries (GBs) in diamond-cubic (dc) silicon (Si) bicrystal under shear are performed. Amorphization initiates when the local resolved shear stress reaches approximately the same value required for amorphization in a perfect single crystal (8.6-9.3GPa) for the same amorphization plane. Since the local stresses at the tip of a dislocation pileup increase when the number of dislocations in the pileup is increased, the critical applied shear stress 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si2.svg"〉〈mrow〉〈msub〉〈mrow〉〈mi〉τ〈/mi〉〈/mrow〉〈mrow〉〈mi〉a〈/mi〉〈mi〉p〈/mi〉〈/mrow〉〈/msub〉〈/mrow〉〈/math〉 for the formation of an amorphous shear band significantly decreases with the dislocation accumulation at the GBs. In particular, when the number of the dislocations in a pileup increases from 3 to 8, the critical shear stress drops from 4.7GPa to 1.6GPa for both the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉9〈/mn〉〈/mrow〉〈/math〉 and 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉19〈/mn〉〈/mrow〉〈/math〉 GBs and from 4.6GPa to 2.1GPa for the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si5.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉3〈/mn〉〈/mrow〉〈/math〉 GB, respectively. After the formation of steps and disordered embryos at the GBs, the atomistic mechanisms responsible for the subsequent amorphous shear band formations near different GBs are found to distinct from each other. For a high-angle GB, such as 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si5.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉3〈/mn〉〈/mrow〉〈/math〉, an amorphous band propagates through the crystalline phase along the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si6.svg"〉〈mrow〉〈mrow〉〈mo〉(〈/mo〉〈mrow〉〈mn〉112〈/mn〉〈/mrow〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉 plane. For the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉9〈/mn〉〈/mrow〉〈/math〉 GB, partial dislocations forming a stacking fault precede the formation of an amorphous band along the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si7.svg"〉〈mrow〉〈mrow〉〈mo〉(〈/mo〉〈mrow〉〈mn〉110〈/mn〉〈/mrow〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉 plane. For the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.svg"〉〈mrow〉〈mtext〉Σ〈/mtext〉〈mn〉19〈/mn〉〈/mrow〉〈/math〉 GB, the one-layer stacking fault along the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si8.svg"〉〈mrow〉〈mrow〉〈mo〉(〈/mo〉〈mrow〉〈mn〉111〈/mn〉〈/mrow〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉 plane transforms into an interesting intermediate phase: a two-layer band with the atomic bonds being aligned along the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si8.svg"〉〈mrow〉〈mrow〉〈mo〉(〈/mo〉〈mrow〉〈mn〉111〈/mn〉〈/mrow〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉 plane (i.e., rotated by 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si9.svg"〉〈mrow〉〈msup〉〈mrow〉〈mn〉30〈/mn〉〈/mrow〉〈mrow〉〈mi〉o〈/mi〉〈/mrow〉〈/msup〉〈/mrow〉〈/math〉 with respect to the atomic bonds outside the band). This intermediate phase transforms to the amorphous band along the 〈math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si8.svg"〉〈mrow〉〈mrow〉〈mo〉(〈/mo〉〈mrow〉〈mn〉111〈/mn〉〈/mrow〉〈mo〉)〈/mo〉〈/mrow〉〈/mrow〉〈/math〉 plane under a further shearing. The obtained results represent an atomic-level confirmation of the effectiveness of dislocation pileup at the nucleation site for various strain-induced phase transformations (PTs), and exhibit some limitations.〈/p〉〈/div〉 〈/div〉 〈h5〉Graphical abstract〈/h5〉 〈div〉〈p〉〈figure〉〈img src="https://ars.els-cdn.com/content/image/1-s2.0-S135964541930535X-fx1.jpg" width="218" alt="Image 1" title="Image 1"〉〈/figure〉〈/p〉〈/div〉