Study of heavy-flavor quarks produced in association with top-quark pairs at <span class="aps-inline-formula"><math display="inline"><mrow><msqrt><mrow><mi>s</mi></mrow></msqrt><mo>=</mo><mn>7</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math></span> using the ATLAS detector