In the range < 35 % Zn, the matrix is a fully alpha phase, which is a ductile phase and strengthened by increasing the Zn content. Above 35% Zn, a brittle beta phase is induced. Consequently, the ductility of the brass significantly decreases.

That's a Very Good Question. Yes it's true that strength and ductility are generally vary inversely , in this particular context Cu is FCC and Zn is HCP. FCC has 12 slip systems while HCP has only 3 slip systems. The more the slip systems higher the ductility and vice versa. FCC metals are very ductile and HCP metals are brittle.

The anamoly lies in tradeoff between ductily and brittleness. While HCP is contributor to strength while FCC to ductility. While former has little influence,but latter has greater influence by which we observe elongation and strength increase in single phase alpha region. Beyond >35%,beta phase steps in the influence of HCP Zn is more predominating than FCC Cu by which elongation decreases and strength increases(alpha + beta) region. Beyond >45%,fully beta prime HCP. Both strength and ductility fall.

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Below 35% Zinc content in brass, it increases yhe ductility of the brass and the strength also increases. With increasing amount of zinc, being a brittle beta phase, the ductility of the alloy begins to decrease while its strength increases. But if the amount of zinc is increased continuously, strength too begins to decrease because the material will begin to become brittle.