A copper-nickel alloy of composition 70 wt% Ni-30 wt% Cu is slowly heated from a temperature of 1300°C (2370°F). (a) At what temperature does the first liquid phase form? (b) What is the composition of this liquid phase? (c) At what temperature does complete melting of the alloy occur? (d) What is the composition of the last solid remaining prior to complete melting? (e) Explain what kind of microstructure would occur if the alloy was very rapidly cooled from 1400°C to RT. (f) How would the microstructure be different if the sample was cooled less quickly but still not slowly enough to allow a fully equilibrium microstructure? Would the alloy be safe to heat to 10 C below the solidus in this condition?

(a) 1345°C

(b) 59 wt% Ni

(c) 1380°C

(d) 79 Wt% Ni

(e) Cored micro-structure

(f) No

Explanation:

(a) Whereby we have the composition of copper given as 70%, we have;

The first liquid phase forms at the point of intersection of the vertical line from the 70% copper composition mark to the α, (α + L) phase boundary which is at 1345°C

(b) The composition of the liquid phase can be found from the horizontal line drawn from the intersection of the 70% composition nickel line and the phase boundary of the α-(α + L) to the intersect with the Liquidos line

The concentration at the point of intersection with the liquidos line = 59 wt%  

(c) The temperature of complete liquidation can be found by extending the vertical 70 wt% Ni line to the liquidos line which gives the temperature of approximately 1380°C

(d) The composition of the last solid remaining can be found by extending an horizontal line to the right of the intersection of the 70 wt% Ni line to the liquidos line to intersect the solidus line and reading off the composition which is found to be 79 Wt% Ni

(e)   Whereby the temperature is cooled very rapidly from 1400°C to RT the composition of the alloy will change due to the cooling rate is faster than the rate at which equilibrium is maintained resulting in segregation or coring

(f) Whereby the cooling rate is less rapidly cooled the composition consists of a substantial amount of liquid and since the temperature to which it would be heated is 10°C below the solidus line, the difference between the composition of the fast cooled alloy and the equilibrium allow will be small, however, there would be some liquid of low melting component present hence it is not safe as complete cooling solidification at the slightly fast cooling rate occurs at about 1240°C which is 105°C lower than the equilibrium solidification temperature

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