Second-Phase Shape: Interfacial Energy Effects GP(Guinier- Preston) Zone in Al – Ag Alloys → negligible contribution to the total free energy A. Fully Coherent Precipitates How is the second-phase shape determined? - If α, β have the same structure & a similar lattice parameter - Happens during early stage of many precipitation hardening - Good match can have any shape spherical 여러가지 다른 종류의 석출물에 대해 어떻게 자유에너지 최소화 시킬 수 있는지 … (G.P. Zone) 지름 약 10 nm 인 GP 대 : Al-rich FCC 기지내 Ag-rich FCC 석출물 형성

2 B. Partially Coherent Precipitates - Coherent or Semi-coherent in one Plane; Disc Shape (also plate, lath, needle-like shapes are possible) 실제 석출물의 모양 disc 모양 아님. 그 이유는 1) 불일치의 변형 에너지가 무시됨 2) 석출물의 방향에 따라 성장속도 다름 - α, β have different structure and one plane which provide close match Fig A section through a γ-plot for a precipitate showing one coherent or semicoherent interface, together with the equilibrium shape (a disc).

3 broad face parallel to the {111}  matrix planes Hcp/Fcc 의 방위관계 가지는 판상 Fig Electron micrograph showing the Widmanstatten morphology of γ’ precipitates in an Al-4 atomic % Ag alloy. GP zones can be seen between the γ’ e.g. at H (x 7000).

4 C. Incoherent precipitates - when α, β have completely different structure Incoherent interfaces - Interface energy is high for all plane spherical shape with smoothly curved interface - Polyhedral shapes : 석출물의 어떤 결정면이 γ-plot 의 변곡점 위에 놓이는 경우 or When the two lattices are in a random orientation (Al 2 Cu)

5 Precipitation Hardening

6 Al-Cu ppt structures

7 GP zone structure Al-Cu ppt structures (a)Bright-field TEM image showing G.P. zones, and (b) HRTEM image of a G.P. zone formed on a single ( ) α plane. Electron beam is parallel to in both (a) and (b).

8 S phase in Al-Cu-Mg alloys ; Lath shape phase in Al-Mg-Si alloys ; Needle shape broad face parallel to the {100}  matrix planes Widmanstätten morphology (habit plane) Θ’ 석출물 단위격자 Matrix 의 단위격자 Al 많이 포함한 기지 ( α) 입방대칭이어서 주어진 결정립 내 여러 방향에서 방위를 만족하는 판상 석출물 존재

9 (Al 2 Cu) - Polyhedral shapes : 석출물의 어떤 결정면이 γ-plot 의 변곡점 위에 놓이는 경우

10 Hardness vs. Time by Ageing Ageing at 130 o C produces higher maximum hardness than ageing at 190 o C. At 130 o C, however, it takes too a long time. Double ageing treatment first below the GP zone solvus → fine dispersion of GP zones then ageing at higher T. How can you get the high hardness for the relatively short ageing time? Age Hardening Overaging : 석출물간 간격 증대로 경도 감소 적정시효시간.    중간상 형성시 커다란 격자변형 수반하고, 소성 변형시 전위의 이동을 방해함 고용강화 최대경도  와  공존할때 미세한 석출물의 분포 Fig Hardness vs. time for various Al-Cu alloys at (a) 130 ℃ (b) 190 ℃

11 Precipitates on Grain Boundaries 1)incoherent interfaces with both grains 2)a coherent or semi-coherent interface with one grain and an incoherent interface with the other, 3) coherent or semi-coherent interface with both grains 다른 방위를 갖는 2 개의 결정립 사이의 입계에서 제 2 상의 형성 Fig Possible morphologies for grain boundary precipitates. Incoherent interfaces smoothly curved. Coherent or semicoherent interface plannar.

12 Precipitates on Grain Boundaries A, B; Incoherent, C; Semi-coherent or coherent Fig An α precipitate at a grain boundary triple point in an α – β Cu-In alloy. Interfaces A and B are incoherent while C is semicoherent (x 310).