The microstructures, mechanical properties (tensile and low cycle fatigue), and thermal aging behavior of the new Cu-Al-Ni-X shape memory alloys have been investigated. The compositions of alloys studied are Cu(13~14)Al-(3~4)Ni-(0.3~0.6)Zr an Cu13Al-3Ni-0.06Si-0.6Zr, having $M_s$ temperature from 35 to 183°C. The $M_s$ temperatures decreased with increasing electron/atom ratio. The decreasing of $M_s$ temperature with grain size refinement was observed. The smaller gain size, the more grain restraint in martensite transformation occurs, thereby decreasing $M_s$ temperature with decreasing grain size.
The hot rolled plates were solution treated a the temperatures of 850°C and 950°C with varying times. A significant grain size refinement occurred by recrystallization treatment. During recrystallization process, a very rapid grain growth took place for the Zr-free alloy, while a delayed grain growth took place for the Zr-added alloys. The results show that the addition of Zr to Cu-13.4Al-3.8Ni was very effective in suppressing the subsequent grain growth. Auger electron analysis showed a small amount of Zr segregated at the grain boundaries, which was mainly responsible for the suppression of grain growth in the Zr-added alloys. Also the drag-effect of precipitations, Y-phase(Zr-rich), was responsible for grain size refinement.
The grain size refinement was very beneficial in enhancing the fracture strength($\sigma_f$) and fracture strain($\epsilon_f$). The increases of transition stress($\sigma_f$), fracture stress($\sigma_f$), and fracture strain($\epsilon_f$) through grain size refinement in the alloys were found to obey a Hall-Petch relation. The highest stress (UTS 830 MPa) and strain (8%) were obtained for the alloy having the composition of Cu-13.4Al-3.1Ni-0.06Si-0.58Zr. The tensile fracture mode of Zr=added alloys was predominantly ductile fracture, while that of Zr-free alloy was mainly intergranular fracture. A change of fracture mode from intergranular to ductile was occurred by the addition of Zr.
The variations of tensile properties(especially $\sigma_t,\sigma_f$) with testing temperatures were also studied. A linear variation of $\sigma_t$ above transformation temperature($M_s$) has been found to obey the Clausius-Clapeyron relation At temperature below $M_s$, $\sigma_t$ increased slowly with decreasing temperatures. The fracture stress($\sigma_f$) appears to increase with decreasing temperatures from $M_s$, while at temperatures above $M_s$, $\sigma_f$ was nearly constant with some scattering. A high recovery(above 90%) occurred at 3% tensile strain when heated to transformation temperature($A_f$).
The low cycle fatigue properties due to grain size refinement were also investigated in the alloy. Coffin-Manson law was obeyed for $\Delta\epsilon_p$ values obtained from different cycle number(N) to failure.
Microstructures of investigated alloys were examined by X-ray and transmission electron microscopy(TEM) analysis. Martensites formed were both a M18R martensite having internal stacking faults and a N2H martensite having internal twins according to the electron-atom(e/a) ratios. The microstructure changed from a M18R to a mixture of M18R and N2H martensite as the e/a ratio increased. As the e/a ratio further increased, a completely N2H single phase was observed. This change of e/a ratio was more sensitive to Al composition compared to Ni composition.
The spot patterns of TEM of the M18R and the N2H martensite were analysed by computer-simulated diffraction analysis method. The major orientation relationships of a twin or two phases were also derived from the transformation matrices ($T^i_j$, $T_j^{i-1}$) using the computer-simulated diffraction analysis. The habit plane in the M18R martensite was {4 4 1} type, while that of the N2H martensite was {1 3 -3} type. The ratio of the two twins in a N2H plate calculated was 2.5~3: 1, which agreed with the experimentally observed ratio of 3: 1.
From TEM examinations and the result of tensile test, it was found that as the volume fraction of a N2H martensite increased, the tensile properties (σ and $σ_f$) decreased. It was found that tensile properties, especially transition and fracture stress, were sensitive to microstructure as well as test temperatures.
A new comprehensive kinetic model has been applied to the analyses of the isothermal and non-isothermal aging kinetics of the Cu-Al-Ni-X shape memory alloys. The derived apparent activation energies of isothermal and non-isothermal agings were 150 ± 5KJ/mol. It appears that the apparent activation energy was related to Al and Ni diffusion. During the isothermal aging from 380 to 480°C, a eutectoid transformation (β → α + \gamma_2$) were occurred.
본 논문의 실험 결과를 요약하면 아래와 같다.
1. 미량합금 원소(Zr)의 첨가와 재결정처리에 의해서 고강도(830MPa)와 양호한 연성 (8%)를 가지는 새로운 Cu-Al-Ni-X계 형상기억 합금을 개발하였다.
2. 기계적 성질의 향상은 결정립 크기와 마르텐사이트 variant의 미세화에 기인했고, Augerelectron spectroscopy에 의한 분석결과 결정립계에 편석된 Zr의 solute drag effect가 결정립의 미세화에 결정적인 역할을 한다는 사실을 확인했다. 또한 석출물인 Y-Phase (Zr-rich)의 drag-effect역시 결정립의 미세화에 기여함도 확인하였다.
3. 인장성질은 결정립의 미세화에 따라 증가하는 Hall-Petch relation에 따랐고 Fracture mode는 입계파괴에서 결정립의 미세화에 따라 연성파괴로 변했다. 또한 인장성질들은 실험온도에 따라 변화를 보였고 특히 변태온도($M_s$)이상에서 Transition stress($\sigma_t$)의 변화는 측정 온도가 증가함에 따라 직선적으로 증가하는 Clausius-Claperyron relation을 따랐다.
4. 연구된 합금들은 3%의 인장 변형률에서도 90%이상의 회복력을 보였다. 이는 미량합금 원소의 첨가에 따른 결정립 미세화가 형상기억능을 거의 감소시키지 않으면서 제반 기계적 성질을 향상시킴을 나타낸다.
5. 미량합금 원소에 따른 결정립의 미세화는 또한 low cycle fatigue properties를 크게 증가 시켰고 failure되는 cycle(N)과 plastic strain($\Dekta\epsilonε_p$)사이의 관계는 Coffin Manson law로 설명할 수 있었다.
6. 연구된 합금들의 변태온도는 unit cell당 electron-atom ratio(e/a)가 증가함에 따라 감소했으며 결정립가 미세해질 수 록 감소하였다.
7. 연구된 합금들은 e/a ratio가 증가함에 따라 M18R 마르텐사이트에서 M18R과 N2H의 mixture structure로 변하고 e/a가 더 증가하면 N2H의 single phase로 됨을 확인할 수 있었고, TEM의 Diffraction Pattern을 Computer-simulated diffraction analysis method를 사용해서 분석한 결과가 TEM의 Diffraction Pattern과 완전히 일치하는 결과를 얻었다.
8. 또한 합금들의 인장성질은 조직의 미세화와 실험 온도뿐만 아니라 N2H와 M18R 마르텐사이트의 부피 분율에도 민감함을 알 수 있었고 N2H의 부피 분율이 증가함에 따라 Transition stress($\sigma_t$)와 Fracture stress($\sigma_f$)가 크게 감소했다.
9. Computer-simulated diffraction analysis method에서 사용되는 transformation matrices($T^i_j,T^{j-1}_j$)을 이용해 여러 상(or twin)사이의 결정학적 관계를 정확히 알아낼 수 있었고 현상학적 이론(B-M, W-L-R theory)를 사용해서 모상($D0_3$)에서 마르텐사이트로 변태시의 결정학적 data를 정확히 예측할 수 있었다.
10. 연구된 합금들의 Isothermal aging과 Non-Isothermal aging시의 상변화를 new comprehensive kinetic equation으로 정확히 분석할 수 있었고, 구한 apparent activation energy는 150 ± 5 KJ/mol로 거의 같은 값을 보였다. 상변화를 X-ray와 TEM으로 확인한 결과 고온 β 상에서 평형 α+$\gamma_2$상으로의 eutectoid transformation 이었으므로, 위의 활성화 에너지는 Al과 Ni의 확산과 관계되는 에너지로 생각된다.
