Ni/$Ni_3$ Al two-phase alloys generally have a structure with cuboidal $Ni_3Al$ phases precipitated coherently in Ni single crystalline matrix (disordered fcc structure), which is an essential structure in Ni-base superalloys. Superalloys are utilized at higher proportion of their actual melting point than any other class of broadly commercial metallurgical materials. Superalloys, which have made much of our very high temperature engineering technology possible, are the materials leading edge of the gas turbines that drive jet aircraft.
We have successfully fabricated thin foils of boron-free $Ni_3Al$ (ordered $L1_2$ structure) single crystals and single crystalline Ni-$Ni_3Al$ two phase alloys by cold rolling. The cold rolling behavior of $Ni_3Al $ single crystals strongly depends on its initial crystallographic orientations. The foils are quite straight and cold reduction above 99% is possible only when the initial rolling direction (RD) is close to <001>. In contrast, when the initial RD is close to <112>, the foils are macroscopically curved and eventually cracked from their side edge causing difficulties in obtaining the thin foils with good qualities. Such an anisotropic rolling behavior has been considered to closely relate to the relative difficulty of sufficient multiple slip activation on more than three slip planes in the heavily deformed $Ni_3Al$ single crystals. The difficulty of the multiple slip activation can be evaluated as the difference of work-hardening rates through the plane strain compression tests of single crystals with various crystallographic orientations. The plane strain compression tests of $Ni_3Al$ single crystals have revealed the increase of the work-hardening rates with the number of the slip planes containing operative deformation modes. These results correspond well to the difficulty of the multiple slip activation deduced from the cold rolling of $Ni_3$ Al single crystal.
However, the origin of hardening is complex. The size and spacing of the precipitates and therefore their volume fraction are important factors. Several distinct trends in microstructure have developed with increase in strength; (1) the volume fraction of $Ni_3Al$ has increased, (2) size of $Ni_3Al$ at first increased then held constant at about one micron, (3) $Ni_3Al$ tends to become more cubic, (4) a secondary precipitate of finely divided cooling $Ni_3Al$ is present.
The work-hardening behavior of the Ni-$Ni_3Al$ two-phase alloys has been studied by many researchers. In the two phase alloys, the size and spacing of the precipitates and therefore their volume fraction are important factors for their work-hardening behavior. Cornwell et al. studied the deformation behavior of the two phase alloy by uniaxial compression tests up to about several per cent of compressive strain. They concluded that plastic deformation occurs only in the Ni matrix containing undeformed $Ni_3Al$ particles yielding a high rate of work-hardening at a very initial stage of deformation, but above some critical shear strain the $Ni_3Al$ particles are also sheared resulting in a decrease of the work hardening rate for the samples containing large $Ni_3Al$ particles. However, detailed mechanisms of the work hardening of the Ni-$Ni3Al$ two phase alloys such as effects of initial crystallographic orientations and imposed strain conditions, which are the most important factors in considering the anisotropic cold rolling behavior, have not been clarified yet.
In this study, the deformation behavior of Ni-$Ni3Al$ Al two phase alloys is examined by the plane strain compression tests of single crystals with various crystallographic orientations. Ni-$Ni3Al$ Al single crystals, where $Ni_3Al$ phases were precipitated in the Ni single crystalline matrix (disordered fcc structure), can be fabricated recently by investment casting method. The deformation behavior is found to strongly depend on the initial crystallographic orientations and morphology of $Ni_3Al$ precipitates. Such an anisotropic deformation behavior is discussed in terms of the difference in deformation of coarse $Ni_3Al$ precipitates as a function of numbers of the slip planes containing operative deformation modes.
Flow behavior is strongly dependent on the initial crystallographic orientations and is found to be divided into two types depending on the operative number of slip planes. The Taylor-Bishop-Hill analysis with correction for lateral widening reveals that the difference in deformation of coarse $Ni_3$ Al precipitates is the one of the most important factors on the deformation behavior at least up strain level of this study. The Ni matrix is mainly deformed at early stage, leading to the decreasing of work hardening rates, and then both phases are deformed simultaneously after critical shear stress level, leading to the increasing of work hardening rates, only when slip(s) were activated on 1 or 2 slip planes. When dislocations are activated on 3 or 4 slip planes, both phases can be deformed at early stage prior to the decreasing of work-hardening and thus the work-hardening rate keep a high level.
Next, plastic flow behaviors of Ni-$Ni_3Al$ single crystals depending on the morphology of $Ni_3Al$l precipitates were studied by the plane strain compression tests. The flow behaviors were strongly dependent on the initial crystallographic orientations in DS18-3 with plates- and rods-like $Ni_3Al$l precipitates rather than DS18-1 alloys with cuboidal $Ni_3Al$ precipitates. For all specimens in DS18-1 alloys and (110)[001]- and (110)[112]-oriented specimens in DS18-3 alloys, the flow stresses were similar at least up to strain level in this study, while the flow stresses were much lower for (100)[011]-, (100)[012]- and (210)[001]-oriented specimens in DS18-3 alloys. Such flow behavior is related to increasing of mean free length of new mobile dislocation by cross-slip. The mean free length can be attributed to channel length of Ni matrix determined by obstacles to dislocation moving. In DS18-1 alloys, the largest $Ni_3Al$ precipitates are the obstacles whether cross-slip can be activated or not. The increasing of mean free length is thought to be not high in this case. However, the obstacles can be transferred from middle size of $Ni_3Al$ cuboids to largest $Ni_3Al$ precipitates by cross-slip in the case of DS18-3 alloys. The increasing of mean free length is much higher than that of DS18-1 alloys resulting in more deformation within Ni matrix and thus lower stress levels. Therefore, the flow behavior of $Ni-Ni3Al$ single crystals was dependent on both crystallographic orientations and morphology of $Ni_3Al$ precipitates simultaneously.
Finally, we fabricated Ni-$Ni_3Al$ thin foil by cold-rolling. It is found that the cold-rolling behavior is strongly dependent on the initial crystallographic orientation rather than morphology of $Ni_3Al$ precipitates. The deformation banding is formed in the case of (100)[001]- and (210)[001]-oriented specimens at 83% reduction in thickness by cold-rolling. In contrast, (110)[001]-oriented specimen keep initial crystallographic orientation without deformation banding. It is well-known that the deformation banding can be formed in order to minimize strong dislocation interactions and strain rate incompatibilities. In the case of (100)[001] and (210)[001] orientation, the normal direction should be shifted toward (110) pole resulting in high strain rate incompatibilities. Therefore, the deformation banding is found in the case of (100)[001]- and (210)[001]-orientated specimens with compensation of shear components. In addition, such serious incompatibilities make parallelograms, which is formed by intense shear localization on two {111} planes at 40% reduction in thickness, more finer composed of random orientation. It can be thus found that the brass-type shear band with high misorientation are inclined 30-35℃ to rolling direction in the case of (100)[001]-oriented specimen at 83% reduction in thickness. In contrast, no brass-type shear band can be found in (110)[001]-oriented specimen because of very low incompatibilities. In the case of (210)[001]-oriented specimen, the incompatibilities are not high as the case of (100)[001]-oriented specimen, and thus lower volume fraction of brass-type shear band can be found.
However, the effects of morphology of $Ni_3Al$ precipitates on the microstructure evolution of Ni-$Ni_3Al$ single crystals during cold-rolling are not serious comparing with the effects of initial crystallographic orientation. Therefore, it can be concluded that the deformation behavior of Ni-$Ni_3Al$ single crystals at serious strain level is strongly dependent on the initial crystallographic orientation rather than the morphology of $Ni_3Al$ precipitates, whereas the initial deformation behavior is related to both crystallographic orientation and the morphology of $Ni_3Al$ precipitates.
Boron이 첨가되지 않은 다결정 $Ni_3$ Al 합금은 강한 입계취성에 의해 박판성형이 매우 어렵다고 인식되어 왔다. 하지만, 최근 일방향응고재를 이용하여 냉간압연만으로 박판성형에 성공하였으며, 압연거동은 초기 결정학적방위에 크게 의존하는 것으로 확인되었다.
초기 압연방위 (RD, rolling direction)가 <001>에 근접한 결정학적방위를 가지는 단결정 잉고트는 99% 이상의 압연율을 가지는 박판의 성형이 가능하였다. 이와는 대조적으로, 초기 RD가 <112>에 근접한 결정학적방위를 가지는 단결정 잉고트는 압연시 박판이 휘거나 크랙이 발생하였다. 이러한 압연거동의 이방성을 평면변형압축시험 (plane strain compression tests)을 통하여 그 원인에 대하여 고찰하였다. 일반적으로 상용화되는 fcc 구조의 금속과는 달리 $Ni_3$ Al 합금은 3개 이상의 슬립면에서는 충분한 슬립이 활성화되기 매우 어렵기 때문에 결정학적방위에 의존하는 이방성이 확인되었다.
최근 정밀주조법 (investment casting)을 이용하여, $Ni_3$ Al 상이 단결정 형태의 Ni 기지에 석출되어 있는 $Ni/Ni_3$ Al 단결정 제조에 성공하였다. $Ni/Ni_3$ Al 단결정은 Ni-기 초내열합금 (superalloy)의 근본이 되는 합금으로써, Ni 기지와 $Ni_3$ Al 석출물이 정합관계를 가지기 때문에 높은 고온강도를 나타낸다. 특히, Ni 단상합금 및 $Ni_3$ Al 단상합금보다 $Ni/Ni_3$ Al 2상합금의 크립강도가 높기 때문에, 고온구조용 재료로서 $Ni_3% Al 단결정보다 실용화의 가치가 더욱 높다. 하지만, 정밀주조법을 이용한 $Ni/Ni_3$ Al 단결정 제조는 최근에 알려진 기술이기 때문에, 이 단결정 자체에 대한 연구뿐만 아니라 박판성형에 앞서 반드시 필요한 결정학적방위에 따른 변형거동의 이방성에 대한 연구도 미흡한 실정이다.
평면변형압축시험을 통하여 $Ni/Ni_3$ Al 단결정의 결정학적방위가 변형거동에 미치는 영향을 분석한 연구결과는 다음과 같다. $Ni/Ni_3$ Al 단결정의 변형거동은 결정학적방위에 크게 의존하며, 활성화되는 슬립면의 수에 따라 크게 2가지 그룹으로 나눌 수 있다. 이는 Ni 기지와 $Ni_3$ Al 석출물의 변형거동의 차이에 기인한다. 활성화되는 슬립면의 수가 2개 이하의 결정학적방위를 가지는 경우, 변형초기에는 가공경화율이 감소하였으며 임계 전단응력이상에서는 가공경화율이 증가하였다. 이는 변형초기에는 $Ni_3$ Al 석출물의 변형이 지연되어 Ni 기지에 의해서만 변형되지만, 임계 전단응력이상에서는 $Ni_3$ Al 석출물의 변형이 시작되기 때문이다. 이와는 대조적으로 활성화되는 슬립면의 수가 3개 이상의 결정학적방위를 가지는 경우, 초기부터 높은 가공경화율을 보이며, 이는 $Ni_3$ Al 석출물이 초기부터 변형되기 때문이다. 또한, 활성화되는 슬립면의 수가 2개인 (210)[001]의 결정학적방위를 가지는 경우, $Ni/Ni_3$ Al 단결정과 $Ni_3$ Al 단결정의 변형거동의 차이를 확인하였다. $Ni/Ni_3$ Al 단결정은 변형초기 Ni 기지에 의해 변형할 시 높은 적층결함에너지에 의해 copper-type의 집합조직을 형성하려는 구동력이 작용하는 반면, $Ni_3$ Al 단결정은 매우 낮은 적층결함에너지를 가지며 brass-type의 집합조직을 형성하려는 구동력이 크기 때문에 서로 다른 슬립계가 활성화됨으로써 나타나는 현상이다.
한편, 평면변형압축시험을 이용하여 미세조직 및 결정학적방위가 $Ni/Ni_3Al$ 단결정의 변형거동에 미치는 영향을 분석하였다. 본 연구에서 수행한 DS18-1 합금의 모든 결정학적방위와 DS18-3 합금에서 cross-slip이 활성화되지 않는 (110)[001]과 (110)[112] 결정학적방위는 서로 유사한 응력거동을 보인 반면, DS18-3 합금 중 cross-slip이 활성화 가능한 (100)[011], (100)[012], (210)[001] 결정학적방위에서 상대적으로 낮은 응력거동이 확인되었다. 이는 $Ni_3Al$ 석출물에 의해 이동이 고착되었던 전위가 cross-slip의 활성화로 인하여 새로이 이동이 가능하기 때문에 나타나는 현상으로, 전위의 이동거리의 증가효과가 $Ni_3$ Al 석출물의 크기 및 분포에 크게 영향을 받기 때문이다. DS18-1 합금에서는, cross-slip의 활성화와 무관하게 0.7～0.8 ㎛의 크기의 $Ni_3Al$ 석출물이 전위 이동의 장애물로 작용하기 때문에 cross-slip에 의한 전위의 이동거리 증가효과는 높지 않다. 그러나, DS18-3 합금에서 전위의 이동의 장애물은, cross-slip의 활성화에 의해 0.1~0.2 ㎛의 중간 크기의 $Ni_3$ Al 석출물에서 가장 조대한 $Ni_3Al$ 석출물로 천이된다. 이로 인해, cross-slip의 활성화에 의한 전위의 이동거리 증가효과가 DS18-1 합금보다 DS18-3 합금에서 현저히 높기 때문에 미세조직 및 결정학적방위에 크게 의존하는 응력거동이 관찰된 것으로 판단된다.
최종적으로, 위의 결과를 바탕으로 냉간압연거동에 미치는 결정학적방위 및 $Ni_3$ Al 석출물에 대한 영향에 대하여 연구하였다. 냉간압연 거동도 평면변형압축시험을 이용한 결과와 유사하게
