• 소성∙가공
• /
• 제5권4호
• /
• pp.327-336
• /
• 1996

The effect of dynamic strain aging on high temperature deformation behaviour of the A-Mg alloy was investigated by strain rate change tests and stress relaxation tests between 20$0^{\circ}C$and 50$0^{\circ}C$. Yield point, short stress transient and periodic discontinuities on the stress-strain curve were considered as an evidence of the effect of dynamic strain aging. With this criterion two distinct strain rate-temperature regimes could be manifested. Dynamic strain aging was considered to be effective in the high temperature-low strain rate regime, whereas dynamic recovery was a dominant deformation mechanism in the low temperature-high strain rate regime. It was found that dynamic strain aging in the high temperature deformation was governed by the mechcanism of diffusion-controlled, viscous dislocation movement.

• 소성∙가공
• /
• 제9권1호
• /
• pp.72-79
• /
• 2000

Dynamic recrystallization (DRX), which may occur during hot deformation, is important for the microsturctural evolution of 304 stainless steel. Especially, the current interest in modelling hot rolling demands quantitative relationships among the thermomechanical process variables, such as strain, temperature, strain rate, and etc. Thus, this paper individually presents the relationships for flow stress and volume fraction of DRX as a function of processing variables using torsion tests. The hot torsion tests of 304 stainless steel were performed at the temperature range of 900~110$0^{\circ}C$ and the strain rate range of 5x10-2~5s-1 to study the high temperature softening behavior. For the exact prediction of flow stress, the equation was divided into two regions, the work hardening (WH) and dynamic recovery (DRV) region and the DRX region. Especially, The flow stress of DRX region could be expressed by using the volume fraction of DRX (XDRX). Since XDRX was consisted of the critical strain($\varepsilon$c) for initiation of dynamic recrystallization (DRX) and the strain for maximum softening rate ($\varepsilon$*), that were related with the evolution of microstructure. The calculated results predicted the flow stress and the microstructure of the alloy at any deformation conditions well.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2000년도 춘계학술대회논문집
• /
• pp.183-186
• /
• 2000

The high temperature deformation behavior of spray-formed Al-19wt%Si-1.87wt%Mg-0.085wt.%Fe alloy was studied by torsion testing in the strain rate range of 0.001-1 sec-1 and in the temperature range of 300-500 $^{\circ}C$. The relationship between stress temperature and strain rate is expressed using the Power law. the behavior of dynamic recrystallization is showed in 300-35$0^{\circ}C$, 1-0.1sec-1 and the behavior of dynamic recovery is showed in 450-50$0^{\circ}C$, 0.01-0.001sec-1 The size of Si particles is mall when the temperature is low and the strain rate is high. The strain rate sensitivity(m) and the apparent activation energy(Q) indicate the dependence on strain rate and temperature for flow stress respectively. The hot ductility is high when m is high and Q is low. The maps of strain rate sensitivity and apparent activation energy suggest the optimum processing conditions.

• 소성∙가공
• /
• 제8권2호
• /
• pp.169-177
• /
• 1999

Different pass strains and rolling temperatures were applied to understand the effects of pass strain and rolling temperature on flow stress and flow strain of AA5083 alloy. The specimens were prepared by conventional casting process followed by hot rolling. Hot torsion tests were conducted at temperature ranges of 350 to 52$0^{\circ}C$ under a strain rate of 1.0/sec. During the process, hot-restoration mechanisms, dynamic recovery(DRV) or dynamic recrystallization (DRX), of the AA5083 alloy were analyzed from the flow curves and deformed microstructures. It was found that while the rolling strain per pass and rolling temperature have little effect on the folw stress, they have significant effect on the failure strain. The DRV was responsible for the hot restoration mechanism of the hot-rolled specimen. heavily elongated grains and small subgrains containing dislocations were obtaned during the hot deformation. This was due to the presence of Al6Mn precipitate in the alloy.

• Parasites, Hosts and Diseases
• /
• 제22권2호
• /
• pp.153-160
• /
• 1984

마우스에 대한 요꼬가와흡충 실험감염에 있어서 충체회수률 등 감수성(susceptibility)이 실험자에 따라 매우 다른 것으로 보고되어 있다. 저자등은 이것이 마우스 strain에 따른 차리일 것으로 생각하고 이 연구를 통하여 확인하고자 하였으며, 또 prednisolone 주입으로 숙주의 면역반응을 저하시킬 때 충체회수률이 어떻게 변동하는가를 관찰하고자 하였다. 요꼬가와흡충 피낭유충은 은어의 근육으로부터 분리한 것을 사용하였으며 마우스 5개 strain (A,DBA,CBH,CSTBL,KK) 총 60마리에 대해서 300개씩의 피낭유충을 감염시키고 1주일 후에 도살하여 충체회수률 및 충체의 크기를 측정하였다. 또 ICR계 마우스 29마리에는 각각 1,800개씩의 피낭유충을 감염시키되 그중 15마리에 대해서는 감염 1주전부터 도살시까지 격일로 prednisolone l0mg/kg를 주입하였고 각각 감염 6시간부터 35일까지의 충체회수율을 관찰하였다. 결과는 다음과 같다. 1. 마우스 5개 strain에 있어서 감염성공률은 25.0∼83.3%의 범위에서 차이를 보였고, 충체국수률은 1.2∼18.9%, 나수충체의 크기는 평균 길이 0.554∼0.683mm, 폭 0.214∼0.244mm이었다. 이들 수치는 KK 및 CSTBL strain에서 다른 3가지 strain보다 높았으며 유의한 차리를 보였다. 2. ICR계 마우스에서 시간별로 관찰한 충체회률은 감염 1일이내에는 38∼66%의 높은 율을 보였으나 1일 이후에는 35일까지 0.7%이하의 낮은 회수률을 보였다. 그러나 prednisolone을 투여한 군에서는 감염후 21일까지도 16∼80%의 높은 충체회수률을 유지하였다. 이상의 결과로 미루어 보아 마우스는 strain에 따라 요꼬가와흡충 감염에 대한 감수성이 다르다 것을 알 수 있었다. 또한 감수성이 낮은 ICR계 마우스도 prednisolone을 주입하면 감염 3주까지 높은 충체회수률을 얻을 수 있음을 알게 되었다.

• 한국재료학회지
• /
• 제17권10호
• /
• pp.521-525
• /
• 2007

The microstructure and mechanical properties of an oxygen free copper processed by accumulative roll bonding(ARB) at low strain rate were studied. The copper sheets were highly strained up to an equivalent strain of ${\sim}6.4$ by ARB process at ambient temperature. The strain rate of the copper during the ARB was $2.6sec^{-1}$. The microstructure and mechanical properties of the ARB-processed copper were compared to those of the specimens processed by ARB at relatively high strain rate ($37sec^{-1}$). The microstructure and mechanical properties of the copper with ARB process was very similar to each other despite of some differences in recovery.

• 분석과학
• /
• 제9권2호
• /
• pp.179-191
• /
• 1996

초내열 718 합금의 고온 변형 거동을 이해하기 위하여 rotating grade의 718 합금을 이용하여 온도 $927{\sim}1066^{\circ}C$, 변형속도 $5{\times}10^{-4}{\sim}5{\times}10^0sec^{-1}$ 범위에서 진변형량 0.7까지 압축실험을 수행하였다. 최대 유동 응력은 변형 속도가 증가하고 시험 온도가 감소함에 따라 증가하였다. 변형 속도 $5{\times}10^{-1}sec^{-1}$을 제외한 대부분의 설험 조건에서 가공 연화현상이 관찰되었다. 가공 연화는 저온, 고변형 속도에서는 주로 동적 회복 및 변형 쌍정에 의해 일어나는 반면 고온, 저변형 속도 조건에서는 동적 재결정에 의해 발생하였으며 $5{\times}10^{-1}sec^{-1}$의 변형 속도 조건에서는 동적 재결정된 결정 입자들의 재가공 경화에 의해 가동 경화현상이 나타났다. 변형 속도 감도(m)는 변형 속도가 낮은 경우에는 0.3 정도로서 주로 동적 재결정에 의해 변형 거동이 나타남을 반영하였으며 고변형 속도에서는 0.1 정도로서 동적 회복과 변형 쌍정의 발생으로 718 합금의 변형이 이루어짐을 알 수 있었다.

• 소성∙가공
• /
• 제17권8호
• /
• pp.579-585
• /
• 2008

Numerous experimental investigations on metallic materials and solid polymers have shown that relaxation behavior is nonlinearly dependent on prior strain rate. The stress drops in a constant time interval nonlinearly increase with an increase of prior strain rate. And the relaxed stress associated with the fastest prior strain rate has the smallest stress magnitude at the end of relaxation periods. This paper deals with the performance of three classes of unified constitutive models in predicting the characteristic behaviors of relaxation. The three classes of models are categorized by a rate sensitivity of kinematic hardening rule. The first class uses rate-independent kinematic hardening rule that includes the competing effect of strain hardening and dynamic recovery. In the second class, a stress rate term is incorporated into the rate-independent kinematic hardening rule. The final one uses a rate-dependent format of kinematic hardening rule.

• 열처리공학회지
• /
• 제21권6호
• /
• pp.307-313
• /
• 2008

The tensile deformation and shape recovery behaviors were studied in Ni-Ti shape memory wires showing different transformation characteristics by annealing at $200{\sim}600^{\circ}C$. Both R phase ${\rightarrow}$ B19' martensitic transformation at lower temperature and B2 ${\rightarrow}$ R phase transformation at higher temperature occurred in the shape memory wires annealed at $200{\sim}500^{\circ}C$. Transformation temperature and heat flow of B19' martensite increase but those of R phase main almost constant even with increasing annealing temperature. In the case of wires annealed and then cooled to $20^{\circ}C$, plateau on stress-strain curves in tensile testing can be observed due to the collapse of R phase variants and the formation of deformation-induced B19' martensite. In the case of wires annealed and then cooled to $-196^{\circ}C$, however, plateau on stress-strain curves does not appear and stress increases steadily with increasing tensile deformation. Comparing shape recovery rate with cooling temperature after annealing, shape recovery rate of the wire cooled to $20^{\circ}C$ is higher than that of the wire cooled to $-196^{\circ}C$ after annealing, and maximum shape recovery rate of 95% appears in the wire annealed at $400^{\circ}C$ and then cooled to $20^{\circ}C$. $R_s$ and $R_f$ temperatures measured during shape recovery tests are higher than $A_s$ and $A_f$ temperatures measured by DSC tests even at the same annealing temperature.

• 열처리공학회지
• /
• 제37권2호
• /
• pp.79-85
• /
• 2024

The effects of annealing heat treatment and the addition of Cu element on the shape memory effect of the NiTi-based alloy were investigated by analyzing differential scanning calorimeter results and characterizing recovery rate through 3D scanning after Vickers hardness test. Through 3D scanning of impressions after Vickers hardness test, the strain recovery rates for specimens without annealing treatment and annealed specimens at 400, 450, and 500℃ were measured as 45.96%, 46.76%, 52.37%, and 43.57%, respectively. This is because as the annealing temperature increases, both B19' and NiTi₂ phases, which can impede martensitic transformation, are incorporated within the NiTi matrix. Particularly, additional phase transformation from R-phase to B19' observed in specimens annealed at 400 and 450℃ significantly contributes to the improvement in strain recovery rates. Additionally, the results regarding the Cu element content indicate that when the total content of Ni and Cu is below 49.6 at.%, the precipitation of fine B19' and NiTi₂ phases within the matrix can greatly influence the transformation enthalpy and temperature range, resulting in relatively lower strain recovery rates in NiTi alloys with a small amount of Cu element produced in this study.