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1.
NiTi合金的形状记忆效应与其微观结构特征密切相关,中子衍射技术可以在力学加载过程中原位观察块体NiTi合金的相变、晶间应变以及孪晶再取向等演化特征.结合两相NiTi合金宏观应力一应变曲线呈现的四种阶段性变形特征,利用原位中子衍射技术对其变形过程中的微观结构演化进行了分析.奥氏体初始体积份额约22%,在低应变硬化阶段,晶面(110)B2和(002)B19,的应变分别突然减小和增大表明出现了应力诱发马氏体相变,奥氏体体积份额迅速减小,产生了(011)II型孪晶;同时初始马氏体也开始发生再取向,随着应变量的增加,开始出现新的{20i}型马氏体孪晶,这种孪晶引起的应变卸载时不能回复.在高应变硬化阶段孪晶变形起主导作用,衍射峰半高宽变化较小;而在应变硬化饱和阶段则以滑移机制为主,大量位错的产生使衍射峰半高宽显著增加. 相似文献
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采用VISAR和X光联合测试技术,利用等厚对称和逆向碰撞法测量了FeMnNi合金高压加卸载历程和相变层裂信息。加载过程中,FeMnNi合金样品发生α→ε相转变,相变波速大于塑性波速,在撞击面上相变波与塑性波合并成单一相变波;卸载过程中,FeMnNi合金样品可能发生了逆相变,形成了除合并相变波在自由面反射中心稀疏波R以外的两道卸载波S1和S2。等厚对称高压加载下,FeMnNi合金样品发生了二次层裂。分析中心稀疏波R、卸载波S1和S2在样品中的传播作用过程,发现样品发生冲击相变和卸载逆转变是导致其等厚对称高压加载下发生二次层裂行为的主要原因。 相似文献
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利用高速拉伸实验机在宽的应变率范围内(0.001–1200 s-1), 研究了NiTi形状记忆合金的宏观力学性能随应变率的变化规律, 并借助透射电子显微镜深入研究了微观结构在不同应变率下的演变机制. 研究发现: NiTi合金马氏体(B19’相)孪晶的解孪晶应力随应变率的升高而近乎线性增大, 表明NiTi 合金解孪晶应力具有正向应变率相关性. 在拉伸应变率为10 s-1的样品微观结构中发现了大量的解孪晶区域, 而当应变率进一步增大到100 s-1和 1200 s-1时, 在样品中没有发现解孪晶区域的存在, 样品微观组织以孪晶形式存在. 该结果表明, NiTi合金的马氏体解孪晶速率应在 10–100 s-1范围内. 在高应变率下(≥qslant10 s-1)均发现了热引发奥氏体相(B2)的存在, 表明随应变率的增加, 拉伸过程由等温过程逐渐变为绝热过程. 此外, 在1200 s-1 的样品差示扫描热量曲线中还发现了一个小肩峰, 表明相变过程由一步相变变为两步相变.
关键词:
NiTi形状记忆合金
高速拉伸
应变率相关性
透射电子显微镜 相似文献
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利用正向加载和逆向加载相结合的实验方法,实验研究了从低压到高压三种不同压力范围内Fe基α相合金(Fe-85.03wt%,Mn-7.63wt%,Ni-7.01wt%,Al-0.3wt%)的冲击相变和卸载逆相变历程及对应加载状态的层裂行为特征.给出了Fe基α相合金含冲击相变和卸载逆相变的加卸载Hugoniot线,发现冲击相变阈值、逆相变阈值和冲击加载压力共同决定其冲击加卸载历程,冲击相变强烈影响其层裂行为,导致样品发生了"异常"层裂.利用获得的冲击加卸载历程从应力波相互作用的角度解
关键词:
α相合金')" href="#">Fe基α相合金
相变
逆相变
层裂 相似文献
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对具有两步完全热弹性的Ni53.2Mn22.6Ga24.2单晶的物性采用多种测量手段进行了表征,特别研究了不同温度下的应力-应变特性.研究表明,热诱发的中间马氏体相变应变远大于马氏体相变应变.在较低的形变温度下,沿单晶母相[001]方向的压应力诱发的是两步马氏体相变,材料表现出赝弹性;在较高的形变温度下,只能观察到一步马氏体相变,材料展现出完全超弹性特性.此外,利用热力学理论分别计算了诱发马氏体相变和中间马氏体相变的临界应力与形变温度的关系,与实验测量得到的结果相符.
关键词:
马氏体相变
形状记忆效应
应变
超弹性 相似文献
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采用第二近邻修正型嵌入原子势的分子动力学方法,建立了共格沉淀相与半共格沉淀相块状/柱状模型,模拟了温度诱发相变和应力诱发相变,分析了Ni4Ti3沉淀相对Ni Ti形状记忆合金相变行为的影响.结果表明,Ni4Ti3沉淀相本征应变诱发的弹性应力场对相变中马氏体变体类型、形核位置、分布等有重要影响.在温度诱发相变时,共格沉淀相促进部分马氏体变体的形核生长,能显著提高Ni Ti超弹性形状记忆合金的马氏体相变开始温度;在应力诱发相变时,Ni4Ti3沉淀相使马氏体早于无沉淀相区域形核,导致了相变应力降低、抑制了马氏体解孪,减小了应力-应变曲线的滞回环. 相似文献
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利用内耗手段研究了淬火空位的演变行为及其对Cu-11.9Al-2.5Mn(wt%)形状记忆合金逆马氏体相变温度的影响.通过对不同冷却方式的样品研究表明,较高的冷却速度可以导致较高的逆马氏体相变峰峰温.而对于水淬样品,相变峰峰温随淬火温度呈非单调变化行为,这可能与有序相及无序相中不同的空位形成能有关.经历热循环以后,逆马氏体相变峰峰温显著降低.
关键词:
形状记忆合金
热处理
马氏体相变
阻尼 相似文献
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我们用最近研制成功的LMA型低频力学谱测试系统对NiTi合金马氏体相进行了在很大频率范围内(0.003~1Hz)的低频等温力学谱和温度谱的测量.我们研究的形状记忆合金NiTi(Ni 50.2 at%)试样长34 mm,直径1 mm细丝.经一定热处理,分别在333 K. 343 K和353 K做了内耗随频率的变化的测量.实验表明:频率越小,内耗越大,也就是内耗随频率减少而增大.同时我们采用阶梯升温的方法在八个温度下每个温度测量三种频率(1 Hz, 0.1 Hz, 0.01 Hz)的内耗,结果清楚地表明:不同频率下,内耗峰都出现在372 K(99℃).而且频率越低,峰高越高.这是具有相变峰的特点:相变峰的峰温不随测量频率不同而变化,相变峰高度随频率减少而增大.我们还测量了在1 Hz与0.5 Hz频率下内耗随温度的变化.本文用马氏体相的位错理论初步讨论了上述实验结果. 相似文献
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The stress-induced phase transformation characteristics of unirradiated and proton beam irradiated NiTi alloy were investigated at different tests temperatures. The wire-shaped NiTi specimens were irradiated by 2?MeV proton beam for 30?min at room temperature to a flux of 1019 protons/m2 s. Engineering stress–strain (S-S) curves of both unirradiated and irradiated specimens were obtained using a materials testing machine at 25, 50, 75 and 100°C. The results indicate a single-stage phase transformation from austenite to martensite (B2–B19′) in unirraidated specimens at all the test temperatures. In contrast, in the case of the irradiated specimens, a two-stage austenite–rhombohedral–martensite (B2–R–B19′) phase transformation is observed at 25 and 50°C. The B2–R–B19′ phase transformation, however, is found to change into B2–B19′ transformation at 75 and 100°C. The stress required to initiate the B19′ phase transformation (σMS) and the plateau range are found to be lower in irradiated specimens compared with those of the unirradiated specimens. The results obtained are discussed on the basis of the formation of Ni4Ti3 precipitates in irradiated specimens and their consequences on the phase transformations. 相似文献
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J. M. Dutkiewicz W. Maziarz T. Czeppe L. Lityńska W. K. Nowacki S. P. Gadaj J. Luckner E. A. Pieczyska 《The European physical journal. Special topics》2008,158(1):59-65
Powder metallurgy technology was elaborated for
consolidation of shape memory NiTi powders. The shape memory alloy was
compacted from the prealloyed powder delivered by Memry SA. The powder shows
Ms = 10°C and As = -34°C as results from DSC
measurements. The samples were hot pressed in the as delivered spherical
particle's state. The hot compaction was performed in a specially
constructed vacuum press, at temperature of 680°C and pressure of 400 MPa. The alloy powder was encapsulated in copper capsules
prior to hot
pressing to avoid oxidation or carbides formation. The alloy after hot
vacuum compaction at 680°C (i.e. within the B2 NiTi stability range)
has shown similar transformation range as the powder. The porosity of
samples compacted in the as delivered state was only 1%. The samples
tested in compression up to ε = 0.06 have shown partial
superelastic effect due to martensitic reversible transform- ation which
started at the stress above 300 MPa and returned back to ε = 0.015 after unloading. They have shown also a high ultimate compression
strength of 1600 MPa. Measurements of the samples temperature changes during
the process allowed to detect the temperature increase above 12°C for
the strain rate 10-2 s-1 accompanied the exothermic martensite
transformation during loading and the temperature decrease related to the
reverse endothermic transformation during unloading. 相似文献
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采用激光速度干涉仪(VISAR)、X射线衍射(XRD)和扫描电镜(SEM)联合测试技术,利用等厚对称加载和逆向加载实验,研究了Fe MnNi合金的冲击相变和层裂行为。结果发现:加载压力大于6.5 GPa时,Fe MnNi合金样品发生α→ε相变;中心稀疏波的卸载作用使内压力降至4~5 GPa时,Fe MnNi合金样品发生ε→α逆相变,并伴有卸载稀疏冲击波形成。分析Fe MnNi合金样品中塑性波、相变波、稀疏波和稀疏冲击波的传播作用过程,发现加载压力大于其相变应力时,等厚对称加载下Fe MnNi合金存在产生层裂行为的物理机制。 相似文献
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Natalia Resnina Sergey Belyaev Alexander Shelyakov Eugeniy Ubyivovk 《Phase Transitions》2017,90(3):289-298
An in situ transmission electron microscopy study of the B2 ? B19′ martensitic transformation in Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy was carried out. It was observed that the sequence of the martensite crystals shrinking on heating differed from the sequence of the martensite crystal appearance on previous cooling. This was shown that strain nanodomain formation on cooling prior to the forward martensitic transformation resulted in accumulation of the elastic energy. This led to the dependences of the elastic energy stored on cooling or released on heating on the volume fraction of the martensite phase became different. In this case, at the same volume fraction of the martensite phase, the configuration of the martensite crystals on cooling and heating was different and it was a reason for a violation of the sequence of the martensite crystal formation on cooling and its shrinking on heating. 相似文献
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T. D. Troev K. Hinode S. Tanigawa M. Doyama 《Applied Physics A: Materials Science & Processing》1977,13(1):105-106
Precision measurements of positron lifetime spectra in Fe-29.5at.%Ni alloy specimens were performed to clarify the nature
of the martensitic transformation in this alloy. The forward transformation did not introduce any defects which behave as
strong trapping sites for positrons. On the other hand, the reverse transformation accompanied the production of defects,
to which the positron lifetime is sensitive, maybe dislocations. From the present results, the nature of the transformation
was discussed. 相似文献
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Microstructural,phase transformation and magnetic properties of Ni-Mn-Ga alloy fabricated by spark plasma sintering
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The microstructural,phase transformation and magnetic properties of Ni-Mn-Ga alloy fabricated using the spark plasma sintering method have been investigated. The results show that both the as-sintered and annealed sintered specimens exhibit typical martensitic transformation behaviours. The martensite of the sintered specimen after annealing exhibits a ferromagnetic nature. Moreover,study of the fracture surface indicates that the transgranular fracture contributes to the higher ductility of sintered Ni-Mn-Ga alloy. In addition,the transformation strain in sintered Ni-Mn-Ga alloy is studied for the first time. 相似文献
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We report on new aspects of martensite stabilization in high-temperature shape memory alloys. We show that, due to the difference in activation energies among various structural defects, an incomplete stabilization of martensite can be realized. In material aged at high temperatures, this gives rise to a variety of unusual features which are found to occur in the martensitic transformation. Specifically, it is shown that both forward and reverse martensitic transformations in a Ni–Mn–Ga high-temperature shape memory alloy can occur in two steps. The observed abnormal behaviour is evidence that, in certain circumstances, thermoelastic martensitic transformation can be induced by diffusion. 相似文献
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G. A. Malygin 《Physics of the Solid State》2003,45(2):345-351
The martensite phase formation in elastic fields of isolated screw and edge dislocations, as well as in planar clusters of like-sign dislocations and in a two-dimensional network of opposite-sign edge dislocations, is quantitatively analyzed within the theory of smeared martensitic transitions. The heterogeneous nucleation of martensite at dislocations is shown to increase the characteristic temperature of the martensitic transition and its temperature smearing. 相似文献