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Shape memory response and microstructural evolution of a severe plastically deformed high temperature shape memory alloy (NiTiHf)

机译：严重塑性变形高温形状记忆合金（NiTiHf）的形状记忆响应和微观结构演变

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NiTiHf alloys have attracted considerable attention as potential high temperature Shape Memory Alloy (SMA) but the instability in transformation temperatures and significant irrecoverable strain during thermal cycling under constant stress remains a major concern. The main reason for irrecoverable strain and change in transformation temperatures as a function of thermal cycling can be attributed to dislocation formation due to relatively large volume change during transformation from austenite to martensite. The formation of dislocations decreases the elastic stored energy, and during back transformation a reduced amount of strain is recovered. All these observations can be attributed to relatively soft lattice that cannot accommodate volume change by other means. We have used Equal Channel Angular Extrusion (ECAE), hot rolling and marforming to strengthen the 49.8Ni-42.2Ti-8Hf (in at. %) material and to introduce desired texture to overcome these problems in NiTiHf alloys. ECAE offers the advantage of preserving billet cross-section and the application of various routes, which give us the possibility to introduce various texture components and grain morphologies. ECAE was performed using a die of 90?? tool angle and was performed at high temperatures from 500??C up to 650??C. All extrusions went well at these temperatures. Minor surface cracks were observed only in the material extruded at 500 ??C, possibly due to the non-isothermal nature of the extrusion. It is believed that these surface cracks can be eliminated during isothermal extrusion at this temperature. This result of improved formability of NiTiHf alloy using ECAE is significant because an earlier review of the formability of NiTiHf using 50% rolling reduction concluded that the minimum temperature for rolling NiTi12%Hf alloy without cracks is 700??C. The strain level imposed during one 90?? ECAE pass is equivalent to 69% rolling reduction. Subsequent to ECAE processing, a reduction in irrecoverable strain from 0.6% to 0.21% and an increase in transformation strain from 1.25% to 2.18% were observed at a load of 100 MPa as compared to the homogenized material. The present results show that the ECAE process permits the strengthening of the material by work hardening, grain size reduction, homogeneous distribution of fine precipitates, and the introduction of texture in the material. These four factors contribute in the increase of stability of the material. In this thesis I will be discussing the improvement of mechanical behavior and stability of the material achieved after various passes of ECAE.
机译：NiTiHf合金作为潜在的高温形状记忆合金（SMA）已经引起了相当大的关注，但是在恒定应力下的热循环过程中，转变温度的不稳定性和明显的不可恢复的应变仍然是一个主要问题。不可恢复的应变和相变温度变化与热循环有关的主要原因可以归因于在从奥氏体到马氏体的相变过程中，由于体积变化相对较大而形成的位错。位错的形成降低了弹性存储的能量，并且在反向转变期间，恢复了减少的应变量。所有这些观察结果都可以归因于相对较软的晶格，该晶格无法通过其他方式适应体积变化。我们已经使用等通道角挤压（ECAE），热轧和锻压来增强49.8Ni-42.2Ti-8Hf（原子％）材料，并引入所需的织构以克服NiTiHf合金中的这些问题。 ECAE具有保留钢坯横截面和应用各种路线的优势，这使我们有可能引入各种纹理成分和晶粒形态。 ECAE使用90？的模具进行。工具角度，并在500？C至650？C的高温下进行。在这些温度下，所有挤压都进行得很好。仅在500℃下挤压的材料中观察到了微小的表面裂纹，这可能是由于挤压的非等温特性所致。据信，在该温度下的等温挤出过程中可以消除这些表面裂纹。使用ECAE改善NiTiHf合金的可成形性的结果是重要的，因为较早的审查使用50％压下率的NiTiHf的可成形性得出的结论是，轧制无裂纹的NiTi12％Hf合金的最低温度为700℃。一次90？施加的应变水平ECAE合格率相当于轧制减少69％。在ECAE处理之后，与均质材料相比，在100 MPa的载荷下，不可恢复的应变从0.6％降低至0.21％，转化应变从1.25％升高至2.18％。目前的结果表明，ECAE工艺允许通过加工硬化，减小晶粒尺寸，均匀分布细小沉淀物以及在材料中引入织构来增强材料。这四个因素有助于增加材料的稳定性。在本文中，我将讨论在ECAE的各种通过之后所获得的材料的机械性能和稳定性的改进。

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    Simon Anish Abraham;

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  • 年度 2006
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