Material Behavior at Cryogenic Temperatures
介绍相关的物理现象和材料在低温下的性能
金属材料在低温条件下的行为贯穿始终 cryogenic testing 在相同类型的测试中，它们在室温下的行为有很大不同. 这是由于当温度达到液氮的温度(-196°C)或当接近液氦的温度(约-267°C)时，各种物理现象变得相关或进一步下降。.
以下是在低温下发生的典型行为的定性描述. This narrative, although very simplified, 试图对实验室试验中显而易见的现象作出解释.
1. Heat Capacity
在低温下最相关的影响之一是定容热容(Cv)。. Under constant-volume conditions, 温度(T)变化的主要贡献是有关物体的内能(U)的变化. Its value can therefore be expressed as:
To a sufficient degree of approximation, 可以说，几乎所有固体在室温上下的摩尔比热都是恒定的，与温度无关. 根据Dulong Petit定律，它近似等于:
Where R is the molar 气体 constant (≈8.314 J mol-1K-1).
This law ceases to apply as we approach cryogenic conditions, 在哪里 quantum phenomena become increasingly relevant. At temperatures below 20 to 30 Kelvin, Dulong和Petit定律不再适用，必须考虑其他模型，如Debye积分.
基本上, 这些统计数据表明，当温度接近绝对零度时，比热趋向于零(或至少像爱因斯坦模型中那样趋向于非常低的值).
As can be deduced from the expression of Cv above, a very low Cv 值引起强烈的温度变化，即使内能∆U变化很小, 在材料中潜在产生热不稳定现象的问题，在足够的强度，以促进结构的变化在晶格中.
2. Reduced mobility of dislocations
低温下的第二个相关现象是固体基体中位错迁移率的降低, 哪个在与温度相关的运输机制中越来越不受欢迎.
这种降低的迁移率进一步有利于在晶格中已经存在的障碍物附近堆积位错.g. in the presence of Lomer-Cottrell dislocations, which can form in FCC-type lattices, such as in austenitic steels).
堆积过程继续进行，直到储存在势垒处的能量超过势垒本身的能量. 在这一点上，随着储存的内能的迅速释放，结构发生坍塌. 如果在堆叠阶段，内能∆U的变化仍然有限, 屏障的崩塌导致能量的大量释放, 与Cv very low at cryogenic temperatures, 结果导致一个快速的温度峰值，可以是几十度的顺序.
释放的能量在大多数被封锁的地点引发雪崩现象, encouraging a repeat of the same phenomenon.
在宏观层面上，上述情况在实验室测试的结果中变得明显可见.
In particular, the graphs of stress and strain for a tensile test show a typical serrated pattern, with strong and frequent oscillations, 每个都与上述能量积累和释放现象相关(图1)。. 这些振荡可以影响很大比例的图形区域, in many cases from 10% to 30% of the total elongation.
3. Hardening of material
A well-known phenomenon that occurs during cyclic fatigue testing is the hardening of the material. 即使在-196°C，这种现象也变得特别相关和清晰可见. 从图2中可以明显看出，在-196°C下的循环载荷耐久性要比在室温下的相同测试好得多.
硬化现象在奥氏体钢中尤为明显, often accompanied by austenite-martensite transformations. It should be noted that, as martensite is ferromagnetic (在哪里as austenite is diamagnetic), 在涉及特别高磁场的应用中，这种转换变得尤为关键.
4. Charpy test at cryogenic temperatures
The last test considered in this article is the Charpy test. Finite element simulations 在米兰进行的测试表明，测试技术通常按照ISO148和ASTM E23的描述使用, which involves cooling the sample to the desired temperature, transferring the sample to the testing machine, then impact testing within 5 seconds. It is not applicable at extremely low temperatures, though.
事实上, 从图中可以看出，仅从杜瓦瓶中提取样品，在5秒后缺口处的温度至少增加了42开尔文, 使钢的有效试验温度达到-228℃左右. The worst case is for aluminum, 在哪里, after 5 seconds, the temperature reaches about 62 Kelvin, thus bringing the test temperature to values around -211° C.
The solution adopted by im体育APP Milan 在极低温度下进行夏比试验是在连续氦流条件下进行试验.
这极大地限制了由于样品从杜瓦瓶转移到测试系统而引起的温升. This allows us to guarantee a nominal temperature of -267° C, with an uncertainty of approximately 20 Kelvin.
To find out more about our cryogenic testing services,请 contact our team of experts today.
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