金属快速加热的非弹性热软化研究

本文利用电阻加热法研究了Ｈ６２黄铜和ＬＹ１２铝合金在短时热冲击下的非弹性热软化性能。发现了两种合金的屈服温度随加热速率提高而明显提高的现象以及其在不同加热速率下的再结晶特点。将Ｊｏｈｎｓｏｎ－Ｍｅｈｌ恒温条件下的再结晶动力议程推广到变温的情况，从理论分析了加热速率高低对金属再结晶过程的影响，并提出了屈服温度上下限的概念。     

加热速率和形变量对IF钢再结晶温度的影响

采用膨胀仪法研究了4种退火加热速率对不同冷形变量的IF钢再结晶温度的影响。IF钢的再结晶温度随加热速率的增加而提高,随变形量的增大而降低,如将膨胀曲线的微分最低点定为再结晶温度,在形变量为ε=1.2的条件下,当加热速率由20℃/h提高到400℃/h时,该钢的再结晶温度由645℃提高到664℃。当加热速率保持不变,形变量由0.8增大至1.2时,再结晶温度下降约10℃左右。     

Q550级桥梁钢动态再结晶行为研究

采用Gleeble-3500热模拟试验机模拟了屈服强度550 MPa级桥梁钢轧板单道次热压缩变形过程,得到了试验钢的真应力-真应变曲线,分析了变形温度和应变速率对动态再结晶行为的影响,并建立了试验钢的再结晶图。结果表明,在较高的变形温度和较低的应变速率下,动态再结晶易进行。实验在动态再结晶激活能为460.14 kJ/mol时,建立了试验钢动态再结晶动力学模型。     

Mg-5.6Zn-0.7Zr-0.8Nd合金高温塑性变形的热/力模拟研究

采用Gleebe-1500热／力模拟机研究了Mg-5．6Zn-0．7Zr-0．8Nd合金在应变速率为0．1，0．01和0．002s^-^3、变形温度为373—673K,最大变形程度60％条件下的高温塑性变形行为．分析了合金流变应力与应变速率、变形温度之间的关系，计算了高温变形时变形激活能和应力指数，并观察了合金变形过程中显微组织变化情况．结果表明：Mg-5．6Zn-0．7Zr-0．8Nd合金在热变形过程中不同温度下流变应力呈现不同形式，分析可知加工硬化、动态回复和动态再结晶在不同温度和不同应变速率下各自起到了重要的作用，合金变形激活能随应变速率增加而升高．在473K温度以上变形，合金发生明显动态再结晶且动态再结晶晶粒非常细小，晶粒尺寸为5—10μm，从而可明显提高合金的塑性．     

AZ31B镁合金热压缩变形的组织变化与变形机制

对铸态AZ31B镁合金在温度280℃～440℃、应变速率0.001s-1～0.1s-1条件下进行热压缩实验,分析变形程度、应变速率和加热温度对其微观组织变化的影响,探讨合金的热压变形机制。实验结果表明,该合金热变形时发生了动态再结晶。变形温度越高、变形速率越小和变形量越大时,动态再结晶进行的越充分;变形温度越低、变形速率越大和变形量越大时,动态再结晶晶粒越细小。该合金的热变形机制是滑移孪晶联合机制。     

多元Nb—Ti—Al合金的热压缩模拟

对多元Nb-Ti-Al合金热压缩后的屈服曲线和显微组织进行了研究.结果表明:合金在室温低应变速率条件下不存在屈服软化现象;在高温大变形速率条件下存在屈服软化现象,且随着试验温度的降低和应变速率的增大,软化程度增大.其中在950℃时发生的屈服软化主要是由组织中溶质的再分配引起;而1050℃和1150℃时的屈服软化主要是由动态再结晶引起.     

15Cr-25Ni-Fe基合金高温塑性变形行为的加工图

在Gleeble-1500热模拟机上对15Cr-25Ni-Fe基合金GH2674进行了热压缩实验，采用动态材料模型的加工图研究了其在950-1200℃和0．001-10S^-1条件下的热变形行为．结果表明：GH2674合金在热变形时呈现两个微观机制不同的动态再结晶峰区．再结晶Ⅰ区：功率耗散效率峰值为38％，峰值对应的温度和应变速率分别为1040℃与10s^-1；再结晶Ⅱ区：功率耗散效率峰值为40％，峰值对应的温度和应变速率分别为1075℃与0．04s^-1．在1075-1100℃温度区间内，可能是晶界相M382的溶解造成该合金的晶粒粗化，这在一定程度上会影响合金的热加工性能．在应变速率小于0．01s^-1、形变温度高于1050℃条件下，合金呈现晶粒急剧粗化现象，进而导致在热变形过程中楔形裂纹的产生；在应变速率高于0．1s^-1、形变温度低于1000℃条件下，合金有出现剪切变形带的趋势．根据上述加工图对GH2674合金的热变形工艺进行了初步设计．     

加热温度对AZ31镁合金薄板组织性能的影响

通过对AZ31镁合金板带在不同加热温度的轧制过程观察，测量了不同轧制温度下板带的力学性能，并对其组织进行了分析。分析结果表明，轧制变形使板带晶粒细化，抗拉强度增加，伸长率降低；随着加热温度的升高，发生部分再结晶行为，AZ31镁合金板带的抗拉强度略有下降，屈服强度下降，伸长率提高，晶粒细化。     

0Cr11Ni2MoVNb钢连续冷却热压缩特性研究

采用Gleeble 3500研究了0Cr11Ni2MoVNb钢在变形速率0.01s-1,温度为1100℃～850℃时的连续冷却热压缩特性,结果表明,加热一次压缩0Cr11Ni2MoVNb钢在950℃变形抗力增加幅度加大,即终锻温度或终轧温度不应低于950℃;加热一次热变形压缩没有发现动态再结晶,加热两次及三次后变形有动态再结晶出现,出现再结晶的温度区间为1000℃～1050℃.     

GH2150合金的热变形行为及动态再结晶规律

采用Gleeble-3800热模拟试验机热模拟压缩试验研究了GH2150合金在不同试验参数下的热变形行为和再结晶演变规律。结果表明，在1000～1200℃范围内，应变速率为0.1～5 s^-1,变形量分别为30%、50%、70%条件下，合金峰值应力随变形温度升高而降低，随应变速率降低而降低。结合真应力-真应变曲线及阿伦尼乌斯公式得到了GH2150合金的热变形本构方程，采用该方程得到的计算结果与实际结果的平均相对误差为4.36%,相关系数R=0.992,具有较好的吻合性。绘制GH2150合金动态再结晶图发现大变形量有利于提高再结晶分数，合金再结晶行为在50%变形量下主要受变形温度影响，在70%变形量下采用低应变速率更有利于再结晶发生。     

Effect of temperature and heating rate on mechanical properties of magnesium alloy AZ31

Effects of temperature and heating rate on the mechanical properties of the tensile specimens of magnesium alloy AZ31 were experimentally investigated using a Gleeble-1500 thermo-mechanical material testing system. The metallurgraphs of the fracture section of the specimens were also experimentally observed and analyzed for exploring their failure mechanism under different temperatures and heating rates. The results show that the higher the temperature, the lower the ultimate strength of the specimens. And the higher the heating rate, the higher the ultimate strength of the specimens. The high temperatures and high heating rates will induce microvoids in the specimens which make the specimens failure under relatively low loads.     

加热速率对CuAlBe形状记忆合金相变的影响

通过差示扫描量热技术研究了加热速率对CuAlBe形状记忆合金的相变特征的影响。结果表明：在不同加热速率下的相变点在一个区域内变化。快速加热使两个相变峰靠近，随着加热速率的降低，两个相变峰逐渐分离。用超低降温速率法分离后的DSC曲线，右端的大峰变成一连串由许多微区依次发生马氏体逆相变所产生的许多小的放热峰。     

Microstructural Characterization of Thermomechanical and Heat-Affected Zones of an Inertia Friction Welded Astroloy

The behaviour of γ’ phase to thermal and mechanical effects during rapid heating of Astroloy, a powder metallurgy nickel-based superalloy has been investigated. The thermo-mechanical-affected zone (TMAZ) and heat-affected zone (HAZ) microstructures of an inertia friction welded (IFW) Astroloy were simulated using a Gleeble thermo-mechanical simulation system. Detailed microstructural examination of the simulated TMAZ and HAZ and those present in actual IFW specimens showed that γ’ particles persisted during rapid heating up to a temperature where the formation of liquid is thermodynamically favored and subsequently re-solidified eutectically. The result obtained showed that forging during the thermo-mechanical simulation significantly enhanced resistance to weld liquation cracking of the alloy. This is attributable to strain-induced rapid isothermal dissolution of the constitutional liquation products within 150 μm from the center of the forged sample. This was not observed in purely thermally simulated samples. The microstructure within the TMAZ of the as-welded alloy is similar to the microstructure in the forged Gleeble specimens.     

Selective hardening in E52100 steel

A process involving the use of an argon thermal plasma jet to facilitate selective hardening of a high- carbon steel (E52100) has been studied experimentally. Cylindrical hollow specimens (approximately 8.0 cm OD by 6.0 cm ID by 9.0 cm long) were treated during this process by rapidly heating the central section of these samples to 870 °C, soaking at 870 °C for 1.0 to 2.0 min and then rapidly quenching the samples. All samples were continuously turned at an angular velocity of about 6 rev/sec during each trial run to promote uniform changes in temperature. A noncontact infrared sensor was also used to monitor and facilitate specimen temperature control throughout the heating and soaking phases of each test trial. The heat treated specimens were sectioned longitudinally, polished, and used to obtain microhardness profiles for several of these heat treated specimens. This work indicated that complete and relatively uniform through hardening was achieved in all of the test specimens. A significant advantage of this process over other more conventional hardening methods involves the very rapid heat transfer rates that are possible between a plasma jet flame and the heated object. An additional and somewhat related advantage is that undesirable overheating of specimen sections (adjacent to the heat treated zone) by conduction can be minimized. This effect may permit the production of selectively hardened steel objects that cannot be produced using more conventional technology.     

热处理22MnB5钢的力学响应及本构建模

22MnB5钢冷轧板材分别在750、850和950 ℃保温30 min水淬后,进行了应变速率分别为0.0005、0.001、0.01和0.1 s^-1的室温拉伸试验。结果表明:随着热处理温度的升高,试验钢的微观组织中板条状马氏体含量增加,其屈服强度和抗拉强度也显著提升;随着应变速率的增加,试验钢的屈服强度和抗拉强度呈现先增加后减小的趋势;最后基于Voce本构模型,通过引入Johnson-Cook本构的应变速率项,构建了可以描述22MnB5钢在不同热处理工艺和应变速率下力学行为的本构方程,其相关系数(R)和平均相对误差(AARE)分别为0.993和3.15%。     

Effect of heating rate and grain size on the melting behavior of the alloy Nb–47 mass% Ti in pulse-heating experiments

The effect of heating rate and grain size on the melting behavior of Nb–47 mass% Ti is measured and modeled. The experimental method uses rapid resistive self-heating of wire specimens at rates between ∼10² and ∼10⁴ K/s and simultaneous measurement of radiance temperature and normal spectral emissivity as functions of time until specimen collapse, typically between 0.4 and 0.9 fraction melted. During heating, a sharp drop in emissivity is observed at a temperature that is independent of heating rate and grain size. This drop is due to surface and grain boundary melting at the alloy solidus temperature even though there is very little deflection (limited melting) of the temperature–time curve from the imposed heating rate. Above the solidus temperature, the emissivity remains nearly constant with increasing temperature and the temperature vs time curve gradually reaches a sloped plateau over which the major fraction of the specimen melts. As the heating rate and/or grain size is increased, the onset temperature of the sloped plateau approaches the alloy liquidus temperature and the slope of the plateau approaches zero. This interpretation of the shapes of the temperature–time curves is supported by a model that includes diffusion in the solid coupled with a heat balance during the melting process. There is no evidence of loss of local equilibrium at the melt front during melting in these experiments.     

快速回火工艺条件下超高强低碳贝氏体钢的组织与性能

采用盐浴热处理试验,结合扫描电镜、透射电镜及室温拉伸试验,研究了快速加热+短时保温快速回火条件下超高强低碳贝氏体钢的组织和性能变化规律。结果表明,快速回火工艺下,超高强低碳贝氏体钢发生碳过饱和贝氏体和马氏体中的碳化物析出、铁素体和马氏体的重构以及微合金析出物的析出等现象,进而影响材料的强塑性;在700℃以下快速回火时,与以板条状贝氏体(LB)组织为主的复相贝氏体钢相比,以粒状贝氏体(GB)组织为主的钢具有更好的回火稳定性;在750～800℃两相区快速回火时,铁素体和马氏体相大量重构,最终形成粗大铁素体和马氏体,抗拉强度大幅提升,屈强强度大幅降低,且以LB组织为主的复相贝氏体钢中重构铁素体晶粒更为粗大,导致其屈服强度更低。     

Loading Mode and Environment Effects on Surface Profile Characteristics of Martensite Plates in Cu-Based SMAs

The present work reports the influence of the loading mode provided during training under constant stress, in bending, applied to lamellar specimens of Cu-Zn-Al shape memory alloys (SMAs). During training, the specimens were bent by a load fastened at their free end, while being martensitic at room temperature and they lifted the load by one-way effect (1WE), during heating up to austenitic field. On cooling to martensite field, the lower concave surface of bent specimens was compressed, and during heating it was elongated, being subjected to a series of tension-compression cycles, during heating-cooling, respectively. Conversely, the upper convex surface of bent specimens was elongated during cooling and compressed during heating, being subjected to compression-tension cycles. Furthermore, 2WE-trained actuators were tested by means of a hydraulic installation where, this time heating-cooling cycles were performed in oil conditions. Considering that the lower concave surface of the specimens was kept in compressed state, while the upper convex surface was kept in elongated state, the study reveals the influence of the two loading modes and environments on the width of martensite plates of the specimens trained under various numbers of cycles. In this purpose, Cu-Zn-Al specimens, trained under 100-300-500 cycles, were prepared and analyzed by atomic force microscopy (AFM) as well as optical and scanning electron microscopy (OM and SEM, respectively). The analysis also included AFM micrographs corroborated with statistical evaluations in order to reveal the effects of loading mode (tension or compression) in different environmental conditions of the specimens, on the surface profile characteristics of martensite plates, revealed by electropolishing.     

GCr15轴承钢在不同加热速率下的奥氏体转变研究

加热速率对GCr15轴承钢铸坯表面组织有较大影响。利用DIL805A热膨胀仪进行热模拟试验,通过分析GCr15轴承钢在连续加热过程中的热膨胀曲线,研究了不同加热速率下的奥氏体转变过程,分析了加热温度对奥氏体转变温度和奥氏体转变量的影响,分析了不同加热速率下奥氏体转变规律和大断面铸坯表面组织。结果表明:GCr15轴承钢中珠光体转变为奥氏体,温度范围约为760~810 ℃;(Fe,Cr)₃C_Ⅱ向奥氏体中的溶解,温度范围约为810~1 100 ℃;奥氏体的成分均匀化温度大于1 100 ℃。若GCr15大断面铸坯表面过热度大,相变后晶粒粗大,相对于内部组织其表面的耐磨性和抗疲劳性下降,且铸坯表面奥氏体浓度均匀性差,后续液析碳化物溶解过程受阻碍,碳化物溶解浓度不均匀,表面的组织性能受到影响。根据J-M-A方程,计算了模型参数,GCr15轴承钢激活能Q约为7.156×10⁵ J/mol,n=0.52,k₀=75。     

Coupled thermodynamic/kinetic analysis of diffusional transformations during laser hardening and laser welding

Several important industrial material processes, such as welding and surface treatments with high energy beams, incorporate rapid thermal cycles characterized by high heating/cooling rates and short dwell times. Computational simulation of the evolution of microstructure under these extreme conditions has received rather limited attention. With the advent of modern computational tools regarding alloy thermodynamics and kinetics, it is possible to simulate the progress of diffusional phase transformations and thus to predict microstructural development. In the present work, moving boundary diffusion problems have been simulated for two cases. In the first case the rapid austenitization during laser transformation hardening of a hypoeutectoid steel was examined. The effects of heating rate, maximum temperature, dwell time and initial microstructure fineness were analyzed. In the second case the aging, dissolution and coarsening of strengthening precipitates in the heat affected zone of laser welds in Al–Mg–Si alloys was examined. The simulation provided the variation of the volume fraction and average size of the strengthening phase during the weld thermal cycle. In both cases the calculations were performed by applying the coupled thermodynamics and kinetics approach, incorporated in the DICTRA program. This kind of simulation provides useful information for the design of the above processes.